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Abstract: During a sustained contraction, electromyographic signals (EMGs) undergo a spectral compression. This fatigue behaviour induces a shift of the mean and the median frequencies to lower frequencies. On the other hand, several studies conclude that the mean/median frequency can increase, decrease or remain constant with an increasing force level. Such inconsistency is embarrassing since the fatigue state may be influenced by the force level. In this paper, we propose a frequency indicator which is sensitive to the force level independently of the fatigue state evaluated at 70% of the maximal voluntary contraction. Ten healthy volunteers participated in the study and both surface EMGs (from the short head of the biceps brachii) and force signals were measured. This study compared force and fatigue effects on the EMGs during short (3-s) isometric contractions at different strength intensities and during a sustained isometric contraction until exhaustion. The EMGs partly show 1/f α spectral behaviours since their power spectral densities may experimentally fit with two linear segments in a log–log representation. The measured “right” slope produces variations of force as 20 times the variations of fatigue. 1/f α Behaviour may be related to stochastic fractals. This fractal indicator is a new frequency indicator that is thus complementary to other known classical frequency indicators when studying force during unknown fatigue states. [Copyright &y& Elsevier]

Abstract: Objective: The objective of this work was to study modifications in motor control through surface electromyographic (sEMG) activity during a very short all-out cycling exercise. Methods: Twelve male cyclists (age 23±4 years) participated in this study. After a warm-up period, each subject performed three all-out cycling exercises of 6s separated by 2min of complete rest. This protocol was repeated three times with a minimum of 2 days between each session. The braking torque imposed on cycling motion was 19Nm. The sEMG of the vastus lateralis was recorded during the first seven contractions of the sprint. Time–frequency analysis of sEMG was performed using continuous wavelet transform. The mean power frequency (MPF, qualitative modifications in the recruitment of motor units) and signal energy (a quantitative indicator of modifications in the motor units recruitment) were computed for the frequency range 10–500Hz. Results: sEMG energy increased (P ⩽0.05) between contraction number 1 and 2, decreased (P ⩽0.05) between contraction number 2 and 3 then stabilized between contraction number 3 and 7 during the all-out test. MPF increased (P ⩽0.05) during the all-out test. This increase was more marked during the first two contractions. Conclusions: The decrease in energy and the increase in the sEMG MPF suggest a large spatial recruitment of motor units (MUs) at the beginning of the sprint followed by a preferential recruitment of faster MUs at the end of the sprint, respectively. [Copyright &y& Elsevier]

Abstract: The aim of the study was to confirm the hypothesis that the longer a contraction is sustained, the larger are the changes in the spatial distribution of muscle activity. For this purpose, surface electromyographic (EMG) signals were recorded with a 13×5 grid of electrodes from the upper trapezius muscle of 11 healthy male subjects during static contractions with shoulders 90° abducted until endurance. The entropy (degree of uniformity) and center of gravity of the EMG root mean square map were computed to assess spatial inhomogeneity in muscle activation and changes over time in EMG amplitude spatial distribution. At the endurance time, entropy decreased (mean±SD, percent change 2.0±1.6%; P <0.0001) and the center of gravity moved in the cranial direction (shift 11.2±6.1mm; P <0.0001) with respect to the beginning of the contraction. The shift in the center of gravity was positively correlated with endurance time (R 2 =0.46, P <0.05), thus subjects with larger shift in the activity map showed longer endurance time. The percent variation in average (over the grid) root mean square was positively correlated with the shift in the center of gravity (R 2 =0.51, P <0.05). Moreover, the shift in the center of gravity was negatively correlated to both initial and final (at the endurance) entropy (R 2 =0.54 and R 2 =0.56, respectively; P <0.01 in both cases), indicating that subjects with less uniform root mean square maps had larger shift of the center of gravity over time. The spatial changes in root mean square EMG were likely due to spatially-dependent changes in motor unit activation during the sustained contraction. It was concluded that the changes in spatial muscle activity distribution play a role in the ability to maintain a static contraction. [Copyright &y& Elsevier]