Development of an apparatus to quantify the volitional muscle performance of rat plantar flexors in vivo

An in vivo animal model was developed to study the effects of voluntary eccentric, concentric, and isometric muscle actions and varying work-rest cycles on muscle performance, behavior, and histological and biochemical response. Using a custom-designed apparatus that was attached to a standard operant chamber, rats were operantly conditioned with food rewards to perform a voluntary lifting task to generate controlled movement of the plantar flexors. An opening in the front panel of the operant chamber allowed the rat to enter a Plexiglas tube that was mounted vertically to restrict the movement of the rat. A load cell was embedded in a platform at the bottom of the tube to measure the dynamic force exerted by the plantar flexors. Inside the tube, a neck ring was supported by a yoke that moved along two vertical shafts via linear bearings. A displacement transducer (LVDT) was attached to the weight pan to measure the range of motion of the lift, and allowed determination of velocity and acceleration of the lifting motion. The apparatus allowed the rat to enter the tube through the opening, insert its neck into the ring, and lift the ring assembly. The entire process was computer automated, and vertical displacement, time during each lift, and dynamic forces exerted during each lift were sampled at 100 Hz via a computer-controlled data acquisition system. This apparatus allows skeletal muscle performance to be studied longitudinally and in a controlled biomechanical environment. The apparatus is well suited to study the effect of chronic voluntary muscle actions and work-rest cycles on behavior and physiological outcomes