EMG-force relationship of the cat soleus muscle studied with distributed and non-periodic stimulation of ventral root filaments.

Distributed stimulation of ventral root (VR) filaments and pseudo-random interpulse intervals (based on a Gaussian distribution with a coefficient of variation of 12.5%) were used to modulate electromyographic activity (EMG) and force of the cat soleus muscle to assess the EMG-force relationship. A protocol consisting of addition and rate modulation of ten VR filaments that contain alpha motoneurones to the soleus muscle was adopted. EMG was measured using indwelling electrodes and forces were measured at the distal tendon using a strain transducer. EMG records obtained using this approach were similar in the time and frequency domains to those obtained during voluntary contractions. Force records obtained from stimulation of single VR filaments showed summation effects typical of irregular interpulse intervals. The overall relationship between integrated rectified EMG (IEMG) and mean force was found 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% and 88% of the maximal tetanic force), the IEMG-mean force relationship was virtually linear. Muscles with a homogeneous fibre type composition, such as the cat soleus muscle, have been reported to have a linear EMG-force relationship.