Relations among motor unit types,generated forces and muscle length in single motor units of anaesthetized 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 amplitude and the tension-time area at muscle lengths extending throughout the physiological length range of the muscle (mean 5.5 mm, standard deviation ±0.8). The muscle lengths at which each tested MU developed its maximal twitch (L _tw) and tetanic (L _te) tensions were determined and compared with the muscle length (L _o) at which the stimulation of all the -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 values of L _tw(L _o+1.6±1.0 mm). Single MUs stimulated at tetanic frequencies 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). For all the units L _te was shorter than L _tw. L _tw and L _te were, respectively, negatively and positively correlated with the developed tension. Optimal length values also appeared to be related to the MU types. The possibility is discussed that the muscle and tendon compliances and the high non-linearities to the applied forces are the main factors which can determine the differences among L _o, L _tw and L _te values. The relationships between MU type and optimal length values are suggested to be, at least partly, an epiphenomenon due to the different contraction strengths of the various MU types. However, the heterogeneous distribution of the MU types is brought into account to explain the dependence of L _tw and L _te values on MU type.