Orthopedics · Medical Literature
Acute Effects of Varying Neuromuscular Electrical Stimulation Amplitude on Quadriceps Isometric Torque and Muscle Thickness in Healthy Young Adults: A Randomized Split-Limb Trial.
Abstract
Despite the relevance of neuromuscular electrical stimulation (NMES) dosimetry, it is unclear whether increasing the self-reported tolerated amplitude enhances acute neuromuscular responses. To investigate the acute effects of varying NMES amplitude and type of induced contraction on quadriceps isometric torque (IT) and muscle thickness. 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. 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). 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.
Clinical relevance
In healthy young adults, does increasing the Aussie current amplitude to 20% above the self-reported tolerance threshold (suprathreshold) produce greater acute quadriceps isometric torque and muscle thickness responses compared with stimulation at the tolerated threshold, and does the type of induced contraction (NMES alone vs. NMES superimposed on maximal voluntary effort) further modify these outcomes? __SNAPSHOT__ {"studyDesign":"Randomized split-limb within-subject repeated-measures trial","setting":"Not reported (institution type and country not specified in abstract)","sampleSize":"n=41 (within-subject design; dominant and non-dominant limbs served as paired units receiving threshold and suprathreshold doses, respectively, in randomized allocation)","population":"Healthy young adult volunteers of both sexes with no reported neuromuscular contraindications; specific age range, body mass, and sex distribution not reported in the abstract","intervention":"Aussie current NMES delivered at suprathreshold amplitude (20% above the individually determined self-reported tolerated amplitude), assessed under three conditions: (1) maximal voluntary isometric contraction (MVIC) without NMES, (2) NMES alone, and (3) NMES superimposed on MVIC","comparator":"Aussie current NMES at the self-reported tolerated amplitude (threshold), applied to the contralateral limb under the same three conditions in separate sessions","primaryOutcome":"Quadriceps isometric torque (IT) measured via portable dynamometer and quadriceps muscle thickness assessed via ultrasonography, with participants seated at 60° (±5°) of knee flexion","followUp":"Acute (single-session measurements); no longitudinal follow-up reported","fundingSource":"Not reported"}
Key findings
The abstract as provided is truncated and does not include the complete results section; therefore, quantitative outcomes, effect sizes, confidence intervals, p-values, and between-condition differences are not fully available for citation. From the partial abstract, it is noted that isometric torque effect sizes under isolated NMES conditions were reported as follows: threshold condition η²=0.49 and suprathreshold condition partial η² value was not fully disclosed in the available text. No absolute torque values, mean differences, confidence intervals, NNT/NNH, or muscle thickness numerical outcomes are reported in the available abstract text. Adverse events were not mentioned. All numerical values beyond those explicitly present in the abstract fragment are not reported and have not been calculated or imputed.
Practice implications
This study addresses an important and practically relevant question in NMES dosimetry: whether exceeding a patient's self-reported amplitude tolerance threshold by a fixed margin (20%) translates into superior acute neuromuscular responses in healthy individuals. The randomized split-limb within-subject design is methodologically appropriate for this comparison, as it controls for inter-individual variability and allows direct limb-to-limb comparisons within the same session. However, the findings are restricted to healthy young adults, and it is not established whether results apply to post-operative patients, older adults, individuals with quadriceps inhibition (e.g., following ACL reconstruction or total knee arthroplasty), or clinical populations where NMES dosimetry is most consequential. The study's acute, single-session design means that neither hypertrophic adaptation nor sustained torque gains can be inferred. The use of ultrasound to measure acute muscle thickness changes (rather than chronic hypertrophy) is a surrogate for acute swelling or architectural change, not a validated predictor of long-term muscle growth. Whether statistically significant between-dose differences, if present, would translate into clinically meaningful rehabilitation outcomes remains unestablished. Importantly, because only the abstract is available, the complete results, discussion, and conclusions cannot be reviewed; this limitation must be acknowledged when applying these findings. __LIMITATIONS__ Abstract truncation: complete results, statistical comparisons, and author conclusions are unavailable; all interpretation is limited to the partial abstract provided.,Healthy young adult sample only: findings cannot be generalized to clinical populations (e.g., post-surgical, elderly, or neurologically impaired patients) in whom NMES amplitude titration is most clinically relevant.,Single-session acute design: no data on sustained training adaptations, hypertrophy, or functional outcomes over time.,Ultrasound muscle thickness as an acute outcome: acute muscle thickness changes may reflect mechanical or hemodynamic responses rather than true structural adaptation, limiting clinical inference about hypertrophy.,Portable dynamometer: the validity and reliability of portable devices for isometric torque measurement may differ from gold-standard isokinetic or fixed dynamometers, introducing measurement uncertainty.,Blinding: participant blinding is not feasible in NMES amplitude studies; assessor blinding status is not reported.,Dominance limb allocation: while randomized, assignment of threshold vs. suprathreshold to dominant vs. non-dominant limbs introduces a potential confound if limb dominance affects baseline neuromuscular responsiveness.,Sex-disaggregated data: results for male vs. female participants are not reported in the abstract, limiting assessment of sex-based heterogeneity in NMES responses.,Funding and conflict of interest: not reported, precluding assessment of potential sponsorship bias.,Clinical significance threshold not defined: whether any observed torque or thickness differences meet a pre-specified minimal clinically important difference is not reported. __BOTTOM_LINE_CATEGORY__ promising_but_preliminary __BOTTOM_LINE__ This randomized split-limb trial in healthy young adults examines whether delivering Aussie current NMES at 20% above the self-reported tolerance threshold produces superior acute quadriceps isometric torque and muscle thickness responses compared with threshold-level stimulation. Because only a truncated abstract is available, definitive conclusions about the direction and magnitude of findings cannot be drawn. Clinicians should await access to the full publication before modifying NMES amplitude dosing protocols, particularly in clinical rehabilitation populations where these results have not been directly tested.