Ahcène Bounceur

Mohammad Hammoudeh

Jameleddine Hassine

Madani Bezoui

Adel Fadhl

Muhamad Felemban

Ahcene Bounceur
email: ahcene.bounceur@kfupm.edu.sa

Ahcene Bounceur

RPA-Code for Secure Binary Sequence Generation from Graph-Based Scanning

Keywords: Random generation, graph, polar-angle, LPCN algorithm, key generation

des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

INTRODUCTION

Generating random numbers from pictures is an emerging field with applications in various domains, including cryptography, security, and data encryption. In today's world, QR codes have become the most commonly employed method for copying information without the need for manual data entry or keyboard input. For publicly accessible, non-sensitive information, QR codes can be utilized without any concerns. However, when these codes are used to conceal or safeguard sensitive data, as far as we are aware, there is currently no established standard code designed to address this particular requirement. In this paper, we introduce a novel approach known as Random-Polar-Angle (RPA) coding for generating random codes that are inherently resistant to easy decipherment.

Generating random numbers from pictures is an evolving field, with a wide range of potential applications. Researchers and practitioners continue to explore new methods and techniques to harness the inherent randomness present in visual data.

Cryptography relies heavily on the generation of secure and unpredictable cryptographic keys. Traditional methods often use pseudorandom number generators, but the security of such sequences can be compromised. Over the years, researchers and cryptographic practitioners have delved into a multitude of techniques and algorithms to produce keys and binary streams that are both highly secure and unpredictable. This pursuit of cryptographic robustness has led to the development of innovative methods that harness mathematical principles, complex algorithms, and even physical phenomena. One such approach revolves around the use of graphbased algorithms, where the structure of a graph is utilized to derive secure binary sequences. In this context, the LPCN (Lowest Polar Angle Connectivity Node) algorithm, as introduced in [14] [START_REF] Saoudi | D-LPCN: A distributed least polar-angle connected node algorithm for finding the boundary of a wireless sensor network[END_REF], stands as a prominent example. By selecting nodes within a graph based on minimal polar angles, the LPCN algorithm introduces an element of randomness and complexity that enhances the security of generated binary sequences. This technique has gained attention for its potential to transform graphical representations into concealed binary codes, thereby bolstering data protection, by transforming the type of the found angle in each iteration to a binary value such as:

• acute angle = 1 • obtuse angle = 0
The rest of the paper is organized as follows. Section 2 presents and discusses the related work. Section 3 describes the LPCN algorithm. The proposed methodology is presented in Section 4. Section 5 presents the implementation and results obtained by applying the proposed method, followed by an assessment of the randomness of the generated sequences. Finally, Section 6 concludes the paper and offers perspectives for further exploration.

LITERATURE REVIEW

There has been a significant amount of work done in the domain of generating random numbers from images. Image steganography [START_REF] Fridrich | Steganography in Digital Media: Principles, Algorithms, and Applications[END_REF] is the practice of embedding hidden messages or data within digital images without altering their visible appearance. This technique is used for covert communication and information security. Chandramouli and Memon [START_REF] Chandramouli | Analysis of LSB based image steganography techniques[END_REF] investigated the "steganographic capacity" of image based steganography techniques. Steganographic capacity refers to the maximum amount of data that can be concealed or embedded within an image without causing noticeable or statistically significant changes to that image. It is typically measured in terms of bits. In other words, it answers the question of how much information can be hidden in an image without making the presence of the hidden data evident. The authors [START_REF] Chandramouli | Analysis of LSB based image steganography techniques[END_REF] analyzed LSB (Least Significant Bits) steganography, which is a specific method for achieving steganographic capacity. In LSB steganography, the hidden data is inserted into the least significant bits of the pixels in an image, while the most significant bits remain unchanged. Since the least significant bits have the least impact on the visual appearance of the image, this method aims to maximize the steganographic capacity while minimizing the noticeable alterations to the cover medium. In order to increase information security, Wang et al. [START_REF] Wang | High-capacity adaptive steganography based on LSB and Hamming code[END_REF] introduced a hybrid steganography method based on least significant bit (LSB) replacement and Hamming code (HLAH). As the sharp areas of the image can accommodate greater alterations compared to the smooth areas, the HLAH method embeds a higher volume of secret messages in edge regions and a minimal amount in smooth regions. Experimental results [START_REF] Wang | High-capacity adaptive steganography based on LSB and Hamming code[END_REF] demonstrate that the HLAH method offers both superior embedding capacity compared to existing techniques and consistently maintains higher image quality. Cheddad et al. [START_REF] Cheddad | Digital image steganography: Survey and analysis of current methods[END_REF] presented a state-of-the-art review and analysis of the different existing steganography techniques. More recently, the utilization of Generative Adversarial Networks (GANs) [START_REF] Goodfellow | Generative adversarial networks[END_REF] in image steganography research has shown significant promise and potential. The GAN [START_REF] Goodfellow | Generative adversarial networks[END_REF] may be trained adversarially to ensure that the stego-images are visually identical to the cover images. This adversarial training makes it challenging for external parties to detect the presence of hidden data. The generator in a GAN [START_REF] Goodfellow | Generative adversarial networks[END_REF] can be used to embed the secret information into an image. Instead of generating random images, the generator can take the cover image and encode the hidden data in a way that the output image appears visually similar to the original one. This process ensures that the embedded information is perceptually indistinguishable from the original image. The GAN discriminator is used to ensure that the generated stego-image appears similar to real images and not distinguishable from authentic images. It helps in maintaining the quality of the stego-image. Liu et al. [START_REF] Liu | Recent Advances of Image Steganography With Generative Adversarial Networks[END_REF] conducted an extensive review of steganography techniques utilizing GANs, systematically categorizing them into three distinct data hiding strategies, encompassing cover modification, cover selection, and cover synthesis. For a comprehensive survey of image steganography, the reader is referred to [START_REF] Inas | Comprehensive survey of image steganography: Techniques, Evaluations, and trends in future research[END_REF].

In pixel-based methods, the randomness is extracted from the pixel values of an image. The noise in an image, such as that from a camera sensor or compression artifacts, can be a source of randomness. Techniques involve analyzing pixel values for entropy, employing hash functions, or applying statistical tests to determine the randomness level. Hu and Han [START_REF] Hu | A pixel-based scrambling scheme for digital medical images protection[END_REF] introduced an efficient and secure pixel-based scrambling method for safeguarding the distribution of digital medical images. In order to efficiently encrypt a substantial volume of digital medical images, their approach employs a straightforward pixel-level XOR operation for image scrambling, incorporating structural parameters of the encryption scheme into the cryptographic key. This cryptographic key is derived from a genuinely random number sequence generated from multi-scroll chaotic attractors.

Visual Cryptography [START_REF] Dyala | An overview of visual cryptography techniques[END_REF] is a technique that encrypts an image into multiple shares, each containing partial information. Combining these shares reveals the original image. The randomness in these shares can be harnessed as a source of random numbers. Gurunathan and Rajagopalan [START_REF] Gurunathan | A stegano -visual cryptography technique for multimedia security[END_REF] drew inspiration from steganography and introduced a technique for concealing a secret message within a cover image. This method divides the cover image into n blocks, each measuring 8x8 pixels, while the secret message is partitioned into n segments. This approach enhances both image quality and the capacity of the hidden message, bolstering its security. In their proposed method [START_REF] Gurunathan | A stegano -visual cryptography technique for multimedia security[END_REF], they utilize Cuckoo Search (CS) to search for approximate and optimal solutions for transforming each block of the message. This is in contrast to seeking a single optimal substitution matrix for the entire cover image. "

