The EMG-Force Relation of the Cat Soleus Muscle

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Abstract

This project was aimed at gaining a better understanding of the relationship between electromyographic (EMG) and force signals of the cat soleus muscle. Distributed stimulation of ventral root (VR) filaments and pseudo-random inter-pulse intervals were used, respectively, to: (1) assess the isometric EMG-force relation, and (2) determine the effects of changes in muscle length on the EMG signals, force, and EMG-force relation; and an attempt was made to (3) describe, and (4) understand the dynamic EMG-force relation on the basis of instantaneous contractile condition during . locomotion. Indwelling (wire) electrodes and tendon force transducers were used. A Protocol consisting of addition and rate modulation of eight or ten VR filaments (N = 3) produced EMG and force records similar to those obtained during voluntary contractions, and yielded non-linear (sigmoid) relations between integrated rectified EMG (IEMG) and mean force. In an intermediate (physiological) stimulation region the IEMG-mean force relation was virtually linear. Similar results were found in most cases for four different muscle lengths (N = 2). Effects of muscle length on EMG were likely to be associated to movement of the recording electrodes relative to each other and to the active motor units. Mean forces increased with increasing muscle length and stimulation. Mean force-length relations obtained using submaximal stimulation levels appeared to be shifted towards longer muscle lengths compared to the force-length relation obtained using supramaximal nerve stimulation. The mean EMG-force relations obtained during locomotion were highly non-linear, but similar in shape across cats (N = 3) and five different speeds. The theoretically predicted activation-time plots (Allinger and Herzog, 1993) and the experimentally measured IEMG-time plots tended to have two bursts. The first IEMG burst was usually higher than the second one, and it appeared to be associated with the initial priming of the muscle for force production at paw contact, and with the peak force observed early during the stance phase. The second IEMG burst appeared to be an attempt to produce the force required to complete the stance phase, when the contractile conditions of the soleus muscle for force production were highly unfavourable.

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