What if tiring out your right hand for nine minutes could make your left hand respond faster and grip harder? This study tested that counterintuitive possibility with behaviour, functional connectivity, and brain chemistry. Motor skill is essential in daily life, from grasping and manipulating objects to safely completing activities of daily living.

Many people lose the ability to perform these tasks through injuries, making restoration clinically important. The open question is how to improve behaviour optimally, both for restoring motor function in stroke survivors and for maximizing performance in healthy populations such as athletes.

Studies suggest that motor performance can be enhanced by increasing corticomotor excitability or decreasing corticomotor inhibition. GABAergic inhibition decreases during motor learning, and the magnitude of that GABA decrease correlates with subsequent motor performance improvement.

Altering motor cortex excitability and inhibition through non-invasive brain stimulation increases motor skill acquisition in neurologically intact adults and functional recovery after stroke. Non-invasive brain stimulation is not widely available, so determining other methods to alter corticomotor function is an important research objective.

A widely accessible approach is a unilateral exercise protocol that induces motor fatigue and may alter the cortical excitation-inhibition balance. At a neurochemical level, unilateral fatiguing exercise decreases M1 GABAA activity.

The study used resting-state functional MRI and resting-state magnetic resonance spectroscopic imaging as two independent measures. The hypothesis was that repeated unilateral handgrip contractions would improve performance in the opposite hand, producing faster response times and enhanced handgrip strength.

The proposed neural relationships involved increased interhemispheric homologous connectivity of M1 and SMA through increased glutamate or decreased GABA in these regions. In panel A, the curved black connection links the two motor regions across the hemispheres, while the colored pathways trace the proposed relationships to the right-hand force change and left-hand behavioral improvement.

In panel B, the timeline shows response-time testing on both sides of the force-matching task, with MVC assessments framing the contraction block before and after scanning. Participants completed two experimental sessions, performing a right-hand force-matching task at either five percent or fifty percent of maximum voluntary contraction in randomized order.

Response times for each hand were assessed before and after entering the MRI. Inside the scanner, functional MRI, magnetic resonance spectroscopic imaging, and three unilateral maximum voluntary contractions with each hand were acquired before and after the nine-minute task.

The study used a within-participant repeated-measures design, with condition order stratified-randomized for sex across the group. The force-matching task used an MRI-compatible hand dynamometer and in-house code. Participants performed visually cued repeated contractions at zero point five hertz, with one second of contraction and one second of rest, at either five percent or fifty percent of maximum voluntary contraction.

Maximum voluntary contraction for each hand was tested immediately before and after the submaximal handgrip task in the MRI. During the nine-minute task, participants performed two hundred seventy handgrip contractions. Participants completed a visually cued response-time task before and after MRI scanning.

Responses were made with the right or left hand, depending on the block, using a button box. Each block contained sixty-four self-paced visual cues in random order, and participants pressed the corresponding button as quickly and accurately as possible.

With each hand separately, participants performed two blocks before and two blocks after the force-matching task. For the left hand, the study reported significantly improved handgrip force and response times after nine minutes of repeated right-hand contractions at fifty percent MVC.

After nine minutes of repeated fifty percent MVC contractions with the right hand, left-hand response times and handgrip force were significantly improved. After nine minutes of repeated fifty percent MVC contractions with the right hand, left-hand response times and handgrip force were significantly improved.

Figure two summarizes behavioural changes after the right-hand force-matching task. In panel A, the fifty-percent MVC condition shows a significant decline in motor performance over the nine-minute task, with a significant difference between conditions.

Panel B shows percent changes in response time for both hands, with a significant condition-by-hand interaction and significant follow-up tests, indicating that the behavioural response depended on both contraction condition and hand. The analysis tested whether the fifty-percent condition altered interhemispheric functional connectivity between left and right M1 and between left and right SMA.

A repeated-measures analysis found a significant condition by region-of-interest by time interaction. There was also a significant main effect of region of interest, but no significant main effect of condition or time. The block identifies increased SMA-SMA connectivity after fifty-percent contractions.

The fifty-percent force-matching task produced both a decrease in force output and an increase in SMA-SMA connectivity, so the study tested whether those effects were related. A greater decline in handgrip performance, indexed by a greater decrease in force output, correlated with an increase in SMA-SMA connectivity.

This relationship was not observed for M1-M1 connectivity. In panel B, the orange SMA–SMA points spread upward in the fifty-percent MVC group, with the mean bar above zero, while the corresponding five-percent group centers closer to zero. Panel C shows the fifty-percent MVC triangles following a downward-sloping fit, whereas the five-percent MVC circles cluster near a nearly flat line.

The analysis found no significant main effects or interactions for GABA in M1 and SMA. However, decreases in left-hand response times significantly correlated with decreases in GABA in left M1, left SMA, and right SMA. The relationship indicated that a greater decrease in GABA in these regions was associated with a greater decrease in left-hand response times.

Figure four begins with an MRSI slice and representative spectra from motor-cortex voxels in both hemispheres, illustrating how GABA relative to total creatine was mapped. The four scatter plots then relate neurochemical changes in SMA and M1 to changes in left-hand response time after the right-hand force-matching task, with circles marking five percent MVC and triangles marking fifty percent MVC.

The reported fifty percent MVC correlations suggest that changes in inhibition-related metabolites were associated with behavioural improvement, while the right M1 Glx relationship was also significant. Right-hand contractions at fifty percent maximum voluntary contraction, but not five percent, produced a decline in right-hand grip force consistent with fatigue, although other interpretations are possible.

Repeated fifty-percent contractions significantly improved left-hand maximum voluntary contractions and response times, an effect not seen in the right hand or in the low-force condition for response times. The improvements were accompanied by neural changes, including a significant interhemispheric interaction between declining M1-M1 connectivity and increasing SMA-SMA connectivity.

Increases in SMA-SMA connectivity correlated with the degree of force decline, while increased right M1-right orbitofrontal cortex connectivity appeared in a voxel-wise analysis. Repetitive fifty-percent right-hand contractions led to behavioural improvements in the opposite hand accompanied by increased SMA-SMA functional connectivity.

The study also found increased functional connectivity between right M1 and the right orbitofrontal cortex. Changes in GABA in left M1 and bilateral SMA were positively related to behavioural improvement, while change in right M1 Glx had a negative relationship with improved response times in the non-contracting left hand.

These results suggest that repetitive fifty-percent unimanual contractions can enhance subsequent motor performance in the opposite hand. The approach may serve as an adjunct therapy to prime the ipsilateral sensorimotor system for rehabilitation in neurological or orthopedic conditions.

Nine minutes of repeated right-hand contractions at 50% maximum voluntary contraction improved left-hand response times and strength, alongside connectivity and neurochemical relationships. The finding suggests a simple way to prime the opposite limb, although the evidence is not a clinical treatment yet.