Annie Hélène Rouard

Effects of fatigue on inter-cycle variability in cross-country skiing.

The aim of the study was to examine the inter-cycle variability in cross-country skiing gait and its evolution with fatigue. Both issues were investigated to understand the flexibility capabilities of the neuromuscular system. Four women and four men skied on a treadmill, up to exhaustion. The angular displacements of the arms and legs movements were obtained for 40s period at the beginning and end of the skiing test. Mean inter-cycle standard deviation (SD(c)), largest Lyapunov exponent (lambda(1)) and correlation dimension (D(c)) were computed for each time series and surrogate counterpart to evaluate the magnitude and nature of the variability. For any experimental time series, lambda(1) was positive, D(c) greater than 1 and both were found to be different from their surrogate counterparts, confirming that the temporal variations of the data had a deterministic origin. More, larger SD(c), D(c) and lambda(1) values were observed at the end of the test, indicating more variability, noise and local dynamic instability in the data with fatigue. Hence, the fluctuations of limb angular displacements displayed a chaotic behavior, which reflected flexibility of the neuromuscular system to adapt to possible perturbations during skiing. However, such chaotic behavior degraded with fatigue, making the neuromuscular system less adaptable and more unstable.

Effect of fatigue on double pole kinematics in sprint cross-country skiing

The aim of the present study was to examine the effect of fatigue (physiological, mechanical, and muscular parameters) induced by a sprint simulation on kinematic parameters (cycle, phases, and joints angles) of the double pole technique. Eight elite skiers were tested for knee extensor strength and upper body power both before and after a three-bout simulation of sprint racing. They were video analyzed during the final part of the test track of bouts 1 and 3 using a digital camera. Results showed that skiers were in a fatigue state (decrease of the knee extensors voluntary force (-10.4+/-10.4%) and upper body power output (-11.1+/-8.7%) at the end of the sprint. During bout 3, the final spurt and cycle velocities decreased significantly (-7.5+/-12.3%; -13.2+/-9.5%; both p<.05). Angular patterns were only slightly modified between bouts 1 and 3 with trunk, hip, and pole angles being significantly greater for the third bout. The decrease of hip and trunk flexion and the lower inclination of the pole during the poling phase suggested a reduced effectiveness of the force application which could lead to a decrease in the cycle velocity.

Dynamics of coordination in cross-country skiing.

The aim of the present study was to identify modes of coordination in cross-country skiing from a dynamical systems perspective. Participants (N=8) skied on a treadmill using classical techniques with varying steepness (i.e., 0 degrees-7 degrees). Coordination was evaluated in terms of the relative frequency and relative phase between upper arms and thighs. Results revealed that the limb movements were systematically attracted towards low integer frequency ratios (i.e., 1:1 and 2:1) and in-phase (phi approximately 0 degrees ) and anti-phase relationships (phi approximately 180 degrees). The increase in steepness produced shifts between the attractive modes of limb movements and a loss of stability was observed during transitions. These results suggest that principles of coordination between limbs in cross-country skiing are akin to those of non-linear coupled oscillators, as documented for a broad range of motor activities. Yet, differences with such classical findings are discussed reflecting the specific biomechanical constraints of cross-country skiing.

Upper- and lower-limb muscular fatigue during the 200-m front crawl

The aim of this study was to investigate how upper- and lower-limb muscle fatigue evolves in a 200-m front crawl swimming race. Surface electromyography signals were collected from the flexor carpi radialis, biceps brachii, triceps brachii, pectoralis major, upper trapezius, tibialis anterior, biceps femoris, and rectus femoris muscles of 10 international-level swimmers; 4 underwater cameras were used for kinematic analysis. In addition, blood lactate was measured before and after the test using capillary blood samples. Swimming speed and stroke length decreased from the beginning to the end of the effort, whereas stroke frequency increased after an initial decrease to maintain speed. Concomitant with the decrease in speed, blood lactate increased to 11.12 (1.65) mmol·L(-1). The changes in stroke parameters were associated with an increase in integrated electromyography (20%-25%) and a decrease in spectral parameters (40%-60%) for all of the upper-limb muscles, indicating the reaching of submaximal fatigue. The fatigue process did not occur regularly during the 8 laps of the 200 m but was specific for each muscle and each subject. Lower-limb muscles did not present signals of fatigue, confirming their lower contribution to swimming propulsion. The test was conducted to individualize the training process to each muscle and each subject.

