Defining the intrinsically disordered C-terminal domain of SSB reveals DNA-mediated compaction

The bacterial single-stranded DNA (ssDNA) binding protein SSB is a strictly conserved and essential protein involved in diverse functions of DNA metabolism, including replication and repair. SSB comprises a well-characterized tetrameric core of N-terminal oligonucleotide binding OB folds that bind ssDNA and four intrinsically disordered C-terminal domains of unknown structure that interact with partner proteins. The generally accepted, albeit speculative, mechanistic model in the field postulates that binding of ssDNA to the OB core induces the flexible, undefined C-terminal arms to expand outwards encouraging functional interactions with partner proteins. In this structural study, we show that the opposite is true. Combined small-angle scattering with X-rays and neutrons coupled to coarse-grained modeling reveal that the intrinsically disordered C-terminal arms are relatively collapsed around the tetrameric OB core and collapse further upon ssDNA binding. This implies a mechanism of action, in which the disordered C-terminal domain collapse traps the ssDNA and pulls functional partners onto the ssDNA

Une nouvelle approche du cyclisme : la transition assis-danseuse comme prétexte à l'étude de l'optimisation du mouvement

The aim of this work has been to deepen the knowledge about the choices spontaneously made by humans in order to realize simple locomotor tasks, with a focus on the pedaling movement. The analysis of the spontaneous transition from the seated to the standing position in cycling was the main topic of this thesis. Little studied in comparison to the walk-run transition, this transition is of interest given the possibilities to constrain the pedaling movement, and because of its abrupt nature making easier the identification of the criteria optimized in the movement. The combination of full-body kinematics, electromyography, inverse dynamics, and the measure of the efforts applied on each of the cyclist's supports on a fully instrumented cycling-ergometer offered a new perspective on the pedaling movement. These methods provide new leads to understand the spontaneous choices made in order to pedal under increasing power-output constraints.L'objectif de ce travail a été d'approfondir la connaissance des choix spontanés effectués par les humains dans le but de réaliser des tâches locomotrices simples, avec un focus sur le mouvement de pédalage. L'analyse de la transition spontanée de la position assise vers celle en danseuse en cyclisme a été le thème central de ces travaux. Peu étudiée en comparaison de la transition marche-course, cette transition est pourtant digne d'intérêt du fait des quelques possibilités de contraindre le mouvement de pédalage, et par sa nature abrupte facilitant ainsi la mise en valeur des critères optimisés lors du mouvement. Les analyses cinématiques, par électromyographie de surface, et par méthode de dynamique inverse du corps complet, ainsi que la mesure des efforts exercés en chacun des points d'appui du cycliste sur un ergocycle entièrement instrumenté ont permis l'analyse du pédalage sous un nouvel angle. La combinaison de ces procédés offre de nouvelles perspectives pour comprendre les choix spontanés effectués pour pédaler sous contrainte incrémentale de production de puissance

A New Approach of Cycling : the Seat-Stand Transition as a Pretext to Study of Movement Optimization

L'objectif de ce travail a été d'approfondir la connaissance des choix spontanés effectués par les humains dans le but de réaliser des tâches locomotrices simples, avec un focus sur le mouvement de pédalage. L'analyse de la transition spontanée de la position assise vers celle en danseuse en cyclisme a été le thème central de ces travaux. Peu étudiée en comparaison de la transition marche-course, cette transition est pourtant digne d'intérêt du fait des quelques possibilités de contraindre le mouvement de pédalage, et par sa nature abrupte facilitant ainsi la mise en valeur des critères optimisés lors du mouvement. Les analyses cinématiques, par électromyographie de surface, et par méthode de dynamique inverse du corps complet, ainsi que la mesure des efforts exercés en chacun des points d'appui du cycliste sur un ergocycle entièrement instrumenté ont permis l'analyse du pédalage sous un nouvel angle. La combinaison de ces procédés offre de nouvelles perspectives pour comprendre les choix spontanés effectués pour pédaler sous contrainte incrémentale de production de puissance.The aim of this work has been to deepen the knowledge about the choices spontaneously made by humans in order to realize simple locomotor tasks, with a focus on the pedaling movement. The analysis of the spontaneous transition from the seated to the standing position in cycling was the main topic of this thesis. Little studied in comparison to the walk-run transition, this transition is of interest given the possibilities to constrain the pedaling movement, and because of its abrupt nature making easier the identification of the criteria optimized in the movement. The combination of full-body kinematics, electromyography, inverse dynamics, and the measure of the efforts applied on each of the cyclist's supports on a fully instrumented cycling-ergometer offered a new perspective on the pedaling movement. These methods provide new leads to understand the spontaneous choices made in order to pedal under increasing power-output constraints

