Document Type : Original Article
Authors
1
Department of Sports Biomechanics, Faculty of Educational Sciences and Psychology, University of Mohaghegh Ardabili, Ardabil, Iran
2
Department of Orthopaedic surgery, Ardebil University of Medical Sciences, Ardebil, Iran
10.30491/tm.2026.581893.1928
Abstract
Introduction: Neuromuscular fatigue can worsen biomechanical deficits after anterior cruciate ligament reconstruction (ACLR), potentially affecting neuromuscular control during running. The electromyographic median frequency (EMGMF) provides an indirect indicator of motor unit recruitment behavior and localized muscle fatigue during dynamic tasks. However, it remains unclear whether fatigue‑induced spectral responses during running differ according to graft type after ACLR. Therefore, this study investigated the effects of a standardized fatigue protocol on EMGMF of lower‑limb muscles during running in individuals with different ACLR graft types.
Method: In this cross‑sectional laboratory study, 53 physically active men (18–30 years) participated, including 38 individuals with unilateral ACLR (SB‑HT, n=12; DB‑HT, n=16; allograft, n=10) and 15 healthy controls. Participants performed overground running at a controlled speed (3.0 ± 0.2 m/s) before and immediately after a standardized treadmill fatigue protocol performed until ≥85% age‑predicted maximal heart rate or Borg ≥17. Surface EMG was recorded from eight lower‑limb muscles (TA, GC, VM, VL, RF, BF, ST, GM). Median frequency was calculated for the early stance and late stance phases of running. A two-way mixed-model repeated‑measures ANOVA examined the effects of fatigue (pre vs. post) and group (SB‑HT, DB‑HT, allograft, healthy).
Result: During early stance, fatigue significantly increased RFMF (p=0.023, ηp²=0.112). Significant group effects were observed for RF (p=0.042, ηp²=0.169), VM (p=0.003, ηp²=0.273), and GM (p=0.024, ηp²=0.190). The allograft group demonstrated higher RFMF than healthy controls (~24%) and higher VMMF than both healthy (~29%) and DB‑HT (~32%) groups. During late stance, fatigue decreased TAMF (p=0.038, ηp²=0.094) and increased GMMF (p=0.026, ηp²=0.108). We observed a significant group‑by‑fatigue interaction for VM (p=0.006, ηp²=0.247), with only the DB‑HT group showing a post‑fatigue increase in MF (p=0.008). No other interaction effects were detected.
Conclusion: Fatigue did not produce a uniform decrease in EMGMF during running. Instead, muscle‑ and phase‑specific spectral adaptations were observed. Increased RFMF during early stance and altered TA and GM responses during late stance suggest adaptive motor unit recruitment strategies to maintain running stability under fatigue. Additionally, graft‑specific differences were evident, with the allograft group showing higher quadriceps MF during early stance and the DB‑HT group demonstrating a fatigue‑related increase in VM during propulsion. These findings indicate that neuromuscular responses to fatigue during running after ACLR may depend on graft type, highlighting the importance of task‑specific neuromuscular assessment in rehabilitation and return‑to‑sport decision-making.
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