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.