The effects of length changes in the in-situ cat soleus muscle on vibromyographic (VMG) signals were assessed using electrical stimulation of the soleus nerve in three adult male cats. Force and VMG signals were measured using an E-shaped force transducer and a miniature, unidirectional accelerometer, respectively. In each test, the soleus nerve was stimulated for 6 s at rates ranging from 4 to 35 Hz, and at two to four ankle angles (80-140 degrees ). The force of the soleus muscle increased with increasing muscle length and stimulation rates. For a given sub-tetanic frequency of stimulation, the root mean square (RMS) values of the VMG signal were larger at intermediate muscle lengths than at both the longest and the shortest muscle lengths. There was a continuous increase in the RMS of the VMG signal with decreasing muscle length when the contraction was tetanic. There was a trend towards an increase in the median frequency (MDF) of the VMG signal with increasing muscle length. The results of this study support the idea that the amplitude and frequency content of the VMG signal during electrically elicited muscle contractions are directly affected by changes in the mechanical properties (i.e. the stiffness and the active and passive tension) of muscle caused by length changes.