EFFECTS OF MUSCLE LENGTH ON THE EMG-FORCE RELATIONSHIP OF THE CAT SOLEUS MUSCLE STUDIED USING NONPERIODIC STIMULATION OF VENTRAL ROOT FILAMENTS

The effects of changing the length of the cat soleus muscle on electro myographic (EMG) signals, muscle force and the corresponding EMG-force relationship were assessed using distributed stimulation of ten ventr al root filaments and irregular interpulse intervals. EMG-force relati onships were first determined for four muscle lengths using a protocol of simultaneous addition and rate modulation of ventral root filament s. In the second test, three submaximal levels of stimulation were app lied at eight muscle lengths. EMG signals were obtained using surface and wire electrodes, and force was measured using a strain transducer. For most muscle lengths, the relationships between integrated EMG and mean force obtained using wire and surface electrodes were sigmoid wi th a linear intermediate region. The effects of muscle length on EMG s ignals were likely to be associated with movement of the recording ele ctrodes relative to each other and to the active motor units. Mean for ces increased with increasing muscle length and with increasing levels of stimulation. Mean force-length relationships obtained using submax imal stimulation were not simply scaled down versions of the force-len gth relationship obtained using supramaximal stimulation of the soleus nerve, but appeared to be shifted towards longer muscle lengths.