Quantitative electromyographic changes assocsiated with muscular weakness.

It has been shown that when a human muscle contracts the mean voltage of the electromyogram recorded from its surface increases with the force of the contraction (Bayer and Flechtenmacher, 1950; Inman, Ralston, Saunders, Feinstein, and Wright, 1952; Lippold, 1952). The relationship between tension and mean voltage is linear provided the contraction is isometric (Lippold, 1952). With an isotonic contraction the voltage depends also on the velocity of shortening of the muscle (Bigland and Lippold, 1954) so that the relationship only remains linear if that is held constant. If a muscle is fatigued by a prolonged isometric contraction the relationship between voltage and tension remains linear but is of a different slope. More electrical activity is associated with a given tension in the fatigued state (Edwards and Lippold, 1956). This is also true if the fatigue results from isotonic exercise (Scherrer, Samson, and Paleologue, 1954). Under standardized conditions it is possible to record similar voltage/tension curves from the same subject on different occasions (Lenman, 1959). The experiments described here were carried out to determine the range of slope for the curves relating mean voltage and tension in the upper limb muscles of healthy subjects and of patients with weak muscles resulting from neurological or muscular disease.