THE EFFECT OF MUSCLE LENGTH ON ELECTRICALLY ELICITED MUSCLE VIBRATIONS IN THE IN-SITU CAT SOLEUS MUSCLE

The effects of length changes in the in-situ cat soleus muscle on vibr omyographic (VMG) signals were assessed using electrical stimulation o f the soleus nerve in three adult male cats. Force and VMG signals wer e measured using an E-shaped force transducer and a miniature, unidire ctional accelerometer, respectively. In each test, the soleus nerve wa s stimulated for 6 s at rates ranging from 4 to 35 Hz, and at two to f our ankle angles (80-140 degrees). The force of the soleus muscle incr eased with increasing muscle length and stimulation rates. For a given sub-tetanic frequency of stimulation, the root mean square (RMS) valu es of the VMG signal were larger at intermediate muscle lengths than a t both the longest and the shortest muscle lengths. There was a contin uous increase in the RMS of the VMG signal with decreasing muscle leng th when the contraction was tetanic. There was a trend towards an incr ease in the median frequency (MDF) of the VMG signal with increasing m uscle length. The results of this study support the idea that the ampl itude and frequency content of the VMG signal during electrically elic ited muscle contractions are directly affected by changes in the mecha nical properties (i.e. the stiffness and the active and passive tensio n) of muscle caused by length changes. (C) 1997 Elsevier Science Ltd.