Random sequence generators typically fall into two categories: true random number generators (TRNGs) and pseudorandom number generators (PRNGs). PRNGs rely on seeds and deterministic algorithms to produce "pseudorandom" numbers, making them faster and suitable when a significant quantity of random-like numbers is needed. In contrast, TRNGs harness non-deterministic sources in conjunction with post-processing functions, such as image noise, to generate randomness. The resulting sequences from TRNGs are recognized for their heightened level of randomness. Zhou et al. [START_REF] Zhou | True random number generator based on mouse movement and chaotic hash function[END_REF] proposed an innovative True Random Number Generator (TRNG) that derives random bits from mouse movements, which are captured as a 2D image. This method offers a practical, versatile, and cost-effective solution for personal computer (PC) platforms. To mitigate regular patterns in mouse movements stemming from user habits, the authors proposed three TRNGs based on chaotic hash functions. Comprehensive experiments were carried out to assess the speed, diffusion, and randomness characteristics of these TRNGs. The results indicate that two of these TRNGs exhibit satisfactory performance and are suitable for implementation on standard PC platforms. Dragan and Mladen [START_REF] Lambić | Pseudo-random number generator based on discrete-space chaotic map[END_REF] introduced a method for generating pseudo-random numbers using a discrete-space chaotic map based on permutation compositions. The randomness of the generated pseudo-random sequences is verified through NIST 800-22 and TestU01 tests. Notably, this method [START_REF] Lambić | Pseudo-random number generator based on discrete-space chaotic map[END_REF] remains unaffected by dynamical degradation, ensuring stable pseudo-random number generation. Its advantages include an extensive key space, efficient memory usage, enhanced security, and extended cycle lengths, making it suitable for memoryconstrained devices. Lakshman et al. [START_REF] Lakshman | A novel logo-inspired chaotic random number generator[END_REF] introduced an innovative and straightforward pseudo-random number generation method that utilizes one-dimensional discrete chaotic maps and a logo image. The method combines sequences from chaotic maps and a logo image to generate real numbers within the range of zero to one, establishing a strong connection between the two. This approach yields numbers that exhibit high sensitivity to minor variations in the underlying keys. The effectiveness of the generator is assessed through a comprehensive set of statistical and security analyses. Rongzhong Li [START_REF] Li | A True Random Number Generator algorithm from digital camera image noise for varying lighting conditions[END_REF] introduced a true random number generator (TRNG) approach using the images taken by web or mobile phone cameras. The proposed TRNG technique uses all three RGB color channels to obtain the random numbers. The study also investigated the physical and statistical properties of the random noise in a digital images and made several approximations to efficiently collect the best random signals from the pixels in the images to map them to random sequences.

Amri et al. [START_REF] Amri | Quantum Random Number Generation Using a Quanta Image Sensor[END_REF] introduced a novel quantum random number generation method leveraging the inherent randomness of photon emissions and the single photon counting capability of the Quanta Image Sensor (QIS). The QIS, consisting of over one billion pixels known as 'jots, ' is optimized for photon counting with sub-micron pitch. This high-speed, low-power QIS technology has the potential to generate high-quality random numbers at a remarkable data output rate, surpassing conventional QRNG technologies.

Kawashima and Mito [START_REF] Kawashima | Random number generation using magnetic domain images of magneto-optical materials[END_REF] proposed a method for generating random numbers from magnetic domain patterns within a magnetooptical (MO) material. By capturing one hundred thousand chaotic magnetic domain images using a transparent polarization microscope, they applied post-processing techniques, including binarization, frequency adjustment, exclusive-OR operations, and offset data reduction, to extract random numbers from the images. These generated random numbers successfully passed the statistical test (NIST SP-800-22).

Yas Abbas Alsultanny [START_REF] Abbas | Random-bit sequence generation from image data[END_REF] introduced a 3-phase approach for generating a highly random bit sequence. In the initial phase, image data is read and partitioned into blocks. The second phase is dedicated to generating a random key, and the third phase conducts statistical tests to assess whether the key sequence exhibits the characteristics expected of a genuinely random sequence. These tests are probabilistic in nature, not deterministic. Users can choose to accept the generated key based on a threshold value, indicating the minimum number of successful tests required. If the key doesn't meet this criterion, the process can be repeated until a satisfactory key is obtained. The algorithm employs five statistical tests: Frequency, Serial, Poker, Runs, and Auto-correlation. According to the author [START_REF] Abbas | Random-bit sequence generation from image data[END_REF], 79% of the generated sequences successfully pass all five statistical tests concurrently, with 94% passing at least four of them.

LPCN ALGORITHM

The LPCN (Least Polar-angle Connected Node) algorithm is used to find a polygon hull of a set of connected points. It starts from a node that belongs to the border. It is given as follows. Let 𝑉 be a set of 𝑛 nodes of a network and ℎ be the number of points of the polygon hull of 𝑉 . The algorithm finds the points 𝑃 0 , 𝑃 1 , ..., 𝑃 ℎ-1 of the convex hull. First of all, we find a point 𝑃 0 with the minimum x-coordinate. Given the point 𝑃 𝑖 , we wish to find the next consecutive point 𝑃 𝑖+1 on the hull, for each 1 ≤ 𝑖 ≤ 𝑘 -1 which is connected to them. This point 𝑃 𝑖+1 is the one that has the least polar angle with respect to 𝑃 𝑖 . When we reach the highest point 𝑃 ℎ-1 , we have constructed the polygon hull of 𝑉 . The algorithm stops once we reach again the point 𝑃 0 . It is given as follows:

1: procedure LPCN1(𝑉 , 𝐸) Figure 3 shows how the algorithm works. Let us consider the graph of Figure 3(a) where 𝑉 = {𝐴, 𝐵, 𝐶, 𝐷, 𝐸, 𝐹, 𝐺, 𝐻 }. First, we choose the node that has the minimum x-coordinate 𝑃 𝑐 . In this example, this node is 𝐴 (𝑃 𝑐 = 𝐴) which is the first node of the border B = {𝐴}.

The end of the algorithm is determined when the next border node 𝑃 𝑘 is equal to the first border node 𝑃 𝑓 . In this example, the first node is also 𝐴 (𝑃 𝑓 = 𝐴). In the first iteration 𝑃 𝑐 is always equal to 𝑃 𝑓 .

Let consider a fictive node 𝐴 ′ that has an x-coordinate smaller than that of 𝐴 (𝑃 𝑝 = 𝐴 ′ ). Note, that other fictive nodes can be considered. Figure 3(b) shows the different starting nodes and their fictive neighbors designed by the gray points and letters with apostrophes.

Next, we have to find the minimum polar angle formed by the edge (𝑃 𝑐 , 𝑃 𝑝 ) (i.e., (𝐴, 𝐴 ′ )) and the edge formed by 𝑃 𝑐 and each edge of its neighbors ℵ(𝑃 𝑐 ) (i.e., (𝐴, 𝐵), (𝐴, 𝐶), (𝐴, 𝐷) and (𝐴, 𝐺)). In this example, the obtained neighbor is 𝑃 𝑘 = 𝐶 (cf. Figure 3(c)). Then, the second edge of the searched border is (𝐴, 𝐶). Therefore, B = {𝐴, 𝐶}.

In the next iteration we will do the same procedure by searching the minimum polar angle formed by the edge (𝐶, 𝐴) and edges formed by 𝐶 and its neighbors, i.e., (𝐶, 𝐵) and (𝐶, 𝐷) except (𝐶, 𝐴), the last found edge. The obtained node is 𝐷 (cf. Figure 3(d)). Therefore, the obtained border is B = {𝐴, 𝐶, 𝐷 }. In the same way we determine the other nodes. We found 𝐸 (cf. Figure 3(e)), 𝐻 (cf. Figure 3(f)), 𝐺 (cf. Figure 3(g)) and finally 𝐴 (cf. Figure 3(h)). Since 𝐴 = 𝑃 𝑓 , we stop the algorithm. The obtained border is B = {𝐴, 𝐶, 𝐷, 𝐸, 𝐻, 𝐴} (cf. Figure 3(i)).

