The purpose of this technical note is to describe an experimental approach that allows the study of mechanical properties of muscles in vivo during normal, unrestrained movements in an animal preparation, and that further allows the investigation of sensorimotor control of joint movements. The preparation contains direct muscle force and EMG measurements of the target muscles (as described previously), and a chronically implanted nerve cuff electrode that may be triggered manually or automatically based on the force output of the targeted muscle. The system has been tested successfully on ankle flexor and extensor muscles of five cats.