Relation of whole muscle contractile properties to source of innervation.

This work investigated the extent to which constituent proportions of fast- and slow-twitch muscle units determine whole muscle contractile properties. We employed several self- and cross-reinnervation models using synergistic homogeneous and heterogeneous muscles. By comparing the present data with previously published single-unit studies on the same animals, we confirmed that the whole muscle data provide a weighted ensemble of single-unit effects. They corroborate conclusions based upon single-unit data concerning the proportions of fast- and slow-twitch units, the degree of recovery, and the alteration of force- and speed-related properties of muscle by nerve. In particular, the shape of the whole muscle isometric twitch was a reliable indicator of the relative proportions of fast- and slow-twitch motor units in the muscle. The half-rise time was an indicator of the presence or absence of fast units, and the half-relaxation time was an indicator of the presence of slow units.