Neuromechanics of Muscle Synergies During Cycling

Abstract

. Muscle synergies have been proposed as building blocks that could simplify the construction of motor behaviors. However, the muscles within synergistic groups may have different architectures, mechanical linkages to the skeleton, and biochemical properties, and these put competing demands on the most appropriate way to activate them for different mechanical tasks. This study identifies the extent to which synergistic patterns of muscle activity vary when the mechanical demands on a limb were altered, and additionally identifies how consistent the spectral profiles of the electromyographic (EMG) intensities were across the different movement tasks. The muscle activities were measured with surface EMG across 10 muscles in the leg during cycling at a range of loads and velocities. The EMGs were quantified by their intensities in time-frequency space using wavelet analysis; the instantaneous patterns of activity identified using principal component analysis, statistically compared and further visualized using the varimax rotation. Variability (35.7%) in the patterns of activity between the muscles were correlated with the torque and velocity of the pedal crank. Anatomic groups of muscles share a common mechanical action across a joint; uncoupling between such muscles was identified in 68.8% of the varimax patterns that encompassed all 10 muscles and 20.8 -29.5% of the activity patterns when the anatomic groups were analyzed separately. The EMG spectra showed greatest heterogeneity for the gastrocnemii. These results show that the activity of muscles within anatomic groups is partially uncoupled in response to altered mechanical demands on the limb.

Early deactivation of slower muscle fibres at high movement frequencies

“…In addition, by using wavelet analysis it is possible to retain both time and frequency information of the EMG signal to evaluate the changes in frequency with respect to muscle excitation duration (von Tscharner, 2000;Wakeling et al, 2002;Wakeling and Rozitis, 2004). In this study we hypothesized that the early derecruitment of slower muscle fibres during muscle relaxation would occur at the highest cycle frequencies, and this would be demonstrated by a relative reduction in the low-frequency component (60-90 Hz) of the EMG intensity at the end of the burst of EMG excitation (Wakeling et al, 2001;Wakeling and Horn, 2009).…”

“…However, progressive developments in EMG decomposition have made it possible to distinguish faster and slower muscle fibres based on the signal frequency with fast and slow muscle fibres producing high and low EMG frequencies, respectively (Gerdle et al, 1988;Solomonow et al, 1990;Elert et al, 1992;Kupa et al, 1995;von Tscharner, 2000;Wakeling and Rozitis, 2004;Wakeling, 2009;Wakeling and Horn, 2009). In addition, by using wavelet analysis it is possible to retain both time and frequency information of the EMG signal to evaluate the changes in frequency with respect to muscle excitation duration (von Tscharner, 2000;Wakeling et al, 2002;Wakeling and Rozitis, 2004).…”

Comparison of human gastrocnemius forces predicted by Hill-type muscle models and estimated from ultrasound images

“…EMG signals were preamplified (gain 1000), band-pass filtered (bandwidth 10-500 Hz; Biovision, Wehrheim, Germany) and sampled at 2000 Hz as described elsewhere (e.g. Wakeling and Horn, 2009;Blake and Wakeling, 2014). Electrodes were placed in the mid-region of the muscle belly after the skin had been shaved and cleaned with isopropyl alcohol solution.…”

“…This is the first study to test Hill-type models against in vivo estimates of time-varying muscle forces from human subjects at a range of mechanical conditions. Electromyographic (EMG) recordings from the gastrocnemii of cyclists have shown that the activation patterns of different muscle fibre types vary with cadence (Citterio and Agostoni, 1984;Wakeling et al, 2006;Wakeling and Horn, 2009). However, while muscles are composed of a mix of slow and fast fibres with different physiological and biomechanical properties, Hill-type models are typically composed of a single contractile element with force-length and force-velocity properties that have been estimated from in vitro data collected from single fibres under maximally activated conditions.…”

“…Here, we tested a traditional Hill-type model, with one contractile element, compared with a differential model, having independent slow and fast contractile elements to account for the contributions of slow and fast muscle fibres. Both models were driven by subject-specific measures of fascicle length, velocity and pennation angle derived from ultrasound images and by activation patterns of slow and fast fibres derived from surface EMG recordings and waveletbased time-frequency decomposition techniques (von Tscharner, 2000;Wakeling and Horn, 2009). We hypothesized that the twoelement model would better reproduce the ultrasound-based estimates of gastrocnemius force at the higher cadences, where preferential recruitment of fast fibres has been reported (Citterio and Agostoni, 1984;Wakeling et al, 2006).…”