Control and quantitation of voluntary weight-lifting performance of rats

The present paper describes an exercise model that produces a voluntary hindlimb weight-lifting response. Each rat was operantly conditioned to enter a vertical tube, insert its head into a weighted ring (either 70 g or 700 g), lift the ring until its nose interrupted an infrared detector, and then lower the ring. Load cells measured the external force generated, and displacement transducers measured the vertical displacement of the ring during each lifting and lowering movement. The apparatus and training procedures were computer automated. Peak force, velocity, work, and power were calculated for each movement. Rats in both groups easily acquired the task after 12-15 training sessions, on average, conducted 5 days/wk. Once rats were trained, the lifting patterns were quite stable during several more weeks of posttraining exercise; however, the lighter 70-g load gave rise to more variable performances across rats. Results demonstrate the utility of quantitating the biomechanics of volitional movements and suggest that the present model can establish and maintain controlled repetitive movements necessary for studies of adaptation and/or injury in muscles, tendon, and bone.