Abstract
Disuse induces a disproportionate loss of muscle force compared with muscle mass, with unclear effects on voluntary muscle activation (VA) and peripheral contractility. Furthermore, the effect of neuromuscular electrical stimulation (NMES) as a disuse countermeasure remains uncertain. We investigated the effects of NMES during bed rest on neuromechanical function to improve our understanding of the mechanisms underlying disuse-induced reductions in muscular force. Young (n = 16, 25 years old) and old (n = 16, 71 years old) adults underwent 5 days of bed rest. One leg received NMES (3 × 30 min/day), while the other served as the control (CON). Maximal isometric knee-extensor strength (MVIC), VA and peripheral muscle contractility were assessed before and after bed rest using the interpolated twitch technique, along with biomarkers of neuromuscular junction instability (C-terminal agrin fragment (CAF)) and muscle damage (creatine kinase (CK)). MVIC decreased in both age groups, regardless of NMES (young: CON, -21.7 Nm and NMES, -23.8 Nm; old: CON, -18.5 Nm and NMES, -16.4 Nm). VA was preserved with NMES, while decreasing in CON legs (young, -8.1%; old, -5.6%) following bed rest. Peripheral contractility (resting doublet twitch force) was reduced in CON and NMES legs in both age groups (young: CON, -4.0 Nm and NMES, -11.5 Nm; old: CON, -5.9 Nm and NMES, -10.8 Nm), with a greater decrease in NMES legs. CAF remained unchanged, whereas CK levels increased in young participants, albeit remaining within the normal range. In conclusion, a decline in neuromechanical function was observed after 5 days of bed rest in young and old adults. Although NMES appeared to preserve VA, peripheral muscle contractility was altered, resulting in reduced MVIC.