Modulation of H Reflex in Response to Voluntary Contraction of the Homologous Muscle in the Contralateral Limb

Several studies reported that exercising one limb produces gains in motor output in the same muscle of the un-exercised, contralateral limb. This phenomenon is called cross education. There are also data to suggest that muscle and brain activation are different when muscles shorten and lengthen and that the amount of cross education may be also different according to the type of muscle contraction. This thesis is an initial effort in the form of a cross sectional study to shed light on the mechanism of cross education. This project examines the hypothesis that spinal excitability varies in the resting limb according to the type and intensity of muscle contraction in the contralateral limb. The purpose of this study was to compare spinal excitability in the right wrist flexors during and after concentric and eccentric contraction of the left wrist flexors at an intensity of 100% and 60% of the maximum. Ten healthy right-handed subjects (5 females, 5 males, mean age 21 [plus/minus] 3 years) performed left wrist flexion on a dynamometer using concentric and eccentric contractions at 20[degree]/s over a 40[degree] range of motion. Statistical analysis showed that spinal excitability decreased ~35% in the left wrist flexors during and for almost 25s after the contraction of the right wrist flexors. Against the hypothesis, there was no main effect of contraction type. During left wrist eccentric contractions at 100% of concentric MVC, right FCR H reflex was 28% less depressed than at 60% concentric MVCs (p=0.02, , F([subscript] 1,9)=8.1). During eccentric contractions at 100% and 60% of eccentric MVCs, the weaker contraction produced 24% higher depression of contralateral H reflex (p=0.101) H reflex of right wrist flexor throughout the trial was different after100% and 60% concentric MVCs (p=0.022). In summary, although longitudinal exercise studies suggested that spinal mechanisms may be involved in cross education, the present data show a long-lasting depression of spinal excitability in the contralateral limb that varies by contraction intensity but not by contraction type.  M.S