Evidence of inability to fully activate human limb muscle

The purpose of this study was to determine whether muscle activation level estimated by twitch interpolation technique was different when an electrica l stimulus was applied during a dynamic force (DF; force rising) task from that when the stimulus was applied during a static force (SF; constant forc e) task. Fourteen subjects performed voluntary SF and DF contractions invol ving isometric elbow flexion at seven voluntary force levels. At each level , the electrical stimulation was applied to the surface of the biceps brach ii muscle when the force was steady (SF task) and when the force was rising (DF task). The voluntary activation level of the biceps brachii muscle dur ing the SF maximal voluntary contraction (MVC) was 98.5% and that during th e DF MVC task was significantly lower (94.5%; P < 0.05). The motoneurons an d/or muscle fibers may become more excitable during the DF task so that the same stimulus can recruit those that are otherwise less excitable during t he SF task. (C) 2000 John Wiley & Sons, Inc. Muscle Nerve 23: 376-384, 2000 .