THE RELATION BETWEEN THE DIRECTION DEPENDENCE OF ELECTROMYOGRAPHIC AMPLITUDE AND MOTOR UNIT RECRUITMENT THRESHOLDS DURING ISOMETRIC CONTRACTIONS

The activation of muscles can be studied by measuring the activity of a representative set of single motor units in a muscle or by measuring the surface electromyographic (EMG) activity of a muscle that results from the contribution of a large number of motor units. In this study we have developed a model showing how the direction dependence of the amplitude of the EMG activity during isometric contractions can be un derstood from the recruitment thresholds of single motor units when fo rce is applied in various directions within a plane. The model predict s that the direction with the largest EMG activity (called the ''prefe rred direction'') corresponds to the direction in which the largest nu mber of motor units is recruited. If one assumes homogeneous activatio n of a population of motor units, this preferred direction can be show n to be equivalent to the direction in which the recruitment threshold of the motor units is smallest. The experimental data show that, for most muscles in the human arm, the amplitude of surface EMG activity f or a constant, isometric force at the wrist was proportional to the co sine of the angle between the muscle's preferred direction and the dir ection of force. As predicted by the model, the preferred direction co incided with the direction in which the recruitment threshold of motor units was smallest. Musculus deltoideus anterior had a more complicat ed directional sensitivity for surface EMG activity that could not be well fit by a single cosine function. This effect could be explained b y the finding of two subpopulations of motor units within that muscle, each with a different recruitment behavior.