This study investigates the hypothesis that EMG measured from a muscle at a given force, length, and low-shortening velocity depends on the contraction history, specifically the distance over which the muscle has shortened. Slow linear horizontal wrist movements (3 cm/s) involving shoulder and elbow rotations towards a test position of 90° elbow flexion were performed. REMG was measured at the test position after wrist displacements over 6.5 and 13 cm. Muscle contraction speed was below 1% of maximum. A constant force (25 N) causing flexion torque in the elbow was exerted by the wrist. Inertial load was minimal. Two main elbow flexors (biceps caput longum and breve) showed significantly higher (14 and 24%) concentric REMG after 13-cm wrist movement than alter 6.5-cm. Eccentric EMG did not differ between the 6.5-and 13-cm conditions. It is concluded that adaptation of muscle activation is required to counteract the effects of contraction history on the force producing capacity of the muscle.
The authors are with the Institute for Fundamental and Clinical Human Movement Sciences in the Faculty of Human Movement Sciences at Vrije Universiteit Amsterdam, van der Boechorststraat 9, 1081 BT Amsterdam.