The Muscle Synergy Hypothesis refers to a concept used to attempt an explanation for animals' ability to carry out very complex movements involving accurate control of many different muscles at the same time.

In humans, for example, a movement commonly thought of as relatively straightforward - such as carrying a liquid-filled cup without spilling the contents - is in reality highly complex. The arm has many 'degrees of freedom', that's to say that the individual parts can easily rotate with respect to each other. Each of these rotations must be controlled by a muscle contraction or relaxation. Thus the Central Nervous System (CNS) must be sending highly accurate control signals to many different muscles (or groups of muscles) at the same time. These signals are not only controlling the strength of the contractions, but also the timing of the signals (and how they vary over time).

It should also be noted that many simple actions - such as turning a door handle - can easily be achieved in a variety of ways, with the arms and hands in different positions, but with the same highly co-ordinated result.

For many decades, researchers having been trying to discover how this extremely complex control system works.

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.

Source : Current Opinions in Neurobiology 19(6): 601.

Despite many years of experimental and theoretical research, the question of how the CNS controls complex muscle-based movements has still not been adequately answered.