Using evoked EMG as a synthetic force sensor of isometric electrically stimulated muscle.

A method for the estimation of the force generated by electrically stimulated muscle during isometric contraction is developed here. It is based upon measurements of the evoked electromyogram (EMG) [EEMG] signal. Muscle stimulation is provided to the quadriceps muscle of a paralyzed human subject using percutaneous intramuscular electrodes, and EEMG signals are collected using surface electrodes. Through the use of novel signal acquisition and processing techniques, as well as a mathematical model that reflects both the excitation and activation phenomena involved in isometric muscle force generation, accurate prediction of stimulated muscle forces is obtained for large time horizons. This approach yields synthetic muscle force estimates for both unfatigued and fatigued states of the stimulated muscle. In addition, a method is developed that accomplishes automatic recalibration of the model to account for day-to-day changes in pickup electrode mounting as well as other factors contributing to EEMG gain variations. It is demonstrated that the use of the measured EEMG as the input to a predictive model of muscle torque generation is superior to the use of the electrical stimulation signal as the model input. This is because the measured EEMG signal captures all of the neural excitation, whereas stimulation-to-torque models only reflect that portion of the neural excitation that results directly from stimulation. The time-varying properties of the excitation process cannot be captured by existing stimulation-to-torque models, but they are tracked by the EEMG-to-torque models that are developed here. This work represents a promising approach to the real-time estimation of stimulated muscle force in functional neuromuscular stimulation applications.

Possible mechanisms for observed pathophysiological variability in experimental spinal cord injury by the method of Allen.

Experimental spinal cord injuries were induced in dogs by dropping calibrated weights through a vented tube onto a small impounder resting on the surgically exposed cord. The motion of the impounder and the drop-mass were recorded by high-speed photography and the resulting data were compared to those obtained from a computer simulation of the dynamics of the injury mechanism. It is concluded that this method of induced spinal cord injuries may yield markedly different degrees of cord compression depending upon the parameters of the animal material and apparatus even when the gm-cm of impact energy is maintained at a constant value. Some approaches to standardization of this injury model are suggested.