Effect of lower leg eccentric actions on the nociceptive flexion (R-III) reflex, foot tremor, and delayed onset muscle pain

Hundreds of published experiments have shown that intense eccentric exercise can cause delayed onset muscle pain but little is known about the effect of eccentric exercise on the nociceptive system that underlies pain perception. The primary purpose of this investigation was to learn whether the R-III reflex threshold, a measure of nociception that can be safely obtained from humans, is altered after completing 100 lower leg eccentric exercise actions that cause delayed onset muscle inflammation and pain. Measures of inflammation (leg volume), pain and physiological tremor, a neurological measure not directly involved in nociception but known to be increased following eccentric exercise, were included to document that the stimulus used here induced the expected muscle injury, pain and change in non-nociceptive neurology. Twenty two young adult females were block randomized to either an eccentric exercise group (n=11) or a no exercise control group (n=11). Measurements were made before and after (10 min, 24-, 48- and 72-hrs post) the conditions. Ten minutes after eccentric exercise the R-III reflex was decreased by 30% and 30 minutes after exercise tremor was increased by 113%. From 24 to 72 hours following eccentric exercise, small increases in leg volume (2.9%), pain intensity and pain affect (mean scores on a 0-100 scale were 15.5 and 14.0) were found as were small decreases in foot tremor (-0.2%) and the R-III reflex threshold (-11.1%). Repeated measures ANOVA revealed that the range of oscillation in the minimum and maximum R-III threshold 24 to 72 hours after eccentric exercise was significantly greater than the control condition (F1,20 = 9.91; p = 0.005; ŋ2 = 0.33; ε = 1.00). Eccentric exercise that induces modest changes in indicators of muscle injury increases the range of oscillation in the R-III reflex threshold 24 to 72 hours after the exercise.