The first version of the algorithm LPCN can not work in some graphs when and edge in the border intersect another edge of the border. In case where the aim is to determine the border nodes, this version can not be used, however, in the framework of this paper, we can use it since the objectif is not to find the border nodes but a sequence of nodes that can be used as a code. The following algorithm 2 presents the second version of the LPCN algorithm (LPCN2) where the intersection is eliminated. It will be used in this paper to compare and discuss the codes obtained from both versions.

For simplicity, by considereing only the three first cases, the proposed algorithm is defined as follows:

1: procedure LPCN2(𝑉 , 𝐸) The LPCN algorithm demonstrates minimal energy consumption [START_REF] Kara | A compute and wait in pow (cw-pow) consensus algorithm for preserving energy consumption[END_REF] [START_REF] Pham | 4 -Radio channel access challenges in LoRa low-power wide-area networks[END_REF], and its computational complexity is polynomial.

METHODOLOGY

In this section, we will present the proposed methodology. First, a graph will be generated randomly by considering three main parameters:

• The area 𝑎 : which represents a rectangular space where the graph will be generated • The number of nodes 𝑛 of the graph • The range 𝑟 which represents the circular area around a given node such that all the nodes belonging to the circle will be connected to this node Figure 4 shows an example of a linear graph with 𝑛 = 8 nodes. When we apply the LPCN algorithm to this graph, starting from node 1, we traverse the following angles. Additionally, each angle's type and corresponding binary value are provided.

• This will lead to the generation of the binary stream 100111. The proposed methodology is composed of two main steps:

• Image Transformation: One of the primary applications of this method is the transformation of an image of the graph into a hidden binary code. By scanning the graph in the image and applying the LPCN algorithm, a binary sequence is generated. This sequence can be embedded into the image in a way that is imperceptible to the human eye, ensuring data security. • The Graph Representation: The first step involves representing the data as a graph, where nodes represent data points or features, and edges represent connections between them. This graph is used as the foundation for generating the binary sequence. • LPCN Algorithm: The LPCN algorithm is employed to traverse the graph and select nodes. At each iteration, the algorithm identifies the node that forms the minimal polar angle with its neighbor from the previous iteration. This unique selection process introduces randomness and complexity into the sequence generation. • Iterative Selection: The LPCN algorithm iteratively selects nodes, forming a sequence of nodes from the graph. These selected nodes are then transformed into binary values, ensuring that the generated binary sequence is both secure and unpredictable.

(a)

(b) (c) (d) (e) (f) (g) (h) (i)

EVALUATION

To measure the quality of the generated sequences of bits, we use the concept of entropy, first introduced by Shannon [START_REF] Shannon | A mathematical theory of communication[END_REF]. Sequences with higher entropy are considered to be of higher quality in terms of randomness and unpredictability.

The entropy of generated sequence can be calculated using the formula:

𝐻 (𝑋 ) = -

CONCLUSIONS AND FUTURE WORK

The LPCN algorithm offers a promising approach to generating secure binary sequences for cryptographic purposes. By leveraging the unique node selection based on polar angles, this method enhances the unpredictability and security of generated sequences.

2 : 4 :: repeat 7 : 9 :Figure 1 :

 24791 Figure 1: LPCN1 algorithm (version 1).

Figure 2 :

 2 Figure 2: LPCN2 algorithm (version 2).

( 1 ,

 1 2, 3): obtuse = 1 • (2, 3, 4): acute = 0 • (3, 4, 5): acute = 0 • (4, 5, 6): obtuse = 1 • (6, 7, 8): obtuse = 1 • (7, 8, 9): obtuse = 1

Figure 3 :

 3 Figure 3: LPCN algorithm illustration.

Figure 4 :

 4 Figure 4: Generate binaries from angles.

  𝑖 ) • log (𝑃 (𝑥 𝑖 )) (1) Where: 𝐻 (𝑋 ) : Entropy of the sequence 𝑛 : Number of possible bit values 𝑥 𝑖 : Each possible bit value 𝑃 (𝑥 𝑖 ) : Probability of each bit value We have generated many graphs with different connection ranges: 60, 100, 140, 180. For each graph, we have executed the LPCN algorithm with (version 1) and without (veresion 2) intersections to generate the binary sequence. Then, we have calculated the corresponging entropy.

Figure 5 :

 5 Figure 5: LPCN algorithm with (a) and without (b) intersections executed on a graph with 60 nodes.

Table 1

 1 presents the computed entropy for binary sequences (numbers) generated from each graph. Instances with intersection (Yes) tend to exhibit slightly lower entropy values compared to those without intersection (No), suggesting a potential influence on the randomness of the sequences. Additionally, the variation in entropy across different graph sizes indicates the sensitivity of entropy to graph characteristics, highlighting the importance of considering both intersection and graph size when assessing the quality of the generated random sequences. In this experiment, the best entropy is obtained by the graphs with 60 and 100 nodes.

Figure 6 :

 6 Figure 6: LPCN algorithm with (a) and without (b) intersections executed on a graph with 100 nodes.

Figure 7 :

 7 Figure 7: LPCN algorithm with (a) and without (b) intersections executed on a graph with 140 nodes.

  𝑃 𝑝 ← fictive point situated in the left of 𝑃 𝑓 𝑃 𝑣 𝑖𝑠 𝑠𝑢𝑐ℎ 𝑡ℎ𝑎𝑡 𝑝𝑎(𝑃 𝑝 , 𝑃 𝑐 , 𝑃 𝑣 ) = 𝑃 𝑝 , 𝑃 𝑐 , 𝑃 𝑗 ) : 𝑃 𝑗 ∈ ℵ(𝑃 𝑐 ) & 𝑃 𝑗 ∉ A}

	2:	𝑃 𝑐 ← point having the minimum x-coordinate
	3:	B ← {𝑃 𝑐 }
	4:	𝑃 𝑓 ← 𝑃 𝑐
	5:
	6:	repeat
	7:	if |ℵ(𝑃 𝑐 )| > 1 then
	8:	A = {𝑃 𝑝 , 𝑃 𝑓 }
	9:	else
	10:	A = {𝑃 𝑓 }
	11:	end if
	12:
	13: min{𝑝𝑎(14: if intersection detected then
	15:	A = A ∪ {𝑃 𝑣 }
	16:	Go to 12
	17:	end if
	18:	B ← B ∪ {𝑃 𝑣 }
	19:	𝑃 𝑝 ← 𝑃 𝑐
	20:	𝑃 𝑐 ← 𝑃 𝑣
	21:	until 𝑃 𝑣 ≠ 𝑃 𝑓
	22:	return B
	23: end procedure


Marta Arzarello

Federica Fontana

Marco Peresani

Carlo Peretto

Ursula Thun Hohenstein

Giorgia Sardelli

Martine Regert

François Orange

Roberto Zorzin

Michel Dubar

Gilles Durrenmath

Alain Carré

l'Epigravettiano recente: ricostruzione sperimentale della «chaîne-opératoire»

The transformation of goethite-and hematite-rich coloring materials at Tagliente rock-shelter (NE Italy) during the Late Epigravettian: an experimental reconstruction of the «chaîne-opératoire»

