Estimating the contribution of muscles to joint torque based on motor-unit activity

Because most joints in the human arm are crossed by a number of muscles which exceeds the number of degrees of freedom for those joints, the motor system can use a variety of muscle activation patterns for the same torque in each joint. We have developed a model to estimate the contribution of individual muscles to the total torque in a joint based on intramuscular EMG recordings. EMG activity recorded with surface electrodes may be contaminated with cross-talk from other muscles. Moreover, it may not be representative for the activation of a muscle when there are several subpopulations of motor units in the muscle. We derive a relation between the recruitment threshold of a motor unit in a subpopulation for force in various directions and the relative contribution by that subpopulation to joint torque. A set of linear equations can then be constructed which relates the contribution of each subpopulation (and therefore of each muscle) to the total joint torque. If the activition of individual subpopulations is modulated differently for forces in various directions, the relative contribution of the individual subpopulations to the total joint torque can be estimated.