Reflex responsiveness of a human hand muscle when controlling isometric force and joint position

Abstract

Objective: This study compared reflex responsiveness of the first dorsal interosseus muscle during two tasks that employ different strategies to stabilize the finger while exerting the same net muscle torque.

Methods: Healthy human subjects performed two motor tasks that involved either pushing up against a rigid restraint to exert a constant isometric force equal to 20% of maximum or maintaining a constant angle at the metacarpophalangeal joint while supporting an equivalent inertial load. Each task consisted of six 40-s contractions during which electrical and mechanical stimuli were delivered.

Results: The amplitude of short and long latency reflex responses to mechanical stretch did not differ significantly between tasks. In contrast, reflexes evoked by electrical stimulation were significantly greater when supporting the inertial load.

Conclusions: Agonist motor neurons exhibited heightened reflex responsiveness to synaptic input from heteronymous afferents when controlling the position of an inertial load. Task differences in the reflex response to electrical stimulation were not reflected in the response to mechanical perturbation, indicating a difference in the efficacy of the pathways that mediate these effects.

Significance: Results from this study suggest that modulation of spinal reflex pathways may contribute to differences in the control of force and position during isometric contractions of the first dorsal interosseus muscle.

Fig 1

Experimental arrangement for the assessment of mechanically and electrically evoked reflexes in the first dorsal interosseus muscle during the force and position tasks. The thumb was restrained in extension and the index finger was placed in a splint attached to a low-friction hinge that allowed only abduction-adduction movements about the metacarpophalangeal joint. The position of the index finger was maintained at 0 degrees of abduction by a rigid metal bar during the force task and subjects produced a target force equal to 20% of MVC using visual feedback from a force transducer located at the proximal interphalangeal joint. The bar was removed during the position task and an equivalent mass was hung from the finger brace at the proximal interphalangeal joint. Subjects maintained the position of the index finger at 0 degrees of abduction using visual feedback from a potentiometer that was aligned with the metacarpophalangeal joint. Heteronymous reflexes were evoked by electrical stimuli applied to the median nerve at the wrist. A hammer attached to the shaft of a servo-controlled torque motor was used to rapidly adduct the index finger by striking the distal end of the finger splint, thereby evoking a stretch reflex.

Fig 2

Representative data illustrating task-dependent variations in the EMG response of the first dorsal interosseus muscle to mechanical perturbation of the index finger in three subjects (panels A-C). Traces represent an average of 24 trials for position (top row) and EMG (bottom row) signals recorded during the force (grey lines) and position (black lines) tasks. Dashed line indicates stimulus onset, which was verified using an accelerometer that detected the stimulus artifact (an initial upward deflection of the position signal). Short-latency (SL) and long-latency (LL) reflex responses are indicated by arrows. The third peak in the EMG record represents a voluntary response which was necessary to resume the target position after each stretch. The voluntary EMG response was greater for the position task in all subjects. The subject in panel A demonstrated no task differences in the magnitude of the SL response, whereas the LL response was greater for the force task. In contrast, the subject in panel B demonstrated a greater SL response during the position task with no corresponding difference in the magnitude of the LL response. The subject in panel C exhibited a minimal SL response during both tasks, but had a heightened LL response during the force task.

Fig 3

EMG response of the first dorsal interosseus muscle to electrical stimulation of the median nerve is shown for a representative subject (same subject as illustrated in Fig 2, panel C). Traces represent an average of 24 EMG responses for the force (grey lines) and position (black lines) tasks. Dashed line indicates stimulus onset. Short-latency (SL) and long-latency (LL) reflex responses are indicated by arrows. SL and LL responses were greater for the position task compared with the force task.

Fig 4

Peak amplitude of the short- (closed symbols) and long- (open symbols) latency reflex responses to electrical stimulation of the median nerve (A) and mechanical perturbation of the index finger (B) during the force (x-axis) and position (y-axis) tasks for each subject. Most reflex responses to electrical stimulation lie above the line of identity, indicating a consistent increase in reflex sensitivity during the position task (P = 0.02). In contrast, reflex responses to mechanical perturbation were highly variable and were not significantly different across tasks (P = 0.69).