Muscle fatigue during cross country sprint assessed by activation patterns and electromyographic signals time-frequency analysis.

The aims of this study were as follows: (i) analysis of activation patterns during the spurt of two heats of a cross country skiing sprint with the double poling technique and (ii) quantification of muscle fatigue of the investigated muscles. Eight elite skiers were tested and surface electromyographic signals (EMG) were recorded from seven muscles of the upper and lower limbs. For each subject and each muscle, the activation intervals were calculated for relying on a double-threshold statistical detector and the average rectified value was calculated on each activation interval. The detected activations were processed by a time-frequency algorithm in order to assess the progression of muscle fatigue. The EMG activation patterns and EMG amplitude highlighted no significant difference between the two spurts, despite a generally lower speed in the second spurt. The frequency analysis showed that upper body muscles are the first to be affected by fatigue and that clear signs of muscle fatigue appear right from the first spurt of the sprint simulation (i.e., biceps and triceps brachii) with a decrease in the instantaneous mean frequency. Biceps brachii activations and fatigue demonstrated the involvement of this muscle in propulsion.

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.

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.

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.

Wrist stabilisation and forearm muscle coactivation during freestyle swimming.

The aim of this study was to evaluate the stabilisation of the wrist joint and the ad hoc wrist muscles activations during the two principal phases of the freestyle stroke. Seven male international swimmers performed a maximal semi-tethered power test. A swimming ergometer fixed on the start area of the pool was used to collect maximal power. The electromyography signal (EMG) of the right flexor carpi ulnaris (FCU) and extensor carpi ulnaris (ECU) was recorded with surface electrodes and processed using the integrated EMG (IEMG). Frontal and sagittal video views were digitised frame by frame to determine the wrist angle in the sagittal plane and the principal phases of the stroke (insweep, outsweep). Important stabilisation of the wrist and high antagonist muscle activity were observed during the insweep phase due to the great mechanical constraints. In outsweep, less stabilisation and lower antagonist activities were noted. Factors affecting coactivations in elementary movements, e.g. intensity and instability of the load, accuracy and economy of the movement were confirmed in complex aquatic movement.

Determinants of the energy cost of front-crawl swimming in children.

The aim of the present study was to define the determinants of the energy cost of swimming (Cs) in children. Eleven healthy children [mean (SD) age: 12.42 (0.53) years] who practised 7.5-8.5 h x week(-1) volunteered to take part in this study. Anthropometric dimensions such as height (H), body mass (BM), hydrostatic lift (HL) and body surface area (SA) were measured. Forty-eight hours later when maximal oxygen consumption (VO(2max)) had been measured during 400 m of front-crawl swimming, Cs was measured over 200 m for three submaximal swimming speeds (0.9, 1.0 and 1.1 m x s(-1)). Oxygen consumption (Douglas bag method), stroke frequency (SF) and stroke length (SL) were calculated during the last 50 m of each 200 m. The mean (SD) VO(2max) of the young swimmers was 2.19 (0.38) l x min(-1) at a maximal aerobic velocity of 1.19 (0.03) m x s(-1). The values of for Cs at 0.9 m x s(-1), 1.0 m x s(-1) and 1.1 m x s(-1) were 29.27 (3.13) ml x m(-1), 30.25 (3.68) ml x m(-1) and 32.91 (3.59) ml x m(-1), respectively. There was a significant increase in Cs with increasing swim speed. In addition, SF increased with velocity when SL remained constant. The values for SF at 0.9 m x s(-1), 1.0 m x s(-1) and 1.1 m x s(-1) were 31.28 (4.36) strokes x min(-1), 34.10 (5.09) strokes x min(-1) and 38.31 (5.90) strokes x min(-1), respectively. No significant correlation was obtained between Cs and the anthropometric or stroking parameters. It was concluded that for young swimmers, anthropometric characteristics, SF and SL are not good predictors of Cs in front-crawl swimming, and that further studies are needed to explore the influence of underwater torque on Cs in prepubertal children.

Hydrodynamics optimization in butterfly swimming: position, drag coefficient and performance.

A kinematic study allowed to define the three most propulsive positions during a butterfly swimming cycle, which were: the end of the external sweep, the end of the internal sweep and the end of thrust. These instantaneous positions were different for the ex-world champion Pankratov when compared to another swimmer. Using manikins and a drag-measuring device, we showed that the end of the internal sweep induced the highest drag values and that Pankratov may reduce energy expenditure by taking up a particular position during the end of the swimming cycle. These results point out the relations between swimming movements, passive drag and swimmers' performance.

Relative contribution of arms and legs in humans to propulsion in 25-m sprint front-crawl swimming.

