EMG-FORCE RELATIONSHIP OF THE CAT SOLEUS MUSCLE STUDIED WITH DISTRIBUTED AND NONPERIODIC STIMULATION OF VENTRAL ROOT FILAMENTS

Distributed stimulation of ventral root (VR) filaments and pseudo-rand om interpulse intervals (based on a Gaussian distribution with a coeff icient of variation of 12.5 %) were used to modulate electromyographic activity (EMG) and force of the cat soleus muscle to assess the EMG-f orce relationship. A protocol consisting of addition and rate modulati on of ten VR filaments that contain alpha motoneurones to the soleus m uscle was adopted. EMG was measured using indwelling electrodes and fo rces were measured at the distal tendon using a strain transducer. EMG records obtained using this approach were similar in the time and fre quency domains to those obtained during voluntary contractions. Force records obtained from stimulation of single VR filaments showed summat ion effects typical of irregular interpulse intervals. The overall rel ationship between integrated rectified EMG (IEMG) and mean force was f ound to be non-linear. At low and high stimulation levels, IEMG tended to increase proportionally more than mean force. In the intermediate stimulation region (i.e. producing forces between approximately 5 % an d 88 % of the maximal tetanic force), the IEMG-mean force relationship was virtually linear. Muscles with a homogeneous fibre type compositi on, such as the cat soleus muscle, have been reported to have a linear EMG-force relationship.