Between-subject variability of muscle synergies during a complex motor skill

Abstract

The purpose of the present study was to determine whether subjects who have learned a complex motor skill exhibit similar neuromuscular control strategies. We studied a population of experienced gymnasts during backward giant swings on the high bar. This cyclic movement is interesting because it requires learning, as untrained subjects are unable to perform this task. Nine gymnasts were tested. Both kinematics and electromyographic (EMG) patterns of 12 upper-limb and trunk muscles were recorded. Muscle synergies were extracted by non-negative matrix factorization (NMF), providing two components: muscle synergy vectors and synergy activation coefficients. First, the coefficient of correlation (r) and circular cross-correlation (rmax) were calculated to assess similarities in the mechanical patterns, EMG patterns, and muscle synergies between gymnasts. We performed a further analysis to verify that the muscle synergies (in terms of muscle synergy vectors or synergy activation coefficients) extracted for one gymnast accounted for the EMG patterns of the other gymnasts. Three muscle synergies explained 89.9 ± 2.0% of the variance accounted for (VAF). The coefficients of correlation of the muscle synergy vectors among the participants were 0.83 ± 0.08, 0.86 ± 0.09, and 0.66 ± 0.28 for synergy #1, #2, and #3, respectively. By keeping the muscle synergy vectors constant, we obtained an averaged VAF across all pairwise comparisons of 79 ± 4%. For the synergy activation coefficients, rmax-values were 0.96 ± 0.03, 0.92 ± 0.03, and 0.95 ± 0.03, for synergy #1, #2, and #3, respectively. By keeping the synergy activation coefficients constant, we obtained an averaged VAF across all pairwise comparisons of 72 ± 5%. Although variability was found (especially for synergy #3), the gymnasts exhibited gross similar neuromuscular strategies when performing backward giant swings. This confirms that the muscle synergies are consistent across participants, even during a skilled motor task that requires learning.

Muscle synergies during bench press are reliable across days

“…To evaluate the between-day reliability of the extracted muscle synergy vectors and synergy activation coefficients, we performed a cross-validation analysis similar to that performed by Frère and Hug [Frère and Hug, 2012] and Muceli et al [Muceli et al, 2010]. In this iterative procedure [1], the muscle synergy vectors extracted in the first test session were recomputed, using the fixed synergy activation coefficients from the second test session.…”

“…walking [Ivanenko et al, 2004;MacLellan et al, 2014;Oliveira et al, 2014], running [Cappellini et al, 2006], pedaling [Dorel et al, 2009;Hug et al, 2010;Hug et al, 2011], rowing [Turpin et al, 2011a;Turpin et al, 2011b], bench press [Kristiansen et al, 2015b], and backward giant swing [Frère and Hug, 2012].…”

Muscle coordination during breaststroke swimming: Comparison between elite swimmers and beginners

“…E was therefore an 8 row by 800 to 2000 columns matrix. Total Variance Accounted For (VAF) was calculated as previously described (Frère & Hug, 2012). We iterated the analysis by varying the number of synergies between 1 and 8 (i.e.…”

Towards an understanding of motor system impairments in individuals with unilateral lateral epicondylalgia

“…number of recorded muscles) and selected the least number of synergies that accounted for >90% of Variance Accounted For (VAF) or until adding an additional synergy did not increase VAF by > 5% (Frère & Hug, 2012). Mean total VAF was calculated as described in (Torres-Oviedo, Macpherson, & Ting, 2006).…”