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 ( R 2 = 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.