Negli ultimi decenni, i materiali coloranti ricchi di goethite ed ematite provenienti da contesti paleolitici sono stati oggetto di numerosi studi finalizzati alla comprensione del loro ruolo nell'ambito dei sistemi tecnici e simbolici dei cacciatori-raccoglitori [START_REF] Salomon | Les matières colorantes au début du Paléolithique supérieur: sources, transformations et fonctions[END_REF][START_REF] Dayet | Materiaux, transformations et fonctions de l'ocre au Middle Stone Age : le cas de Diepkloof Rock Shelter dans le contexte de l'Afrique australe[END_REF]. Questi materiali sono particolarmente abbondanti lungo la sequenza stratigrafica epigravettiana di Riparo Tagliente. Il riparo (Stallavena di Grezzana, Verona) è collocato nel massiccio pre-alpino dei Monti Lessini (250 m s.l.m.) e la sua sequenza stratigrafica (~4.5 m di profondità) risulta formata da due depositi: il più antico è attribuibile al Musteriano e all'Aurignaziano (MIS 3-4; da 60.000 yr BP a 32.000-30.000 yr BP), mentre il deposito più recente è riferibile a un'occupazione epigravettiana di età tardoglaciale (da 17.219-16.678 cal yr BP a 14.535-13.472 cal yr BP) (Fontana et alii 2009). L'eccezionale quantità di frammenti di ocra gialla e rossa rinvenuta nel sito è già stata oggetto di precedenti studi, effettuati con lo scopo sia di valutare l'entità e la distribuzione di questi materiali nei livelli archeologici indagati sia di individuare le possibili aree di approvvigionamento (Fontana et alii. 2009;Cavallo et alii 2015Cavallo et alii , 2017)). Partendo da questi lavori si è impostato un nuovo programma di ricerca volto alla ricostruzione delle strategie di approvvigionamento e delle tecniche di processamento di questi materiali, incrementando al tempo stesso le conoscenze sulle loro proprietà fisico-chimiche. Il programma di ricerca include analisi macroscopiche e Over the last decades goethite-and hematite-rich materials from Palaeolithic contexts have been the object of several studies aimed at defining their role in the technical and symbolic systems of hunter-gatherer's groups [START_REF] Salomon | Les matières colorantes au début du Paléolithique supérieur: sources, transformations et fonctions[END_REF][START_REF] Dayet | Materiaux, transformations et fonctions de l'ocre au Middle Stone Age : le cas de Diepkloof Rock Shelter dans le contexte de l'Afrique australe[END_REF]) . These materials are particularly abundant along the Late Epigravettian stratigraphic sequence of Tagliente rock-shelter (Stallavena di Grezzana, Verona). This site is located in the Lessini pre-Alpine massif at an altitude of 250 m above the sea level and contains a thick stratigraphic sequence (over 4.50 m deep) formed by two deposits: the most ancient one is referred to the Mousterian and the Aurignacian (MIS 3-4; from 60.000 yr BP to 32.000-30.000 yr BP) while the most recent attests for a Late Epigravettian occupation occurred during the Late Glacial (from 17.219-16.687 cal yr BP to 14.535-13.472 cal yr BP) (Fontana et alii 2009). The exceptional quantity of yellow and red fragments of ochre discovered in the site was the object of previous studies which were carried out in order to both record the presence and distribution of such colouring materials within the explored archeological layers and detect the geological procurement areas (Fontana et alii 2009;Cavallo et alii 2015Cavallo et alii , 2017)). Starting from these earlier works we have established a new research program, aimed at contributing to the reconstruction of the supply strategies and the processing techniques of such materials, while increasing our knowledge on their chemical and physical properties. This program has involved both macroscopic, physical and chemical fisico-chimiche (XRPS, SEM-EDXS e PLM) su campioni archeologici e geologici, associate ad un programma sperimentale. In questo lavoro si focalizza l'attenzione su quest'ultimo aspetto e sui primi risultati desunti da tale attività. I prodotti della sperimentazione rappresentano un importante corpus comparabile con il record archeologico al fine di comprendere i gesti e gli schemi concettuali attuati dai gruppi epigravettiani durante il trattamento meccanico e chimico e ipotizzare la genesi della grande quantità di frammenti ocracei rinvenuti a Riparo Tagliente. Le materie coloranti ricche di goethite utilizzate durante la sperimentazione sono state raccolte a Ponte di Veja, giacimento geologico collocato a ~8 km dal sito archeologico. Tale scelta è supportata da un precedente studio che aveva portato a identificare questo deposito come principale sito di approvvigionamento sfruttato dai gruppi epigravettiani (Cavallo et alii 2015). Tutti i noduli geologici sono stati suddivisi in differenti classi in conformità a specifici criteri fisici. In seguito, una parte dei campioni è stata trattata termicamente a diverse temperature (200°C, 400°C e 600°C), al fine di indagare il processo di trasformazione della goethite in ematite. Infine sono state testate sei differenti tecniche con lo scopo di ridurre in polvere i campioni geologici naturali e calcinati: macinazione (broyage), abrasione (abrasion), raschiatura (raclage), incisione (rainurage), frammentazione (concassage) e scheggiatura (taille). Queste tecniche hanno richiesto l'utilizzo di macine, pestelli, strumenti litici in selce e tavolette abrasive. In fase di analisi, i prodotti dell'attività sperimentale sono stati studiati secondo differenti metodi: le polveri ottenute sono state setacciate tramite setacci a differenti maglie (2 mm, 1.4 mm, 1 mm, 0.5 mm, 0.1 mm e 0.06 mm); i blocchi testati tramite raclage, rainurage e abrasion sono stati osservati al microscopio digitale (fig. 1), mentre tutti i frammenti prodotti durante le differenti attività sono stati registrati in un database Access seguendo gli stessi criteri utilizzati per i campioni archeologici. In conclusione, l'approccio multidisciplinare applicato ha consentito di acquisite una notevole quantità di dati che ci ha portato a formulare ipotesi interpretative riguardanti il record analyses (XRPS, SEM-EDXS and PLM) on archaeological and geological samples as well as the development of an experimental protocol. In this paper we will focus on the first results of the experimental program carried out. The products of the experimental activity represent an important corpus that can be compared with the archaeological record in order to understand the gestures and conceptual schemes implemented by the Epigravettian groups during the mechanical and chemical treatments, as well as to interpret the origin of the high quantity of residues recovered in Riparo Tagliente. The goethite-rich coloring material used for the experimentation was collected at Ponte di Veja, a deposit located at ~8 km from the Tagliente rock-shelter. A previous study had already allowed identifying this deposit as the main sourcing site exploited by the Epigravettian groups of Tagliente rock-shelter (Cavallo et alii, 2015). All the geological nodules were classified according to specific physical criteria. During experimentation part of the goethitebased nodules has undergone a process of heat treatment at different temperatures (200°C, 400°C and 600°C) in order to investigate the process of transformation of goethite into hematite. Six different techniques were then tested for reducing to powder the geological samples both natural and calcined: grinding (broyage), abrasion (abrasion), scraping (raclage), scoring (rainurage), fragmentation (concassage) and knapping (taille). These techniques require the use of grindstones, pestles, knapped lithic tools and abrasive tools (tablettes d'abrasion). During analysis the products obtained from experimentation were studied with different methods: the powder generated during the activities was sieved with sieves of different mesh size (2 mm, 1.4 mm, 1 mm, 0.5 mm, 0.1 mm, 0.06 mm); the raw materials tested by raclage, rainurage and abrasion were studied by digital microscope (fig. 1) while all the fragments were recorded in an Access database following the same criteria used for the archaeological samples. In conclusion, the multidisciplinary approach applied has led to the acquisition of a rich corpus of data which has allowed proposing an interpretation for the rich and varied archaeological record of Riparo archeologico di Riparo Tagliente.

Parole chiave: materiali coloranti, goethite, ematite, Epigravettiano recente, chaîneopératoire.

Tagliente.

Key-words: : colouring materials, goethite, hematite, Late Epigravettian, chaîne-opératoire.

Figura 1 .

 1 Figura 1. : a) Campione geologico C37 raschiato tramite uno strumento litico al fine di ottenere della polvere colorante (Giorgia Sardelli e Jean-Denys Strich); b) Immagine acquisita al microscopio digitale in cui sono visibili le strie formatesi durante il raclage effettuato sul campione C37 (Giorgia Sardelli).

Figure 1 .

 1 Figure 1. a) Goethite-rich geological sample (C37) scored by stone tool in order to produce powder (Giorgia Sardelli and Jean-Denys Strich); b) Image acquired by digital microscope of the incisions made by scoring on the sample C37 (Giorgia Sardelli).


