An electrohydraulic apparatus for the measurement of static and dynamic properties of rabbit muscles

The apparatus described in this communication enables the force-velocity relationship to be determined for whole rabbit muscles in vivo and their resistance to fatigue to be assessed at specified rates of external work. The ergometer generates constant-velocity motion, controlling a force of up to 50 N, over a range of velocity up to 500 mm/s and a distance of 20 mm. This distance corresponds to the range of shortening of the rabbit tibialis anterior muscle from full plantar flexion to full dorsiflexion of the foot, equivalent to approximately 28% fiber shortening. Activated muscles can be allowed to shorten at constant velocity from any point on their isometric force trajectory. Cyclic releases for fatigue testing can be made at rates up to 30 releases/min over a period of 6-8 h. The timing of the release and return strokes of the ergometer is under the control of a digital programmer that also synchronizes the delivery of activating stimuli to the muscle nerve and trigger signals to the recording equipment. An electrohydraulic design was chosen because it is simpler to engineer than an electromagnetic actuator, is reliable in continuous cyclic use, and can be assembled, at least in part, from available industrial components.