Although there is growing evidence that these modules have a neural origin, some authors argue that these are a result of biomechanical constraints. These are considered coordinative primitives that have a neural origin and are structured in the brainstem or spinal cord. Recent research suggest that muscle synergies represent motor modules encoded in spinal cord and brainstem, and controlled by motor cortical areas and integrating sensory information that activate groups of muscles to generate a specific motor output. The muscle synergies allow the CNS to control smaller number of variables, simplifying the construction of motor behaviors. Such mechanism would consist in the presence of low-dimensional elements, muscle synergies, that decrease the computational burden and, hence, would allow a more efficient control from the CNS. It has been suggested that there might be a mechanism that deals with the many degrees of freedom available in the neuromusculoskeletal system. The complexity of the CNS to control all the involved elements is not completely understood yet. When performing a motor task, the Central Nervous System (CNS) has to control the biomechanical redundancy established by infinite neuromuscular interactions, in a way that all muscles involved can lead to the desired joint moments and assure that the task is successfully performed. Thus, the mathematical procedure for the extraction of muscle synergies through nonnegative matrix factorization (NMF) may be considered a reliable method to study muscle coordination adaptations from muscle strength programs. The present findings revealed that the muscle synergies extracted during the power clean remained stable across sets and days in unexperienced participants. Inter-day muscle synergy vectors had moderate similarity, while the variables regarding temporal activation were still strongly related. Intra-day VAF, muscle synergy vectors, synergy activation coefficients and individual EMG profiles showed high similarity values. Three muscle synergies accounted for almost 90% of variance accounted for (VAF) across sets and days. Twelve unexperienced participants performed four sets of power cleans in two test days after strength tests, and muscle synergies were extracted from electromyography (EMG) data of 16 muscles. Therefore, the aim of the study was to evaluate intra- and inter-day reliability of a strength training complex task, the power clean, assessing participants’ variability in the task across sets and days. The reliability of the method has been proposed, although it has not been assessed previously during a complex sportive task. Muscle synergy extraction has been utilized to investigate muscle coordination in human movement, namely in sports.
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