Acute Effects of Varying Neuromuscular Electrical Stimulation Amplitude on Quadriceps Isometric Torque and Muscle Thickness in Healthy Young Adults: A Randomized Split‐Limb Trial.

INTRODUCTION: Despite the relevance of neuromuscular electrical stimulation (NMES) dosimetry, it is unclear whether increasing the self-reported tolerated amplitude enhances acute neuromuscular responses. OBJECTIVE: To investigate the acute effects of varying NMES amplitude and type of induced contraction on quadriceps isometric torque (IT) and muscle thickness. METHODS: A randomized split-limb within-subject repeated-measures design was employed in healthy young adults. IT and quadriceps muscle thickness were the outcomes. Bilateral measurements were obtained using participants seated at 60° (± 5°) of knee flexion. Muscle thickness was assessed via ultrasound and IT using a portable dynamometer. The Aussie current was applied at two doses: the self-reported tolerated amplitude (threshold) and 20% above this value (suprathreshold). Doses were randomly applied to dominant and non-dominant limbs across three assessment conditions in separate sessions: (1) IT at maximal voluntary effort without NMES, (2) IT during NMES alone, and (3) IT during NMES combined with maximal voluntary effort. RESULTS: Forty-one volunteers of both sexes participated. Isolated NMES (threshold = 0.49 and suprathreshold = 0.66 N m kg-1) elicited significantly lower IT than other conditions, whereas maximal voluntary IT (threshold = 2.82 and suprathreshold = 2.84 N m kg-1) did not differ from maximal voluntary IT performed concurrently with NMES (threshold = 3.03 and suprathreshold = 2.91 N m kg-1). The suprathreshold condition resulted in a greater increase in RF muscle thickness. Exploratory regression analysis showed that RF thickness was the only variable significantly associated with IT (R2 = 0.54). CONCLUSIONS: Acute Aussie NMES did not increase IT compared with maximal voluntary contraction, and isolated NMES elicited lower IT. However, suprathreshold stimulation acutely increased RF thickness.