Tendon organ discharge during voluntary movements in cats.

1. The discharge activity of tendon organ afferents was recorded during voluntary movements in cats. 2. The eight tendon organ afferents in our sample all fired during isotonic movements involving active muscle shortening. 3. Firing rates usually exceeded 100 sec-1, even up to the highest muscle shortening velocity observed, 1.8 resting lengths per second (lr/sec). 4. We suggest that during voluntary, isotonic movements involving muscle shortening at velocities exceeding 0.2 lr/sec, the net action of muscle afferents on homonymous motoneurones is often inhibition. 5. These observations on tendon organs, taken together with previous findings on muscle spindles, indicate that in normal fast movements the role of the large muscle afferents is to signal dynamic functions of muscle length and force.