Effects of 17 days bedrest on the maximal voluntary isometric torque and neuromuscular activation of the plantar and dorsal flexors of the ankle

Maximal voluntary isometric torque values of the ankle plantar (Tim,PF) and dorsal flexors (Tim,DF) were assessed in eight healthy adult males at 5 degrees and 15 degrees of dorsal flexion (DF) and at 5 degrees, 15 degrees and 25 degrees of plantar flexion (PF) with the knee at right angles, before (two times), during (three times) and after (three times) 17 days of 6 degrees head-down tilt bedrest (BR). Integrated electromyograms (iEMG) were also recorded from the gastrocnemius medialis and tibialis anterior. Tim,PF and the iEMG of the gastrocnemius medialis were significantly larger (by 14% and by 27%, respectively) at the end of recovery than before BR. This was probably the consequence of training and/or habituation leading to: (1) increased activation of the plantar flexors; and (2) decreased co-activation of the antagonist muscles. Neither Tim,DF nor the tibialis anterior iEMG changed significantly. The effects of BR on muscle performance were evaluated as follows. The net torque generated by a given muscle group was assumed to be the algebraic sum of the torque generated by the agonists and by the antagonists. Thus, for the plantar flexors Tim,PF = alpha iEMGGm - beta iEMGTa, where: (1) iEMGGm and iEMGTa are the iEMGs of gastrocnemius medialis and of tibialis anterior during maximal PF; and (2) the constants alpha and beta represent the electromechanical coupling of the plantar (alpha) and dorsal (beta) flexors. Similarly for the dorsal flexors: Tim,DF = beta iEMGTa - alpha iEMGGm, where iEMGTa and iEMGGm are the iEMGs of tibialis anterior and gastrocnemius medialis during maximal DF. Torque and iEMG values were assessed for all subjects under all experimental conditions. Thus, since the biomechanical leverage of the system was constant, alpha and beta could be calculated. During BR, alpha decreased by 25% and it dropped by a further 30% during recovery. In contrast, beta remained almost unchanged. This suggests that, in spite of training and/or habituation, BR significantly impaired the maximal isometric performance of the plantar flexors, an effect that continued during the initial 10 days of recovery.