Abstract
Recent studies have reported evidence that the motor system may rely on a modular organization, even if this behavior has yet to be confirmed during motor adaptation. The aim of the present study is to investigate the modular motor control mechanisms underlying the execution of pedaling by untrained subjects in different biomechanical conditions. We use the muscle synergies framework to characterize the muscle coordination of 11 subjects pedaling under two different conditions. The first one consists of a pedaling exercise with a strategy freely chosen by the subjects (Preferred Pedaling Technique, PPT), while the second condition constrains the gesture by means of a real time visual feedback of mechanical effectiveness (Effective Pedaling Technique, EPT). Pedal forces, recorded using a pair of instrumented pedals, were used to calculate the Index of Effectiveness (IE). EMG signals were recorded from eight muscles of the dominant leg and Non-negative Matrix Factorization (NMF) was applied for the extraction of muscle synergies. All the synergy vectors, extracted cycle by cycle for each subject, were pooled across subjects and conditions and underwent a 2-dimensional Sammon's non-linear mapping. Seven representative clusters were identified on the Sammon's projection, and the corresponding eight-dimensional synergy vectors were used to reconstruct the repertoire of muscle activation for all subjects and all pedaling conditions (VAF > 0.8 for each individual muscle pattern). Only 5 out of the 7 identified modules were used by the subjects during the PPT pedaling condition, while 2 additional modules were found specific for the pedaling condition EPT. The temporal recruitment of three identified modules was highly correlated with IE. The structure of the identified modules was found similar to that extracted in other studies of human walking, partly confirming the existence of shared and task specific muscle synergies, and providing further evidence on the modularity of the motor system.
Highlights
The study of the neuro-physiological mechanisms underlying movement production has a long fascinating history (Bernstein, 1967)
In the last decade the scientific community has been focusing its attention on the possibility of simplifying the role of the central nervous system (CNS) for the production of movement, by hypothesizing that the complex muscle coordination shown during the execution of a variety of motor acts relies on a simple combination of motor modules (d’Avella et al, 2003)
RESULTS pedal force profiles are shown for the two pedaling conditions, and the corresponding possible changes in the EMG profiles and the underlying muscle synergies are reported as potential signs of neuromuscular adaptations to an altered pedaling technique
Summary
The study of the neuro-physiological mechanisms underlying movement production has a long fascinating history (Bernstein, 1967). Muscle synergies have been investigated in motor tasks like running (Cappellini et al, 2006), postural responses (Torres-Oviedo and Ting, 2007), pedaling (Hug et al, 2010), walking in normal and pathologic conditions (Ivanenko et al, 2004; Clark et al, 2010; Monaco et al, 2010) and upper limb reaching (d’Avella et al, 2008; Cheung et al, 2012) From this background it emerges that the muscle synergies paradigm seems to fairly represent the neural strategies underlying the control of movement, with motor modules characteristic for each task and robustly shared among different subjects, in terms of both temporal and spatial organization of the muscle activity. The existence of a few shared and task-specific muscle synergies during the execution of different movements in the freely moving frogs (d’Avella and Bizzi, 2005) and the existence of separate modules during the coordination of locomotion with voluntary actions (Ivanenko et al, 2005), provides further evidence of modularity
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