Laura Pavlin

Florian Fernandez

Rauhiti Lowgreen

Isabelle Ohresser

Jean-Paul Cristol

Cécile Turc-Baron

DMEM Henri Bernardi
email: henri.bernardi@inra.f

Robin Candau

Laura Pavlin1

Feasibility and early effects of bed-cycling eccentric training: potential clinical applications

Keywords: bedridden patients, exercise rehabilitation, muscle atrophy, negative work

rehabilitation method. This modality permits to achieve high work-loads at low metabolic cost [START_REF] Hoppeler | Moderate load eccentric exercise; a distinct novel training modality[END_REF]. Energy required for muscle lengthening is four to six-fold lower than for shortening [START_REF] Abbott | The physiological cost of negative work[END_REF]Bigland-Ritchie & Woods,1976;[START_REF] Perrey | Comparison of oxygen uptake kinetics during concentric and eccentric cycle exercise[END_REF] thanks to mini-cycles of myosin binding without adenosine triphosphate (ATP) consumption (Vale & Milligan,2000) and to the wrapping of titin around actin although the exact role of titin in force generation and transmission is subject to debate [START_REF] Oranchuk | Scientific basis for eccentric quasi-isometric resistance training: a narrative review[END_REF]. Series and parallel elastic components including tendon and other connective tissues like endo, peri and epimysium are also decisive for the eccentric strength production (Magnusson & Kjaer,2019;[START_REF] Oranchuk | Scientific basis for eccentric quasi-isometric resistance training: a narrative review[END_REF].

Recent works reported the feasibility of moderate load eccentric bed cycling during dialysis and the use of eccentric-overload resistance exercise as a potential treatment to combat decline in physical function in people with chronic kidney disease (CKD; [START_REF] Pavlin | Does the interference phenomenon affect strength development during same-session combined rehabilitation program in hemodialysis patients?[END_REF]. [START_REF] Gollie | Preliminary study of the effects of eccentric-overload resistance exercise on physical function and torque capacity in chronic kidney disease[END_REF].Eccentricexercisewasshowntobefeasibleforpatientswithcardiorespiratory limitations [START_REF] Besson | Eccentric training in chronic heart failure: feasibility and functional effects. Results of a comparative study[END_REF]P. La Stayo et al.,2014).In the elderly, higher improvements in physical function were evidenced following an eccentric training compared to a concentric mode. This study showed that physical fitness and maximal concentric contraction strength of the knee extensors increased more after progressive eccentric training than after progressive concentric training [START_REF] Chen | Superior effects of eccentric to concentric knee extensor resistance training on physical fitness, insulin sensitivity and lipid profiles of elderly men[END_REF]). In the same population, improvements of strength and physical function were similar after an eccentric and a high-velocity training, whereas total work was lower in the eccentric training programme [START_REF] Leszczak | Early adaptations to eccentric and high-velocity training on strength and functional performance in community-dwelling older adults[END_REF]. In a systematic review, Douglas concluded that eccentric resistance training may be more particularly beneficial than concentric resistance training to improve strength, power and speed performance [START_REF] Douglas | Chronicadaptationsto eccentric training: a systematic review[END_REF].However, eccentric modality is underused in rehabilitation, because contractions are often associated with micro-damages in fibres structure causing Delayed Onset Muscle Soreness (DOMS; Proske & Morgan,2001;Vogt & Hoppeler,2014). It is noteworthy that muscle fibres can be preserved, as progressive workload increase confers an early protective effect to muscles and tendons [START_REF] Chen | Superior effects of eccentric to concentric knee extensor resistance training on physical fitness, insulin sensitivity and lipid profiles of elderly men[END_REF][START_REF] Lin | Muscle damage protection by low-intensity eccentric contractions for the knee extensors and flexors[END_REF]McHugh,2003). When intensity was based on rate of perceived exertion(RPE), no pain has been reported [START_REF] Besson | Eccentric training in chronic heart failure: feasibility and functional effects. Results of a comparative study[END_REF]P. La Stayo et al.,2014). Fractioned practice seems to promote this 'repeated bouteffect'(RBE), allowing naive subjects to perform eccentric contractions without DOMS (Steineretal.,2004;Vogt & Hoppeler,2014). Feasibility and efficiency of eccentric training were evidenced in chronically ill patients but were not explored in a lying position (Hoppeler,2016;P.C.LaStayo et al.,2010;P. La Stayo et al.,2014;[START_REF] Steiner | Eccentric endurance training in subjects with coronary artery disease: a novel exercise paradigm in cardiac rehabilitation[END_REF].Compared to eccentric exercise performed in the standing, sitting or semirecumbent postures, the supine position has several special characteristics. Since the strength produced by the vastus lateralis, which is part of the Quadriceps muscle, depends on the knee and hip flexion angle, body position is a crucial parameter for cycle training efficiency [START_REF] Bordoni | Anatomy, bony pelvis and lower limb, thigh quadriceps muscle[END_REF]. During supine exercise where the active musculature is a tor above the level of the heart, the gravitational assistance to muscle blood flow is absent and muscle perfusion pressure is reduced resulting in slowed tissues oxygenation [START_REF] Goulding | Effect of priming exercise and body position on pulmonary oxygen uptake and muscle deoxygenation kinetics during cycle exercise[END_REF][START_REF] Jones | Effects of "priming" exercise on pulmonary O 2 uptake and muscle deoxygenation kinetics during heavy-intensity cycle exercise in the supine and upright positions[END_REF][START_REF] Macdonald | Alveolar oxygen uptake and femoral artery blood flow dynamics in upright and supine leg exercise in humans[END_REF]. This posture dependent alteration in active muscles perfusion could explain why performance on cycle ergometer is improved in the standing position (Astrand & Saltin,1961). Moreover, during concentric recumbent cycling, it was highlighted that pedalling effort was more equally distributed between the front and rear thigh muscle groups compared to standard upright position. It was also evidenced that cardiovascular response differs between positions, recumbent cycling being associated with lowers heart rate and systolic blood pressure, resulting in lower perceived exertion. This difference is probably due to the horizontal leg position, facilitating blood return to the heart and reducing physiological stress [START_REF] Wehrle | Power output and efficiency during supine, recumbent, and upright cycle ergometry[END_REF]. Given that ESRD disease patients often suffer from high blood pressure and cardiac issues, supine position seems to be associated with lower physiological stress and cardiovascular effort at the same work level than up right position. These results obtained for concentric cycling show that physiological and psychological responses to recumbent eccentric cycling may differ from those to upright or semi-recumbent positions. In ERSD patients, dialysis procedure progressively removes fluid overload accumulated in different cellular compartments, including general blood circulation. This decrease in circulating volume declines blood pressure as well as cardiac input and organ perfusion [START_REF] Flythe | Blood pressure and volume management in dialysis: conclusions from a kidney disease: improving global outcomes (KDIGO) controversies conference[END_REF], making supine position the only safe enough for those highly deconditioned patients. By decreasing gravitational vertical force, supine position facilitates venous blood return and diminishes hypotension risk and its consequences on heart function during fluid removal. Thus, a better understanding of eccentric cycling in this lying position is needed for exercise practice during dialysis procedure. Furthermore, despite the benefits of eccentric cycle training, the great heterogeneity of existing protocols and the variety of physiological responses in relation to the solicited muscle groups make it difficult to propose the best training procedures that optimally combine management of DOMS and muscular beneficial effects in terms of strength, power and coordination. (Douglaset al., 017;[START_REF] Vogt | Eccentric exercise: mechanisms and effects when used as training regime or training adjunct[END_REF].

Facing an important need for bed practice exercise facilities, we designed a study protocol to explore the feasibility and the early efficiency of bed-cycling eccentric training, highlighting the early effects of eccentric cycling on healthy volunteers trained in a lying position similar to that adopted by bedridden patients. Progressive increment of workload and exercise duration was planned to practice eccentric cycling safely without DOMS. Power, force and speed outputs, RPE, DOMS and heart rate frequency were monitored during all sessions whereas parameters of power-speed relationship, maximal voluntary contraction force and neural activation during maximal contraction were evaluated before and after training.

