Differences in Hebbian stimulation effects between biceps and triceps brachii in humans.

Animal and human studies indicate that monosynaptic corticospinal connections are more prevalent in biceps than triceps brachii motoneurons. Based on this evidence, we hypothesized that Hebbian stimulation, which targets corticospinal-motoneuronal connections, would enhance corticospinal excitability more in the biceps than the triceps brachii. To test this hypothesis, we assessed motor-evoked potential (MEP) size using transcranial magnetic stimulation (TMS) at resting motor threshold (MEP-RMT) and maximum stimulator output (MEP-100%) immediately and up to 30 min poststimulation. During Hebbian stimulation, 180 paired pulses were delivered, with corticospinal volleys evoked by TMS arriving at corticospinal-motoneuronal synapses 1-2 ms before antidromic potentials from brachial plexus electrical stimulation. Central and peripheral conduction times were similar between muscles. We found that both MEP-RMT and MEP-100% increase in the biceps and triceps immediately and up to 30 min poststimulation. The increase in MEP-RMT was greater in the biceps compared with the triceps, whereas MEP-100% changes did not differ between muscles. Since the maximum MEP size was larger in the biceps than in the triceps, we conducted a control experiment testing responses at an intermediate size between MEP-RMT and MEP-100% (MEP-Control), ensuring similar baseline sizes between muscles. Notably, Hebbian stimulation continued to produce a greater increase in MEP-Control in the biceps than in the triceps. These findings suggest that Hebbian plasticity enhances corticospinal excitability more in the elbow flexor than extensor muscles, emphasizing the need to consider muscle-specific innervation patterns when future studies assess the therapeutic effect of this technique in individuals with motor impairment.NEW & NOTEWORTHY Differences in corticospinal projections to elbow flexor and extensor motoneurons are well established. Our findings demonstrate that Hebbian stimulation, based on spike-timing-dependent plasticity, enhances corticospinal excitability more in the biceps brachii than in the triceps brachii. These results highlight the importance of incorporating muscle-specific innervation patterns when designing plasticity-driven therapeutic strategies.