Mechanisms Underlying Muscle Synergy Studied in Man

One simple mechanism that could account for the co-contraction of synergistic muscles is that it is brought about by the sharing of common excitatory drive. To test this hypothesis we have performed cross-correlation analysis of motor unit discharges recorded during steady voluntary contractions in man (Bremner, Datta & Stephens, 1989; Bremner, Baker & Stephens, 1991b; Carr, Harrison & Stephens, 1994). The presence of a shared input is indicated by a narrow central peak in the cross-correlogram constructed from the times of occurrence of motor unit spikes in the two muscles. An example is shown in Figure 1A for gastrocnemius and soleus (2–3 units in each record). Correlogram peaks have also been found for bilateral homologous muscles such as masseter, diaphragm and rectus abdominis which are normally active together to produce movements symmetrical about the mid-line. Correlogram peaks have not been found when recording from muscle pairs that do not share a common action about a common joint or axis (Gibbs, Topham, Mackenzie, Harrison & Stephens, 1994b). From these experiments a simple new generalisation has emerged for the organisation of synaptic drive to motoneurones which we call the sharing principle — motoneurones innervating muscles that share a common mechanical action share a common presynaptic input. The converse is also true — motoneurones innervating co-contracting muscles that do not share a common action do not share a common input (Gibbs, Harrison & Stephens, 1995). This extends to different motoneurone pools the generalisation envisaged by Henneman to govern the organisation of inputs to a single motoneurone pool — motoneurones innervating a single muscle share common reflex inputs (reviewed by Henneman, 1980). This is suported by the results of cross-correlation analysis which show that motoneurones innervating the same muscle share common presynaptic input (Bremner, Baker & Stephens, 1991a); a mechanism that ensures that during individual muscle contractions motor units act synergistically.