2|METHODS 2.1|Participants, study protocol and design

The study procedures complied with the Declaration of Helsinki and its later amendments on human experimentation and were approvedby the national ethics committee. 11 healthy volunteers, including 6males, were recruited and gave written informed consent. They werenormal active people without medical contraindication to practicephysical activity and he was asked to participants to keep theirphysical activity, their dietary and sleep habits during the 3-weekstraining. Participants were evaluated before and after the fivetraining sessions at increasing intensity during a 3 weeks period.Sample size for before-after study was estimated by G*Power(G*Power 3.1.9.7, Heinrich-Heine Universität Düsseldorf, Düsseldorf,Germany) in an a priori way by using repeated measures ANOVA Considering a type I error rate of 0.05, a type II error rate of 0.2 (Cohen,1988), an effect size of 0.5 (medium size effect, [Čretniket al.,2022;[START_REF] Roig | The effects of eccentric versus concentric resistance training on muscle strength and mass in healthy adults: a systematic review with meta-analysis[END_REF]), the calculated sample size was 10. Wetook a small margin of 10% to ensure this number of 10measurements and involved 11 participants.

2.2|Home-made eccentric ergometer

The construction of an ergometer was necessary due to the lack of commercially available eccentric ergometers. Inspired by a guide previously published (Elmer & Martin,2013), the ergometer was designed with a direct-drive motor for electric bikes fixed on a patient lift. The chain was linked to a SRM crankset equipped with bike pedals. To follow the requested target speed, the participants had a live speed display in front of them. Eccentric workload was measured by using bicycle SRM power meter (Elmer & Martin,2013;[START_REF] Harrison | The acute physiological responses of eccentric cycling during the recovery periods of a high intensity concentric cycling interval session[END_REF][START_REF] Leong | Chronic eccentric cycling improves quadriceps muscle structure and maximum cycling power[END_REF]Lewis,2012;[START_REF] Lewis | Eccentric cycling emphasising a low cardiopulmonary demand increases leg strength equivalent to workload matched concentric cycling in middle age sedentary males[END_REF].

2.3|Training procedure

Before exercise, subjects were equipped with a heart rate transmitter belt and cycling shoes. They were installed on the eccentric ergometer and asked to avoid arm or trunk movements. During each session participants were asked to perform passive anti-clock wise cycling (i.e., without braking phase) and to keep the speed rotation between 50 and 70 rpm during the fivemin warm-up. The intensity of the effort as well as the workload were not fixed in advance and the same task of braking to reach the target speed as quickly as possible was asked to each participant. During the first session, considered as a familiarization session, ergometer speed was successively set at 50, 60, 70 and 80 rpm and subjects were asked to brake the pedals movement until the speed decreased by 5 rpm during 15 s, before 30 s of cool-down in passive cycling. First acclimatization session permitted for everyone to adjust the force applied to the pedals to achieve the goals previously explained and with the help of the advisor supervising the training. The RPE value given by the participants was used as feedback to better understand their feeling about bed-eccentric cycling at different speeds. Exercise was repeated twice for each speed. During the second session, ergometer speed was successively set at 50, 60, 70 and 80 rpm and subjects were asked to brake the pedals movement until the speed decreases by 10 rpm during 15 s, before 30 s of cool-down in passive cycling. Exercise was repeated three times for each speed. The third session ergometer speed was successively set at 50, 60, 70 and80 rpm and subjects were asked to brake the pedals movement until the speed decreases by 10 rpm during 15 s, before 30 s of cool-down in passive cycling. Exercise was repeated four times for each speed. The fourth session ergometer speed was successively set at 50, 60,70 and 80 rpm and subjects were asked to brake the pedals movement until the speed decreases by 10 rpm during 15 s, before15 s of cool-down in passive cycling. Exercise was repeated five times for each speed. The fifth and last session ergometer speed was successively set at 50, 60, 70 and 80 rpm and subjects were asked to brake the pedals movement until the speed decreases by 10 rpm during 15 s, before 15 s of cool-down in passive cycling. Exercise was repeated 6 times for each speed. At the end of each session participants were allowed to cool down in passive cycling for 5 min (Figure1). In this training procedure, we considered that resistive force to decrease the speed by 10 rpm was steady whatever the rotating speed. Since the power output is the product of resistive force and velocity, power output increases with pedalling velocity.

2.4|Eccentric power and speed outputs

Power developed to brake and rotation speed of the pedals were monitored with a SRM 6 power control device (Schoberer Rad Meβ technik GmbH; [START_REF] Gardner | Accuracy of SRM and power tap power monitoring systems for bicycling[END_REF].

2.5|RPE in training procedure

The RPE was requested for each exercise of each session (Figure1).Participants were asked to quote their effort using a revised Borgscale (0 to 10 scale [START_REF] Williams | The borg rating of perceived exertion (RPE) scale[END_REF]) and confirmed the value with an adjective: zero effort, extremely easy...very hard and extremely hard. The RPE scale was explained as follow:' Given that 0 corresponds to any effort, like lying on a bed, and 10 to the most strenuous exercise close to exhaustion, how would you rate this exercise'. Rating combined all the sensations and feelings of physical stress.

2.6|Delayed onset muscle soreness

A general rating of muscle soreness was assessed using a 10-pointscale. A score of 0 corresponded to a rating of no pain, while a score of 9 corresponded to a rating of unbearable pain [START_REF] Mccormack | Clinical applications of visual analogue scales: a critical review[END_REF]. Subjects were requested to rate discomfort in quadriceps and calf regions of the legs, particular attention was paid concerning joint pains. Patients were asked to report any pain in the hip, knee and ankle joints. Soreness measures were collected 48 h following the session.

2.7|Concentric power-speed relationship

Before and immediately after the end of the training programme, subjects performed sprints on an instrumented ergometer (Monark818E, Monark Exercise AB). After 5 min of warm-up, five consecutive sprints were recorded during 4 s with a sampling frequency of200 Hz. Increasing loads were set successively at 0.2, 0.4, 0.6, 0.8 and1 N.kg-1. Maximal power output was extracted from the power-speed relationship with a third-order polynomial regression according to the method previously published [START_REF] Arsac | Muscle function during brief maximal exercise: accurate measurements on a friction-loaded cycle ergometer[END_REF].

2.8|Maximal voluntary isometric contraction force

Before and immediately after the end of the training programme, maximal voluntary isometric contraction force (MVC) was measured during knee extension with a handheld dynamometer (Microfet2,Hoggan Health). Dynamometer was fixed behind the subject and an inelastic strap was used to link the leg, according to the method previously published [START_REF] Souweine | Standardized method to measure muscle force at the bedside in hemodialysis patients[END_REF]. MVC of quadriceps of dominant leg was measured seated, hanging legs, without dorsal support with the knee held at 90° from extension, and arms crossed in front of chest. Before measurements, subjects were allowed toper form two trial contractions. After 1 min rest, subjects performed three maximal contractions of 4 s separated by 1 min rest. The maximal value was retained as MVC (100%). During MVC measurement, participants are seated with the hip angle near 90°. Supervisor ensured that the subjects are sitting naturally with a hip angle close to 90°, without any particular constraint, and does not make any upper body movement that could alter their postures (Garnieret al., 2022).

2.9|Neural activation during maximal isometric contraction

Skin on the quadriceps muscles of the left leg was shaved and prepared to ensure that skin impedance was lower than 5 kΩ.Disposable surface electrodes (EL501, Biopac Systems Inc.) were positioned on the vastus medialis, rectus femoris and vastus lateralis with an intra pair distance of 20 mm, according to the method previously published [START_REF] Rainoldi | A method for positioning electrodes during surface EMG recordings in lower limb muscles[END_REF]. Subjects performed two contractions at 25%, two contractions at 50% and two contractions at 75% of MVC [START_REF] Drechsler | Changes in muscle strength and EMG median frequency after anterior cruciate ligament reconstruction[END_REF] and electromyography (EMG)was assessed during maximal isometric knee extension. Surface electromyography was recorded with a three-ways research system (MP150, Biopac Systems Inc.). The root mean square (RMS) method was used for smoothing with the Acq Knowledge software. EMGi--force relationships were obtained from the values of the four contraction forces, using a linear regression model.

