Elbow flexor inhibition as a function of muscle length

Muscle inhibition (MI) in human knee extensors increases with increasing maximal voluntary force as a function of knee angle. It was speculated that this angle-dependent MI was modulated by force-dependent feedback, likely Golgi tendon organ pathways. Such angle-dependent MI is of clinical and theoretical importance. The purpose of this study was to determine MI in human elbow flexors for maximal voluntary contractions. Muscle inhibition, elbow flexor force, and electromyographic (EMG) activity were measured in 31 volunteers at elbow angles between 30degrees and 120degrees. MI and elbow flexor EMG were the same at all elbow angles. Maximal isometric forces were greatest at the 70 angle, and never fell below 70% of the peak force at any of the tested angles. From these results it is concluded that force-dependent modulation of MI did not occur in the elbow flexors, possibly because maximal isometric force remained relatively close (within 30%) to the peak force. In contrast, force-dependent modulation of MI occurred in the knee extensors at the most extended angles, when the average knee extensor force had dropped to 50% or less of the maximal knee extensor force. It is likely that human maximal voluntary contractions are not associated with a given activation. Rather, activation appears to be modulated by force-dependent feedback at force levels below 70% of the absolute peak force, which manifests itself in a change of MI that parallels the level of maximal isometric force in voluntary contractions.