Extra forces evoked during electrical stimulation of the muscle or its nerve are generated and modulated by a length-dependent intrinsic property of muscle in humans and cats.

Extra forces or torques are defined as forces or torques that are larger than would be expected from the input or stimuli, which can be mediated by properties intrinsic to motoneurons and/or to the muscle. The purpose of this study was to determine whether extra forces/torques evoked during electrical stimulation of the muscle or its nerve with variable frequency stimulation are modulated by muscle length/joint angle. A secondary aim was to determine whether extra forces/torques are generated by an intrinsic neuronal or muscle property. Experiments were conducted in 14 able-bodied human subjects and in eight adult decerebrate cats. Torque and force were measured in human and cat experiments, respectively. Extra forces/torques were evoked by stimulating muscles with surface electrodes (human experiments) or by stimulating the nerve with cuff electrodes (cat experiments). In humans and cats, extra forces/torques were larger at short muscle lengths, indicating that a similar regulatory mechanism is involved. In decerebrate cats, extra forces and length-dependent modulation were unaffected by intrathecal methoxamine injections, despite evidence of increased spinal excitability, and by transecting the sciatic nerve proximal to the nerve stimulations. Anesthetic nerve block experiments in two human subjects also failed to abolish extra torques and the length-dependent modulation. Therefore, these data indicate that extra forces/torques evoked during electrical stimulation of the muscle or nerve are muscle length-dependent and primarily mediated by an intrinsic muscle property.