2.10|Statistical analysis

Statistical analyses were performed using StatView®. Each quantitative parameter was presented as mean ± standard error of the mean(SEM) or as median and quartiles if normality was not verified. Descriptive analyses were carried out for subjects' characteristics and DOMS values. When appropriate, a repeated measures ANOVA or a Friedman ANOVA was performed. When tests found a significant interaction effect, a Tukey's or a pairwise Wilcoxon post hoc test was performed. A result was considered as significant when p value was lower than *0.05, **0.01 and ***0.001. To determine the meaning-fulness of the effect of eccentric exercise, effect size (ES, Cohen'sd) was calculated as the difference between preand post-exercise divided by the pooled standard deviation for maximal voluntary contraction force, maximal power output and integrated EMG signal [START_REF] Cohen | Statistical power analysis for the behavioral sciences second edition[END_REF]. According to the original Cohen's scale, effect sizes of0.2, 0.5 and 0.8 were considered to be small, moderate and large effects.

3|RESULTS

3.1|Subjects

Characteristics of the 11 subjects are presented in Table1. Briefly, mean age was 40.2 ± 4.0 years and mean body mass index (BMI) was23.0 ± 0.7 kg.m-².

3.2|Tolerance to training: Heart rate frequency, RPE, delayed onset muscle soreness

Subjects were highly compliant with training procedures, easily performed reverse pedalling and all exercises were well-tolerated. Heart rate (HR) at rest was 67 ± 1 bpm and during exercise, peak HR increased to 97 ± 1 bpm (Figure2). Peak HR did not exceed 100 bpm. Mean increase was about 29 ± 1 bpm between resting and exercise.

The RPE is depicted on Figure3a, and the familiarization session (Session 1) was not taken into account for data analysis. Inter-individual variability of perceived exertion was high for the same exercise, but values were consistent between sessions for the same subject. RPE significantly increased with speed in every training session. It remained low (<5/10) but increased when subjects had to brake to 70 rpm, the hardest task. No effect of training was observed on subjects' RPE under 60 rpm, but training significantly decreased RPE by approximately 17.5% when braking to 70 rpm. Moreover, global RPE for one entire session, including all exercise speeds, was significantly lower with training. Six subjects experienced DOMS. As presented in Table2, all sessions generated DOMS for 1 to 3 subjects. DOMS values remained low, as maximal score was 3. For each session, mean DOMS value was very low (<1), because most of the subjects did not feel any muscular or joint pain.

3.3|Effects on concentric sprint-cycling performance

Maximal power output was extracted from power-speed relationship (Figure4a). A third-order polynomial regression was used to fit all power-speed relationship curves. Training significantly increased maximal power output (Figure4b). Mean improvement was about8.5%, corresponding to a 74W increase in the maximal power developed during sprints and a small effect size (ES = 0.25).

3.4|Effects on isometric contraction force

Results of MVC presented on Figure4c,dshow that MVC significantly improved after training. Mean improvement was about78N, corresponding to a 17% increase in force from baseline. Integrated EMG during MVC shows a significant increase (+22%)after training and a moderate effect size (ES = 0.59).

Relative EMG-Force relationship shifts above the curve obtained before the programme of eccentric cycling and the area under the EMGi-Force curve was significantly increased by 5% after training(p< 0.05, ES = 0.55) (Figure5).

4|DISCUSSION

The ergometer was designed for hospital bed-cycling. Previously developed eccentric ergometers were developed for semi-recumbent practice [START_REF] Elmer | Construction of an isokinetic eccentric cycle ergometer for research and training[END_REF][START_REF] Elmer | Construction of an isokinetic eccentric cycle ergometer for research and training[END_REF]Hoppeler,2016;[START_REF] Lewis | The feasibility of eccentric cycling as a training modality for increasing quadriceps strength at low cardiovascular cost[END_REF][START_REF] Vieira | Eccentric cycle exercise in severe COPD :feasibility of application[END_REF]. Due to the stabilized position of the subjects in the bed, they had no difficulty to stay in a lying position with a minimal action of arms and trunk. Whereas in our study, healthy subjects could develop an intense mean power output near of 500W, in the literature patients with reduced physical capacity will develop noticeable lower power [START_REF] Aagaard | Role of the nervous system in sarcopenia and muscle atrophy with aging: strength training as a counter measure[END_REF][START_REF] Green | Torque, power and muscle activation of eccentric and concentric isokinetic cycling[END_REF][START_REF] Hoppeler | Moderate load eccentric exercise; a distinct novel training modality[END_REF][START_REF] Meyer | Eccentric exercise in coronary patients: central hemodynamic and metabolic responses[END_REF], thus limiting upper body movement. With most of patients carrying a high cardiovascular risk as end-stage renal disease patients undergoing haemo-dialyses, the HR frequency increase during exercise must be limited. We report a modest 30 bpm increase, and peak HR did not exceed 100 bpm. Our results are in accordance with Chasland's study [START_REF] Chasland | Eccentric cycling: a promising modality for patients with chronic heart failure[END_REF], and consistent with previous observations [START_REF] Lewis | The feasibility of eccentric cycling as a training modality for increasing quadriceps strength at low cardiovascular cost[END_REF][START_REF] Perrey | Comparison of oxygen uptake kinetics during concentric and eccentric cycle exercise[END_REF]. Power outputs reached in the present study were relatively high compared to classical concentric cycling exercise, extending the results of previous studies performed in a semirecumbent position (P. C. [START_REF] Lastayo | Eccentric ergometry: increases in locomotor muscle size and strength at low training intensities[END_REF][START_REF] Lastayo | The positive effects of negative work: increased muscle strength and decreased fall risk in a frail elderly population[END_REF].Eccentric power outputs are rarely measured, workload being usually estimated from oxygen consumption or from a concentric performance [START_REF] Laroche | Is it possible to individualize intensity of eccentric cycling exercise from perceived exertion on concentric test[END_REF][START_REF] Steiner | Eccentric endurance training in subjects with coronary artery disease: a novel exercise paradigm in cardiac rehabilitation[END_REF]. Compared to our data, power outputs were lower during familiarization sessions, due to lower rotation speeds After five training sessions, participants achieved higher work-load for a given RPE. The relationship between PO and RPE was improved and gains were higher in eccentric (+29.6% in maximal eccentric PO) compared to isometric conditions (+17% in MVC) and were even greater compared to concentric type exercises (+8% in maximal concentric power). It was previously suggested that gains were specific to contraction mode and movement velocity [START_REF] Roig | The effects of eccentric versus concentric resistance training on muscle strength and mass in healthy adults: a systematic review with meta-analysis[END_REF]. These functional gains specific to the eccentric type appear valuable in a clinical context since eccentric movements are required to support body weight, particularly during downhill activities to ensure balance and slow down. It was demonstrated that efficiency of eccentric training to decrease fall risk was at least equal (P. Lastayoet al., 2017) or higher (P. C. [START_REF] Lastayo | The positive effects of negative work: increased muscle strength and decreased fall risk in a frail elderly population[END_REF] to traditional concentric training by specifically improving eccentric strength. The transferability of strength gains after eccentric training is of major importance for frail patients. Major isometric strength gains following eccentric training were demonstrated [START_REF] Julian | Eccentric cycling is more efficient in reducing fat mass than concentric cycling in adolescents with obesity[END_REF][START_REF] Roig | The effects of eccentric versus concentric resistance training on muscle strength and mass in healthy adults: a systematic review with meta-analysis[END_REF] but to our knowledge, this is the first report of an increased MVC from the third week after only five training sessions in non-athletes. Similar values were found in a recumbent position with longer programmes [START_REF] Lastayo | The positive effects of negative work: increased muscle strength and decreased fall risk in a frail elderly population[END_REF](Lastayo et al., , 2010[START_REF] Lastayo | Eccentric exercise in rehabilitation: safety feasibility, and application[END_REF]. However, leg muscle power was shown to be a better determinant of physical performance than isometric strength [START_REF] Bean | A comparison of leg power and leg strength within the In CHIANTI study: which influences mobility more? The Journals of Gerontology[END_REF]Reid &Fielding,2012). Eccentric training in young individuals improved maximal cycling power after 8 weeks of training [START_REF] Leong | Chronic eccentric cycling improves quadriceps muscle structure and maximum cycling power[END_REF].In our study, maximal PO during sprints was significantly improved, extending previous results that early adaptive responses occur following eccentric exercise in a healthy population.

