The case for and against muscle synergies

Abstract

A long standing goal in motor control is to determine the fundamental output controlled by the CNS: does the CNS control the activation of individual motor units, individual muscles, groups of muscles, kinematic or dynamic features of movement, or does it simply care about accomplishing a task? Of course, the output controlled by the CNS might not be exclusive but instead multiple outputs might be controlled in parallel or hierarchically. In this review we examine one particular hypothesized level of control: that the CNS produces movement through the flexible combination of groups of muscles, or muscle synergies. Several recent studies have examined this hypothesis, providing evidence both in support and in opposition to it. We discuss these results and the current state of the muscle synergy hypothesis.

Figure 1

Schematic describing the muscle synergy hypothesis. (A) shows a schematic for synchronous synergies. Two synergies (w) are scaled (c) and summed to produce the observed pattern of muscle activations (m). These synchronous synergies are fully described by the balance of activation across the muscles in each synergy: any temporal structure is specified by the scaling coefficients (c). (B) shows a schematic illustrating time-varying synergies. Two time-varying synergies (w) are shown. Each synergy specifies a weighting coefficient for one of three muscles, indicated by the color of the bar and a temporal profile for these weightings. An observed pattern of muscle activations is created by scaling each synergy (c), temporally shifting them (t), then adding them together linearly, as indicated in the right of the figure. The purple and green waveforms indicate the contribution from each of the original synergies to the observed response. (B) is adapted by permission from Macmillan Publishers Ltd: Nature Neuroscience [17], copyright 2003.