Phase-dependence of elbow muscle coactivation in front crawl swimming.

Propulsion in swimming is achieved by complex sculling movements with elbow quasi-fixed on the antero-posterior axis to transmit forces from the hand and the forearm to the body. The purpose of this study was to investigate how elbow muscle coactivation was influenced by the front crawl stroke phases. Ten international level male swimmers performed a 200-m front crawl race-pace bout. Sagittal views were digitized frame by frame to determine the stroke phases (aquatic elbow flexion and extension, aerial elbow flexion and extension). Surface electromyograms (EMG) of the right biceps brachii and triceps brachii were recorded and processed using the integrated EMG to calculate a coactivation index (CI) for each phase. A significant effect of the phases on the CI was revealed with highest levels of coactivation during the aquatic elbow flexion and the aerial elbow extension. Swimmers stabilize the elbow joint to overcome drag during the aquatic phase, and act as a brake at the end of the recovery to replace the arm for the next stroke. The CI can provide insight into the magnitude of mechanical constraints supported by a given joint, in particular during a complex movement.

Upper limb joint forces and moments during underwater cyclical movements.

Sound inverse dynamics modeling is lacking in aquatic locomotion research because of the difficulty in measuring hydrodynamic forces in dynamic conditions. Here we report the successful implementation and validation of an innovative methodology crossing new computational fluid dynamics and inverse dynamics techniques to quantify upper limb joint forces and moments while moving in water. Upper limb kinematics of seven male swimmers sculling while ballasted with 4kg was recorded through underwater motion capture. Together with body scans, segment inertial properties, and hydrodynamic resistances computed from a unique dynamic mesh algorithm capable to handle large body deformations, these data were fed into an inverse dynamics model to solve for joint kinetics. Simulation validity was assessed by comparing the impulse produced by the arms, calculated by integrating vertical forces over a stroke period, to the net theoretical impulse of buoyancy and ballast forces. A resulting gap of 1.2±3.5% provided confidence in the results. Upper limb joint load was within 5% of swimmer׳s body weight, which tends to supports the use of low-load aquatic exercises to reduce joint stress. We expect this significant methodological improvement to pave the way towards deeper insights into the mechanics of aquatic movement and the establishment of practice guidelines in rehabilitation, fitness or swimming performance.

A limit-cycle model of leg movements in cross-country skiing and its adjustments with fatigue

Using dynamical modeling tools, the aim of the study was to establish a minimal model reproducing leg movements in cross-country skiing, and to evaluate the eventual adjustments of this model with fatigue. The participants (N=8) skied on a treadmill at 90% of their maximal oxygen consumption, up to exhaustion, using the diagonal stride technique. Qualitative analysis of leg kinematics portrayed in phase planes, Hooke planes, and velocity profiles suggested the inclusion in the model of a linear stiffness and an asymmetric van der Pol-type nonlinear damping. Quantitative analysis revealed that this model reproduced the observed kinematics patterns of the leg with adequacy, accounting for 87% of the variance. A rising influence of the stiffness term and a dropping influence of the damping terms were also evidenced with fatigue. The meaning of these changes was discussed in the framework of motor control.