The force-EMGi relationship curve shifted above pretraining curve. As a 5% higher neural activation is involved to develop the same MVC after training, it indicates that a slight impairment in neuromuscular efficiency occurs during isometric contractions. Excitationcontraction coupling efficiency was previously shown to be transitorily altered after training in eccentric condition, probably due to changes in membrane excitability, conduction velocity or impaired release of Ca2+ [START_REF] Clos | Concentric versus eccentric cycling at equal power output or effort perception: neuromuscular alterations and muscle pain[END_REF][START_REF] Nasrabadi | Muscle fiber conduction velocity of the vastus medilais and lateralis muscle after eccentric exercise induced-muscle damage[END_REF].However, here, the great isometric strength gains suggest that local nervous adaptations like an improved muscular recruitment exceed contractile function impairment. These results also mean that strength gains result in part from a neural drive optimization through a better motor units recruitment/a higher discharge rate of active motor units or from diminished inhibitory reflex loops. According to Kraemer, these improvements could also be related to the synchronized activation of agonistic, synergistic and antagonistic muscles [START_REF] Kraemer | Strength and power training: physiological mechanisms of adaptation[END_REF]. Motor learning could have improved neural drive of leg muscles to resist against the dynamometer, increasing production force capacity through an efficient activation of muscle sinvolved in movement execution [START_REF] Brueckner | Associations between practice-related changes in motor performance and muscle activity in healthy individuals: a systematic review[END_REF]Wulfet al., 2010).

In this study direct fatigue-related markers were not evaluated or measured, we used heart rate and RPE to evaluate exhaustion. RPE and neuromuscular fatigue were shown to be strongly correlated during endurance test [START_REF] Cruz-Montecinos | Perceived physical exertion is a good indicator of neuromuscular fatigue for the core muscles[END_REF]. Lastly, [START_REF] Zhao | Validity of using perceived exertion to assess muscle fatigue during resistance exercises[END_REF] provide evidence that RPE measure was efficient for muscle fatigue evaluation during resistance exercises (Zhao et al.,2 022). As RPE provides perceptual difficulty, it was previously successfully used in frail population like elderly to assess indirect fatigue through perceptual training intensity [START_REF] Morishita | Relationship between the rating of perceived exertion scale and the load intensity of resistance training[END_REF].Workload was progressively increased, but 6 subjects out of 11subjects experienced moderated DOMS. They reported pain felt during daily eccentric movements like going downstairs. Subjects are differently sensitive to DOMS, and immobilized patients might be more fragile. There is no evidence that gender or age are determinants of DOMS sensitivity, but subjects performing downhill running and regular cycling did not report any pain whereas less active people were subjected to DOMS. These practices can induce a RBE and may explain the differential subjects' sensitivity. Some factors seem to promote DOMS and importance of speed was previously suggested [START_REF] Hoppeler | Moderate load eccentric exercise; a distinct novel training modality[END_REF]. Low velocity could bead vised to facilitate coordination and avoid damage. High velocities induce a disorganized muscular work in favour of a greater elastic component and joint solicitation [START_REF] Elmer | Joint-specific power absorption during eccentric cycling[END_REF]. Nevertheless, in other studies, subjects also experienced DOMS after the first session in a comparable extent of our group whereas rotation speed was below 60 rpm (Besson et al.,2 013;[START_REF] Leong | Chronic eccentric cycling improves quadriceps muscle structure and maximum cycling power[END_REF].

Workload has a major role and must be measured to ensure proper intensity and even though DOMS pain values were low, our programme can be improved. Our training procedures emerged from current recommendations, but most of the procedures are poorly described (Roig et al.,2 008). As familiarization sessions are key steps to confer RBE, a periodization model was suggested to safely introduce eccentric [START_REF] Harris-Love | Eccentric exercise program design: a periodization model for rehabilitation applications[END_REF]. Further studies conducted on healthy volunteers including these recommendations, controlling precisely workload and specially focused on DOMS, are needed to establish the mechanisms underpinning RBE and adequate eccentric intensity to avoid DOMS [START_REF] Hyldahl | Mechanisms and mediators of the skeletal muscle repeated bout effect[END_REF]. Although our study demonstrated the feasibility and effectiveness of eccentric exercise in the lying position, the application of such protocols on patients suffering from pathologies such as renal failure will require adaptations and further studies [START_REF] Hoppeler | Moderate load eccentric exercise; a distinct novel training modality[END_REF][START_REF] Lastayo | Eccentric exercise in rehabilitation: safety feasibility, and application[END_REF]. Indeed, the tolerance to exercise being strongly dependent on the concerned population, physiotherapy prescriptions will require a personalization of the physical training programmes with adapted familiarization sessions. The parameters to be adjusted may concern the intensity of the exercise, its duration or the number of repetitions per session. Another simple way to individualize exercise intensity would be to recommend an RPE to be achieved during exercise phases on the Borg scale.

5|CONCLUSION

In summary, our study shows that bed-cycling eccentric training is feasible, that adaptations to bed-cycling eccentric training occur in the early stage of practice and that strength and power gains following eccentric cycling are transferable to others contraction modalities, that is, concentric and isometric. Interestingly, most of the daily movements include an eccentric, a concentric and an isometric phase, and daily falls in frail people are most of the time attributable to a lack of balance during the eccentric phase. Eccentric movements are also prominent during downhill exercises like going down the stairs or sit on a seat, and the inability to brake the body weight considerably limits functional independence and quality of life. Thus, the specificity of eccentric regimen might constitute a promising exercise mode not only to fight against muscle atrophy linked to end-stage renal disease but also for many clinical applications as haemo-dialyses since training can be safely perform in a lying position and is efficient on muscle function improvement. Transferability of gains to concentric and isometric modalities will allow to improve whole leg function and positively impacts physical abilities in healthy and frail populations.

  Mean power outputs (PO) are presented in Figure3b. Workloads achieved during eccentric cycling were high no matter the exercise and study time-point, and significantly increased with speed. At the beginning of the study, mean value obtained was about 50 W and maximal value plateaued at 540 W. At the end of training, minimal value was close to100 W and maximal value reached 700 W. Workload was significantly different in-between the sessions at 60 rpm (210.1 ± 38.5 W compared to293.8 ± 19.7 W). No significant change was found at other speeds. Lower work reproducibility corresponded to the fastest speed. The relationships between PO and RPE during session 2 and 5 are depicted in Figure3c.Slope of linear fitting curve significantly increased (p< 0.001, +139%) after training programme.

ACKNOWLEDGEMENTS

We would like to sincerely thank participants for their involvement in this study. We are also grateful to AIDER Santé staff for reading the manuscript and helpful discussions and also to the members ofUMR866 for support. We thank M. Combettes for ergometer assembly and C. Dubois for her helpful English corrections in the manuscript submitted for publication. This work was supported by grants from the Association Installation à Domicile Épurations Rénales (AIDER Santé) and the Institut National de Recherche pour l'Agriculture, l'Alimentation et l'Environnement (INRAE) and L. P. was a recipient of a stipend from AIDER Santé and INRAE.

DATA AVAILABILITY STATEMENT

The data that support the findings of this study are available from thecorresponding author upon reasonable request.

CONFLICT OF INTEREST

The authors declare no conflict of interest.