Eight male subjects were asked to swim 25 m at maximal velocity while the use of the arm(s) and legs was alternately restricted. Four situations were examined using one arm (1A), two arms (2A), one arm and two legs (1A2L) and both arms and legs (2A2L, normal swim) for propulsion. A significant mean increase of 10% on maximal velocity was obtained in 1A2L and 2A2L compared to 1A and 2A. A non-significant 4% effect was obtained in 1A. This study focused on the actual contribution of leg kick in the 10% gain in maximal velocity. It was clear that the underwater trajectory of the wrist was modified by the action of the legs (most comparisons P < 0.001). Therefore it was thought that the legs enhanced the generated propulsive force by improving the propulsive action of the arm. The arm action was quantified by selecting typical phases from the filmed trajectory of the wrist, namely forward (F), downwards (D) and backwards (B). Although there was a tendency for individual changes in kinematic parameters (F, D and B) to occur with individual changes in velocity when 2A was compared to 2A2L, no relationship was found between the relative changes in F, D and B and relative changes in velocity. This was illustrated by describing the responses of three individuals who could represent three patterns of contribution by legs and arms to propulsion in high speed swimming.

Muscular activations during repetitions of sculling movements up to exhaustion in swimming.

The purpose of this study was to examine the influence of the repetition of sculling movements of the upper limb on muscular electrical activities during an exhaustive test in front crawl. Six upper limb muscles activities of nine swimmers were recorded, with telemetric EMG data acquisition system using active surface electrodes, during a 4 x 100 m front crawl test conducted to exhaustion. The pattern of the movement was analysed from views obtained by recordings of two underwater cameras. Four phases in the stroke were identified from the hand coordinates in the frontal plane (down-sweep, insweep, outsweep and recovery). Raw EMG were rectified, integrated (IEMG) and normalized for each subject and for each muscle with respect to the highest IEMG obtained during the strokes and the phases. Results indicated that the repetition of the stroke up to exhaustion was not associated with an increase in IEMG for the total stroke and its phases excepted for the most activated muscle. The different sculling movements appeared to be clearly identify by the EMG approach whatever the trial. The contribution of the different muscles remained the same through the different repetitions up to exhaustion. The larger muscular recruitments were obtained during the insweep phase when important antagonist activities were observed. It would be interesting to observe the EMG in a next 100 m repetition when the swimmer could not sustain the same velocity.

The frontcrawl downsweep: shoulder protection and/or performance inhibition.

EMG of 6 shoulder and arm muscles was measured in nine good frontcrawl swimmers using active electrodes and a telemetric EMG data acquisition system. The raw EMG was low pass filtered, rectified and the integrated values were used as a measure of muscular intensity. The selection of muscles combined 2 mono-articular and 2 bi-articular shoulder muscles and 2 muscular primarily acting on elbow & wrist. Kinematic analysis allowed for a detailed time and movement pattern fractionation and the muscle activity within these phases was analysed for 4 x 100 m at maximum effort up to exhaustion. The combination of time distribution and muscular activity suggested that the downsweep is the longest in time but with the lowest intensity of all 4 transient phases of the cyclic arm movement. This is assumed to be effective for decreasing the load of the shoulder joint. However, it could also be considered that if the insweep with its higher muscular participation could start earlier in the pull phase while shortening the downsweep. The higher, but more constant loading of the shoulder might improve performance.

Cocontraction in the elbow and shoulder muscles during rapid cyclic movements in an aquatic environment.

The purpose of this study was to document the cocontraction patterns of the combined wrist-elbow-shoulder joint muscles during maximal-effort swimming. The subjects, nine high-performance athletes, swam 4 x 100 m at maximal effort with a 45 s rest after each 100 m run. Electromyographic (EMG) recording of six muscles was performed with surface electrodes and telemetry. The quality of the arm movement was verified with two video cameras and the movement was divided into four phases: the initial press, the inward scull, the outward scull and the air recovery. Eccentric work and antagonist co-activation of the arm extensors varied within the motion cycle but were mostly present during the inward scull; i.e. it is assumed that coactivation is transient movement and is phase-dependent despite the rapidity of the movement. These data suggest that both eccentric work and cocontractions are important features of rapid cyclic repetitive movement with the m. biceps and the m. brachioradialis as prime movers.

Influences of sex and level of performance on freestyle stroke: an electromyography and kinematic study.

Surface Electromyography (EMG) of the upper arm was performed on 50 subjects (30 males and 20 females). The following six muscles were studied: M. flexor carpi ulnaris, M. brachioradialis, M. biceps brachii, M. triceps brachii and two parts of the M. deltoidus (anterior and medialis). A cinematographical analysis was synchronized with the EMG's recorded during swims at maximal speed. The kinematics data were: hip displacement, stroke rate and five stroke phases measured from the arm-trunk angles. EMG measurements were normalized for each subject and analysed using the statistical method of principal component analysis. The best swimmers, at the same relative effort, had a greater hip displacement, lower electrical muscular activity and more selective recruitment, indicating greater efficiency. The role of each muscle during the different phases was in agreement with previous investigations (2, 11, 13, 14, 19). The swimmer's technique, particularly the position of the upper arm during the push phase, influenced these results. Time parameters of the stroke were not significantly different for the various swimmer's levels. The conclusions were similar, for the females, however, with a worse displacement of the hip.