Equivalent bilateral early latency cutaneous reflex amplitudes during graded contractions in right handers

Abstract

Functional asymmetries exist in right handers due to asymmetries at multiple levels of the nervous system. Spinally-mediated contributions have been assessed using the Hoffmann reflex but have provided varying results. The purpose of this paper was to explore possible spinally-mediated contributions from cutaneous receptors to the asymmetry in strength seen in right handed individuals. Asymmetries were assessed by maximal voluntary contraction force during wrist flexion, extension, and handgrip. Peak muscle activation (EMGmax) was measured in the flexor (FCR) and extensor (ECR) carpi radialis and normalized to the maximal evoked motor responses (M-Max) elicited via median (MED) or radial nerve stimulation applied proximal to the elbow joint. Cutaneous reflexes were evoked by trains of stimulation (5x1ms @ 300Hz) delivered to the superficial radial (SR) or MED at the wrist during graded ipsilateral wrist extension contractions of 5, 10, 25, and 50% EMGmax. Results indicate that asymmetries exist for both strength and maximal muscle activation normalized to M-Max. Early latency (50 – 70 ms) cutaneous reflex amplitudes are larger on the dominant (right) side. However, when normalized to background EMG these differences disappear. Thus, when considering the same relative input, cutaneous afferent pathways have similar bilateral effects during ongoing muscle activity.