Modification of the spontaneous seat-to-stand transition in cycling with bodyweight and cadence variations

International audienceWhen a high power output is required in cycling, a spontaneous transition by the cyclist from a seated to a standing position generally occurs. In this study, by varying the cadence and cyclist bodyweight, we tested whether the transition is better explained by the greater power economy of a standing position or by the emergence of mechanical constraints that force cyclists to stand. Ten males participated in five experimental sessions corresponding to different bodyweights (80%, 100%, or 120%) and cadences (50 RPM, 70 RPM, or 90 RPM). In each session, we first determined the seat-to-stand transition power (SSTP) in an incremental test. The participants then cycled at 20%, 40%, 60%, 80%, 100%, or 120% of the SSTP in the seated and standing positions, for which we recorded the saddle forces and electromyogram (EMG) signals of eight lower limb muscles. We estimated the cycling cost using an EMG cost function (ECF) and the minimal saddle forces in the seated position as an indicator of the mechanical constraints.Our results show the SSTP to vary with respect to both cadence and bodyweight. The ECF was lower in the standing position above the SSTP value (i.e., at 120%) in all experimental sessions. The minimal saddle forces varied significantly with respect to both cadence and bodyweight.These results suggest that optimization of the muscular cost function, rather than mechanical constraints, explain the seat-to-stand transition in cycling

Elastic energy in locomotion: Spring-mass vs. poly-articulated models

International audienceThe human is often modeled as a Poly-Articulated Model (PAM) with rigid segments while some authors use a Spring Mass Model (SMM) for modeling locomotion. These two models are considered independent, and the objective of this study was to link them in order to enlighten the origin of the elasticity in locomotion.Using the characteristics of the two models, a theoretical relationship demonstrates that the variation of elastic energy of the SMM equals the variation of the internal kinetic energy minus internal forces work of the PAM. This theoretical relationship was experimentally investigated among 19 healthy participants walking and running on a treadmill.The results showed that the equality is verified except during the double support phase at 0.56 m s−1, at high walking speeds (1.67 and 2.22 m s−1) or during the aerial phase of running.The formal relationship showed that the global stiffness of the SMM is directly related to the work of the internal forces of the PAM, and thus, to the characteristics of the musculoskeletal system. It also showed the relevance of taking into account the participation of each joint in the global stiffness. Finally, the coordination of internal forces work to produce a global stiffness may be considered as a new criterion of movement optimization for clinical purposes or motion planning for humanoid robot

Transferability between Isolated Joint Torques and a Maximum Polyarticular Task: A Preliminary Study

International audienceThe aims of this study were to determine if isolated maximum joint torques and joint torques during a maximum polyarticular task (i.e. cycling at maximum power) are correlated despite joint angle and velocity discrepancies, and to assess if an isolated joint-specific torque production capability at slow angular velocity is related to cycling power. Nine cyclists completed two different evaluations of their lower limb maximum joint torques. Maximum Isolated Torques were assessed on isolated joint movements using an isokinetic ergometer and Maximum Pedalling Torques were calculated at the ankle, knee and hip for flexion and extension by inverse dynamics during cycling at maximum power. A correlation analysis was made between Maximum Isolated Torques and respective Maximum Pedalling Torques [3 joints x (flexion + extension)], showing no significant relationship. Only one significant relationship was found between cycling maximum power and knee extension Maximum Isolated Torque (r=0.68, p<0.05). Lack of correlations between isolated joint torques measured at slow angular velocity and the same joint torques involved in a polyarticular task shows that transfers between both are not direct due to differences in joint angular velocities and in mono-articular versus poly articular joint torque production capabilities. However, this study confirms that maximum power in cycling is correlated with slow angular velocity mono-articular maximum knee extension torque

An algorithm to decompose ground reaction forces and moments from a single force platform in walking gait

International audienceIn walking experimental conditions, subjects are sometimes unable to perform two steps on two different forceplates. This leads the authors to develop methods for discerning right and left ground reaction data while they are summed during the double support in walking. The aim of this study is to propose an adaptive transition function that considers the walking speed and ground reaction forces (GRF). A transition function is used to estimate left and right side GRF signals in double support. It includes a shape coefficient adjusted using single support GRF parameters. This shape coefficient is optimized by a non-linear least-square curve-fitting procedure to match the estimated signals with real GRF. A multiple regression is then performed to identify GRF parameters of major importance selected to compute the right and left GRF of the double support. Relative RMSE (RMSER), maximum GRF differences normalized to body mass and differences of center of pressure (CoP) are computed between real and decomposed signals. During double support, RMSER are 6%, 18%, 3.8%, 4.3%, 3%, and 12.3% for anterior force, lateral force, vertical force, frontal moment, sagittal moment and transverse moment, respectively. Maximum GRF differences normalized to body mass are lower than 1 N/kg and mean CoP difference is 0.0135 m, when comparing real to decomposed signals during double support. This work shows the accuracy of an adaptive transition function to decompose GRF and moment of right and left sides. This method is especially useful to accurately discern right and left GRF data in single force platform configurations

