Motor unit heterogeneity with respect to speed and fatiguability in cat muscles after chronic stimulation or paralysis

Chronic stimulation of fast-twitch muscles in rabbit (Salmons & Vrbova, 1969; reviewed by Pette & Vrbova, 1985) and cat (Eerbeek et al., 1984; reviewed by Kernell & Eerbeek,1989) has been shown to change contractile properties toward those of slow muscles. Activation with total claily amounts of activity of> 30%, 5% and <0.5% of the day, corresponding roughly with the activity of slow (S), fast-fatigue resistant (FR) and fast fatiguable (FF) units, appeared to convert the muscle contraction characteristics toward those of the predicted motor unit (MU) type (Eerbeek et al., 1984; KernelI et al., 1987a,b; Westgaard & Lomo, 1988). If activity causes complete conversion, the prediction would be that all MUs in the stimulated musc1es would become homogeneous in their properties. We therefore asked 2 questions: 1) what are the effects and the time course of increasing or decreasing activity on muscle speed and endurance? and 2) do MUs become homogeneous with respect to force, speed and endurance as predicted if activity converts muscles rather than modulates their properties within an adaptive range (Ausoni et al., 1990; Westgaard & Lomo, 1988).