Abstract A system for the remote recording of electromyogram (EMG) and muscle length in freely moving cats is described. The length of M. triceps surae is monitored using an implanted mercury-in-rubber length-gauge, whose robust construction gives it an in vivo life expectancy of in excess of 4 weeks. A small (10 × 25 × 25 mm) telemeter attached to the animal's head amplifies and transmits the EMG and encoded length-gauge signals. The resistance variations of the gauge, resulting from changes in its length, are converted by the telemeter into a frequency modulated (FM) audio signal (f c = 15 kc/sec) which is in turn frequency modulated for ratio transmission (f c = 98 Mc/sec). This FM-FM technique eliminates orientation artefacts in the received signal. The step cycle data are analyzed in accordance with previous classifications of the flexion and extension phases. Both M. soleus and gastrocnemius are active throughout the stance phase, with peaks at start and end, but soleus is also moderately active during the early part of the flexion (swing) phase.
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