Modela-r as a Froude and Strouhal dimensionless numbers combination for dynamic similarity in running

International audienceThe aim of this study was to test the hypothesis t hat running at fixed fractions of Froude (Nfr) and Strouhal (Str) dimensionless numbe rs combinations induce dynamic similarity between humans of different sizes. Ninet een subjects ran in three experimental conditions, i) constant speed, ii) similar speed (N fr) and iii) similar speed and similar step frequency (Nfr and Str combination). In addition to anthropometric data, temporal, kinematic and kinetic parameters were assessed at each stage to measure dynamic similarity informed by dimensional scale factors and by the decrease of dimensionless mechanical parameter variability. Over a total of 54 dynamic parameters, dynamic similarity from scale factors was met for 16 (mean r = 0.51), 32 (mean r = 0.49) and 52 (mean r = 0.60) parameters in the first, the second and the third experimental conditions, r espectively. The variability of the dimensionless preceding parameters was lower in the third condition than in the others. This study shows that the combination of Nfr and Str, computed from the dimensionless energy ratio at the center of gravity (Modela-r) ensures d ynamic similarity between different-sized subjects. The relevance of using similar experiment al conditions to compare mechanical dimensionless parameters is also proved and will hi ghlight the study of running techniques, or equipment, and will allow the identification of abnormal and pathogenic running patterns. Modela-r may be adapted to study other abilities requiring b ounces in human or animal locomotion or to conduct investigations in comparat ive biomechanics

Walking dynamic similarity induced by a combination of Froude and Strouhal dimensionless numbers: Modela-w

International audienceThe aim of this study was to assess the accuracy of a new dimensionless number associating Froude (Nfr) and Strouhal (Str) called Modela-w to induce walking dynamic similarity among humans of different sizes. Nineteen subjects walked in three experimental conditions: (i) constant speed, (ii) similar speed (Nfr) and (iii) similar speed and similar step frequency (Modela-w). The dynamic similarity was evaluated from scale factors computed with anthropometric, temporal, kinematic and kinetic data and from the decrease of the variability of the parameters expressed in their dimensionless form. Over a total of 36 dynamic parameters, dynamic similarity from scale factors was met for 11 (mean r = 0.51), 22 (mean r = 0.52) and 30 (mean r = 0.69) parameters in the first, the second and the third experimental conditions, respectively. Modela-w also reduced the variability of the dimensionless preceding parameters compared to the other experimental conditions. This study shows that the combination of Nfr and Str called Modela-w ensures dynamic similarity between different-sized subjects and allows scientists to impose similar experimental conditions removing all anthropometric effects

Can muscle coordination explain the advantage of using the standing position during intense cycling?

International audienceObjectives: When compared to seated, the standing position allows the production of higher power outputs during intense cycling. We hypothesized that muscle coordination could explain this advantage. To test this hypothesis, we assessed muscle activity over a wide range of power outputs for both seated and standing cycling positions. Design: Nine lower limb muscle activities from seventeen untrained volunteers were recorded during cycling sequences performed in the seated and the standing positions at power outputs ranging from ∼100 to 700 W at 90 ± 5 revolutions-per-minute (RPM). Methods: Integrated electromyography activity (iEMG), temporal patterns of the EMGs, and muscle synergies were analyzed. Results: Muscle activity was underlain by four muscle synergies in both positions. Muscle synergies were similar in the two positions (Pearson's r = 0.929 ± 0.125). The activation patterns of knee and ankle extensor muscles and their associated synergies had different timings in the two positions (differences of ∼2-10% of cycle). No major timing changes were observed with power output (<2% of cycle). Differences in iEMG between the two positions depended strongly on power output in all but the calf muscle (medial gastrocnemius). Conclusions: The number and structure of the muscle synergies play a minor role in the advantage of using the standing position when cycling at high power-outputs. However, the standing position is favorable in terms of iEMG at power outputs 500-600 W due to position-dependent modulations of muscle activation levels. These data are important for understanding the determinants of the seat-stand transition in cycling