Modulation of upper limb joint work and power during sculling while ballasted with varying loads

ABSTRACT The human musculoskeletal system must modulate work and power output in response to substantial alterations in mechanical demands associated with different tasks. In particular, in water, upper limb muscles must perform net positive work to replace the energy lost against the dissipative fluid load. Where in the upper limb are work and power developed? Is mechanical output modulated similarly at all joints, or are certain muscle groups favored? This study examined, for the first time, how work and power per stroke are distributed at the upper limb joints in seven male participants sculling while ballasted with 4, 6, 8, 10 and 12 kg. Upper limb kinematics was captured and used to animate body virtual geometry. Net wrist, elbow and shoulder joint work and power were subsequently computed through a novel approach integrating unsteady numerical fluid flow simulations and inverse dynamics modeling. Across a threefold increase in load, total work and power significantly increased from 0.38±0.09 to 0.67±0.13 J kg–1, and 0.47±0.06 to 1.14±0.16 W kg–1, respectively. Shoulder and elbow equally supplied >97% of the upper limb total work and power, coherent with the proximo-distal gradient of work performance in the limbs of terrestrial animals. Individual joint relative contributions remained constant, as observed on land during tasks necessitating no net work. The apportionment of higher work and power simultaneously at all joints in water suggests a general motor strategy of power modulation consistent across physical environments, limbs and tasks, regardless of whether or not they demand positive net work. Summary: A novel approach integrating inverse dynamics and numerical fluid flow simulation reveals that humans modulate upper limb joint work in water just as they do at the lower limb on land.

Breaststroke swimmers moderate internal work increases toward the highest stroke frequencies

A model to predict the mechanical internal work of breaststroke swimming was designed. It allowed us to explore the frequency-internal work relationship in aquatic locomotion. Its accuracy was checked against internal work values calculated from kinematic sequences of eight participants swimming at three different self-chosen paces. Model predictions closely matched experimental data (0.58 ± 0.07 vs 0.59 ± 0.05 J kg(-1)m(-1); t(23)=-0.30, P=0.77), which was reflected in a slope of the major axis regression between measured and predicted total internal work whose 95% confidence intervals included the value of 1 (β=0.84, [0.61, 1.07], N=24). The model shed light on swimmers ability to moderate the increase in internal work at high stroke frequencies. This strategy of energy minimization has never been observed before in humans, but is present in quadrupedal and octopedal animal locomotion. This was achieved through a reduced angular excursion of the heaviest segments (7.2 ± 2.9° and 3.6 ± 1.5° for the thighs and trunk, respectively, P<0.05) in favor of the lightest ones (8.8 ± 2.3° and 7.4 ± 1.0° for the shanks and forearms, respectively, P<0.05). A deeper understanding of the energy flow between the body segments and the environment is required to ascertain the possible dependency between internal and external work. This will prove essential to better understand swimming mechanical cost determinants and power generation in aquatic movements.

Different Muscle-Recruitment Strategies Among Elite Breaststrokers

PURPOSE To investigate electromyographical (EMG) profiles characterizing the lower-limb flexion-extension in an aquatic environment in high-level breaststrokers. METHODS The 2-dimensional breaststroke kick of 1 international- and 2 national-level female swimmers was analyzed during 2 maximal 25-m swims. The activities of biceps femoris, rectus femoris, gastrocnemius, and tibialis anterior were recorded. RESULTS The breaststroke kick was divided in 3 phases, according to the movements performed in the sagittal plane: push phase (PP) covering 27% of the total kick duration, glide phase (GP) 41%, and recovery phase (RP) 32%. Intrasubject reproducibility of the EMG and kinematics was observed from 1 stroke cycle to another. In addition, important intersubject kinematic reproducibility was noted, whereas muscle activities discriminated the subjects: The explosive PP was characterized by important muscle-activation peaks. During the recovery, muscles were likewise solicited for swimmers 1 (S1) and 2 (S2), while the lowest activities were observed during GP for S2 and swimmer 3 (S3), but not for S1, who maintained major muscle solicitations. CONCLUSIONS The main muscle activities were observed during PP to perform powerful lower-limb extension. The most-skilled swimmer (S1) was the only 1 to solicit her muscles during GP to actively reach better streamlining. Important activation peaks during RP correspond to the limbs acting against water drag. Such differences in EMG strategies among an elite group highlight the importance of considering the muscle parameters used to effectively control the intensity of activation among the phases for a more efficient breaststroke kick.