Voluntary activation of the different compartments of the flexor digitorum profundus.

Flexor digitorum profundus (FDP), the sole flexor of the fingertips, is critical for tasks such as grasping. It is a compartmentalized multitendoned muscle with both neural and mechanical links between the fingers. We determined whether voluntary activation (VA), the level of neural drive to muscle, could be measured separately in its four compartments, whether VA differed between the fingers, and whether maximal voluntary contraction (MVC) force and VA changed when the non-test fingers were extended from full flexion to 90° flexion to partially "disengage" the test finger. Transcranial magnetic stimulation (TMS) of the motor cortex was used to measure VA, in a position in which only FDP generated force at the fingertip. Despite differences among the fingers in MVCs, VA for each finger was ∼92% (n = 8), with no differences between fingers. When the test finger was partially disengaged by extending the other fingers to 90° flexion, performance was more variable both within and between subjects. MVCs decreased significantly by about 25-40% for the four fingers. However, VA was not significantly changed (n = 6) and was similar for the four fingers. In both positions, there were strong linear relationships between the voluntary forces and the superimposed twitch sizes, indicating that the method to measure VA was very reliable. Our results indicate that maximal VA is similar for all four compartments of FDP when force production by the other fingers is unconstrained. When altered mechanical connections between the compartments decrease voluntary force output there is little difference in neural drive.