# Practicing Active Control of Thumb Force Alters the Excitability of Anterior Horn Cells of the Spinal Cord

## TL;DR

Practicing thumb force control reduces spinal cord cell excitability, suggesting a link between active muscle control and neural adaptation.

## Contribution

This study demonstrates that active force control in the thumb alters spinal anterior horn cell excitability, as measured by F waves.

## Key findings

- The F/M amplitude ratio decreased after force control tasks but not after non-force control tasks.
- No changes in motor performance were observed after either task.
- The decrease in F/M amplitude ratio suggests reduced excitability of spinal anterior horn cells following active force control.

## Abstract

Background and objective

The H-reflex amplitude of certain muscles, such as the wrist and ankle, decreases immediately after performing a force control task that actively controls muscle output. However, this was unknown for the fingers. Therefore, the effect of force control exercises on the excitability of anterior horn cells of the spinal cord in the thumb was investigated using F waves.

Materials and methods

The participants were 15 healthy subjects. The exercise was to control the palmar abduction of the finger on the non-dominant hand side to 20% MVC (maximum voluntary contraction: MVC). In the force control task, visual information was used to actively control muscle output. In the non-force control task, participants were instructed to hold the thumb in a palmar abduction position against a constant external load, using isometric muscle contraction to maintain the posture without visual feedback or force modulation. Each task was practiced 10 times for 90 seconds, and the F wave was derived from the short abductor pollicis brevis muscle pre- and post-assignment at rest.

The F/M amplitude ratio and frequency of appearance were used as analysis items; the F/M amplitude ratio was defined as the average of the vertex-to-vertex amplitudes of the 30 F waves obtained in one stimulus divided by the maximum amplitude value of the M wave, expressed as a percentage. Persistence was defined as the number of F waves that appeared for the number of stimuli in one trial and expressed as a percentage.

Motor performance was assessed by performing controlled movements of 20% MVC of the thumb palmar abduction pre- and post-task with visual deprivation.

Results

In the force control task, the F/M amplitude ratio decreased post-task compared to pre-task. In the non-force control task, no differences were found between pre- and post-task. There were no differences in motor performance before and after practice for both tasks.

Conclusion

In the force control task, there was a decrease in the amplitude F/M ratio post-task. Furthermore, this decrease in the amplitude F/M ratio was thought to be due to active force control.