RELATIONS AMONG MOTOR UNIT TYPES, GENERATED FORCES AND MUSCLE LENGTH IN SINGLE MOTOR UNITS OF ANESTHETIZED CAT PERONEUS LONGUS MUSCLE

The active length-tension curves of identified single motor units (MUs ) belonging to peroneus longus muscle (PL) of anaesthetized adult cats were obtained by eliciting isometric single twitches and tetani. The recorded responses were evaluated by measuring the peak tension amplit ude and the tension-time area at muscle lengths extending throughout t he physiological length range of the muscle (mean 5.5 mm, standard dev iation +/-0.8). The muscle lengths at which each tested MU developed i ts maximal twitch (L(tw)) and tetanic (L(te)) tensions were determined and compared with the muscle length (L(o)) at which the stimulation o f all the alpha-axons, innervating PL and contained in L7 ventral root , developed their maximal twitch tension. The mean of single MU L(tw) values was at L(o) + 1.08 +/- 1.1 mm. Slow MUs showed the longest valu es of L(tw) (Lo( )+ 1.6+/- 1.0 mm). Single MUs stimulated at tetanic f requencies presented their L(te) at values shorter than L(o) (L(o)-2.8 +/- 1.7 mm). Slow MUs had the shortest L(te) (L(o)-3.4 +/- 1.5 mm). F or all the units L(te) was shorter than L(tw). L(tw) and L(te) were, r espectively, negatively and positively correlated with the developed t ension. Optimal length values also appeared to be related to the MU ty pes. The possibility is discussed that the muscle and tendon complianc es and the high non-linearities to the applied forces are the main fac tors which can determine the differences among L(o), L(tw) and L(te) v alues. The relationships between MU type and optimal length values are suggested to be, at least partly, an epiphenomenon due to the differe nt contraction strengths of the various MU types. However, the heterog eneous distribution of the MU types is brought into account to explain the dependence of L(tw) and L(te) values on MU type.