Task Specific Effects of Unilateral Eccentric and Concentric Exercise on Spinal Excitability of the Contralateral Homologous Plantar Flexors.

Abstract

It is well established that resistance training increases the size and strength of the trained muscles. It is also known that unilateral muscle contractions can produce strength gains in the non-exercised contralateral homologous muscle. This effect of training a muscle and having strength gains on the analogous, opposite side muscle is called cross education. Cross education tends to be greater during eccentric contractions, when the muscle is actively lengthening, compared with concentric contractions when the muscle is actively shortening. The mechanisms behind the strength gains of cross education are less clear. It has been suggested that a change in excitability at the spinal level may mediate cross education. The purpose of this study was to compare spinal excitability in the resting right plantarflexors before, during, and after bouts of unilateral eccentric and concentric contractions of the left plantarflexors. We hypothesized that unilateral plantarflexion facilitates spinal excitability in the resting contralateral plantar flexors, and the facilitation will be task-specific according to the type of muscle contraction. This hypothesis is based on the observation that contraction of a remote muscle increases reflex excitability produced in a remote muscle, a phenomenon known as Jendrassik maneuver. Instead of a chronic training study, the present experiment used one exercise session of each contraction type but explored in detail the magnitude and time course of responses in the resting, contralateral right plantarflexor muscles. Subjects participated in two exercise treatments in one day, separated by 10 min of rest. Subjects performed eccentric and concentric contractions, at 90% of maximal voluntary concentric contraction. Each treatment consisted of 5 sets of 10 repetitions, with 120 s of rest between sets, followed by 5 contractions with 120 s of rest between each contraction. During the protocol, H-reflexes were evoked during each contraction over the exercise bouts, every 5 s for 120 s in the between-set rest periods, and every 5 s for 120 s during the follow-up after the 5th exercise bout. Against expectations, spinal excitability decreased ~20% relative to baseline during each of the 5 exercise bouts and returned, in each bout, to baseline in about ~30-35 s after each contraction. In addition, this recovery to baseline was extended in the follow-up so that spinal excitability actually became facilitated and increased ~20% relative to baseline. The data seem to suggest that the somatotopic organization of spinal excitability is more complex than previously thought and it may be inhibitory between pairs of the same muscles during contraction. The data thus suggest that spinal mechanism during exercise is probably not a primary mechanism to mediate cross education but it remains to be determined if the facilitatory after-effects are associated with cross education.