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Muscles within muscles: Do spatio-neural mechanisms underpin force production?

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Our conception on the function of skeletal muscles is historically bound to their anatomy. The active contribution of skeletal muscles to a certain movement is typically based upon their attachment sites and the body joints they span. Such contribution has been considered to be proportional to the size of ...

Our conception on the function of skeletal muscles is historically bound to their anatomy. The active contribution of skeletal muscles to a certain movement is typically based upon their attachment sites and the body joints they span. Such contribution has been considered to be proportional to the size of active muscle volume. Specifically, the temporal and the spatial summation of discharges issued by the nervous system to a collection of muscle fibres are key determinants of the magnitude and timing of force vectors. Of less general concern, though not of less physiological relevance, is the association between the location of activated fibres within the muscle volume and the resulting direction of the lumped force vector. Direct, empirical evidence on the shaping of motor unit recruitment by force direction first appeared in the early eighties, when the likelihood of recruitment was shown to depend on the force direction both in humans and cats. Physiologically, this evidence suggested the fractionation of pools of motor neurons (i.e., the formation of task-groups) according to segmental kinematics rather than to individual muscles.

The notion of fractionation is not in conflict with the parsimonious view that body movements are controlled through low level, motor modules. Activation of a specific set of key muscles has been consistently appreciated in response to external perturbation, both in healthy and in centrally and peripherally injured subjects. These muscle synergies posit an asset accounting for chief issues related to the neural control of movement, e.g., limited central resources, redundancy and the number of degrees of freedom. On one hand one could argue that fragmenting individual muscles into populations of motor units increases the number of degrees of freedom to be controlled. On the other hand, fractionation might constitute a flexible means through which body kinematics are efficiently coded in supraspinal levels. In this case, individual muscles would participate in different synergies, depending on the population of motor units active. Quantifying the spatial distribution of the fibres of activated motor units may therefore reveal finer principles of intramuscular functional and/or mechanical specialisation than have previously been appreciated.

Comprehensive assessment of muscle fractionation is currently methodologically challenging. Intramuscular electromyograms sample from markedly small muscle volumes. Arrays of surface electrodes provide spatial representations of muscle activity, though their pick-up volume is limited in depth direction. Although ultrasound imaging provides a deep-superficial muscle view, sources of tissue movement might be of passive and/or active origin. Magnetic resonance imaging is expensive and, in certain circumstances (e.g., dynamic contractions), its use might be not viable. Notwithstanding these technical limitations, results emerging from different centres have collectively supported the notion of fractionation.

This research topic therefore focuses on collecting evidence either supporting or refuting the existence of spatio-neural mechanisms underpinning the control of muscles and, ultimately, of movement. Original manuscripts addressing the activity of motor units and/or of muscle and its sub-volumes in relation to body kinematics are strongly welcome. Methodological articles highlighting new insights into the quantification and interpretation of muscle activity are further encouraged.


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