Effects of neuromuscular electrical stimulation on Achilles tendon load

Tendon loading assessed by shear wave tensiometry provides insight into tendon health and performance. Comparing loads during voluntary contractions and neuromuscular electrical stimulation (NMES) may inform rehabilitation strategies. This study compared Achilles tendon loading during voluntary plantarflexion (PF) and NMES-evoked contractions of the triceps surae in healthy individuals. Twenty participants (30.7 ± 7.8 years; 10 male and 10 female) performed ankle PF under four conditions using a dynamometer: maximal voluntary isometric contraction (MVIC), maximal tolerated NMES-evoked contraction (ESTIM), voluntary contraction at 20% MVIC torque (VOL20), and NMES-evoked contraction at 20% MVIC torque (ESTIM20). Achilles tendon loading was assessed using shear wave tensiometry. The MVIC generated the highest PF torque values (means ± SD, 78.8 ± 17.2 N·m), followed by ESTIM (29.6 ± 9.7 N·m), VOL20 (18.1 ± 4.8 N·m), and ESTIM20 (16.7 ± 3.9 N·m), respectively. Normalized shear wave speeds at MVIC [(NMES-evoked shear wave speed or 20% MVIC shear wave speed × voluntary shear wave speed^-1) × 100] were not significantly different between MVIC (100%) and ESTIM (82.1 ± 28.5%) nor between VOL20 (62.6 ± 24.6%) and ESTIM20 (68.9 ± 28.6%), despite differences in PF torque. In conclusion, Achilles tendon loading did not differ between MVIC and ESTIM despite a 62.3% lower torque in ESTIM. At 20% torque output, tendon loading was similar between VOL20 and ESTIM20 conditions. These findings challenge the assumption that increases in PF torque produce proportional increases in Achilles tendon load and may reflect contribution from synergistic muscles or compensatory activation at higher force levels.NEW & NOTEWORTHY This work demonstrates that shear wave tensiometry offers a practical physiological tool to quantify Achilles tendon loading during different modes of muscle activation. We show that NMES can safely generate controlled contractions, enabling tendon load assessment even when voluntary activation is limited by pain or weakness. The ability to monitor and modulate tendon loading without requiring high voluntary effort provides new opportunities for mechanophysiological evaluation in clinical and athletic populations.