Goal-directed modulation of stretch reflex gains is reduced in the non-dominant upper limb

Most individuals experience their dominant arm as being more dexterous than the non-dominant arm, but the neural mechanisms underlying this asymmetry in motor behaviour are unclear. Using a delayed reach task, it has recently been demonstrated that there is a strong goal-directed tuning of stretch reflex gains in the dominant upper limb of human participants. Here, we used an equivalent experimental paradigm to address the neural mechanisms that underlie the preparation for reaching movements with the non-dominant upper limb. We found only minor goal-directed differences in the short latency stretch reflex of the non-dominant limb. There were consistent effects of load, preparatory delay duration and target direction on the long latency stretch reflex. However, by comparing stretch reflex responses in the non-dominant arm with those previously documented in the dominant arm, we demonstrate that goal-directed tuning of short and long latency stretch reflexes is markedly weaker in the non-dominant limb. The results indicate that the motor performance asymmetries across the two upper limbs is partly due to the more sophisticated control of reflexive stiffness in the dominant limb, likely facilitated by the superior goal-directed control of muscle spindle receptors. Our findings therefore suggest that independent fusimotor control plays a role in determining performance of complex motor behaviours and support existing proposals that the dominant arm is better supplied for executing more complex tasks, such as trajectory control. Significance Statement Most of us routinely rely on the dominant arm to perform more complex and demanding motor tasks. However, the mechanisms enabling the superior motor performance of the dominant limb are unclear. A better understanding of the motor asymmetry across the two arms might provide key insight into core sensorimotor principles. Here, we show that goal-directed tuning of short and long latency stretch reflexes in the non-dominant arm is markedly weaker than in the dominant arm. Our results highlight that the more sophisticated control of reflexive stiffness in the dominant limb, likely facilitated by superior fusimotor control, partly underpins the laterality of motor performance.