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

Abstract

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 human subjects, is altered after completing lower leg eccentric exercise that causes delayed onset muscle pain. It was hypothesized that the R-III reflex threshold would decrease in the injured leg of the exercise group after eccentric exercise compared to the uninjured leg of the control group. 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. Initial investigations (n = 30 to 31 adults tested) examined the day-to-day reliability of lower leg volume and active foot tremor at 90 and found both to be highly reliable (ICC 3,5 = .972 and .821, respectively). In the primary experiment, 22 young adult females were block randomized to either an eccentric exercise group (n=11) or a no exercise control group (n=11). Before and after the exercise or control condition (10 min, 24-, 48- and 72-hrs post) measurements were made of the R-III reflex threshold, physiologic foot tremor, leg volume and muscle pain. From 24 to 72 hours following eccentric exercise, small increases in leg volume, pain intensity and pain affect were found as was a small decrease in foot tremor and R-III reflex threshold. 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 in the control condition (F1,20 = 9.91; p = 0.005; n2 = 0.33; e = 1.00). It is concluded that eccentric exercise inducing 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.