Motor neuron excitability: the Hoffmann reflex following exercise of low and high intensity.

The current exercise literature lacks physiological evidence for a stress reduction induced by high intensity aerobic work. The purpose of this study was to investigate the effectiveness of high intensity aerobic work on stress reduction and to evaluate the currently tenable hypotheses in view of the data on the response to high intensity work. The subjects participated in a control trial and two 20-min treadmill exercise trials at low and high (40% and 75% VO2max) intensity. The H- and M-wave responses to artificial stimulation of the tibial nerve (Angel and Hofmann, 1963) were used to assess spinal cord activation level which constituted the major dependent variable in the study. Low intensity exercise showed a 12.8% reduction in the H max/M max ratio (P less than 0.05) while high intensity exercise reduced the H max/M max ratio by 21.5% (P less than 0.0002). The two exercise treatment levels were also significantly different from each other (P less than 0.02). These data provide objective evidence for the efficacy of high intensity exercise in stress reduction as measured by the Hoffmann reflex and provide indirect evidence for a possible thermal hypothesis for exercise-induced stress reduction.