Individuals with ACLR exhibited lower amplitudes in lower extremity muscles during the early- to mid-stance phase, higher amplitudes during the late-stance phase, and a shift toward recruiting slower frequency contents in the quadriceps and MG during a 30-minute run.
Key Findings
Results
Individuals with ACLR showed lower muscle activation amplitudes in all measured lower extremity muscles during the early- to mid-stance phase of running compared to healthy controls.
Twenty individuals with ACLR and 20 matched healthy controls participated in the study.
All between-group amplitude differences during early- to mid-stance were statistically significant (all p < 0.05).
EMG data were collected during the stance phase over a 30-minute running protocol.
Functional linear models were used to compare EMG amplitude patterns across the entire stance phase waveform.
Results
Individuals with ACLR demonstrated higher muscle activation amplitudes in VM, ST, BF, TA, and MG during the late-stance phase of running compared to healthy controls.
All between-group amplitude differences during late-stance were statistically significant (all p < 0.05).
The pattern suggests a temporal shift in muscle activation timing in the ACLR group relative to controls.
RF was not listed among the muscles showing higher late-stance amplitudes in the ACLR group.
Comparisons were made using functional linear models applied to the full EMG signal during stance phase.
Results
From the 5th to the 25th minute of running, EMG amplitude increased in all muscles but decreased in RF and BF in specific stance regions.
The time-based comparison was made between the 5th and 25th minutes of the 30-minute run.
All time-related amplitude changes were statistically significant (all p < 0.05).
Amplitude decreases were observed in RF and BF only in specific regions of the stance phase, not uniformly across stance.
This temporal analysis examined how muscle activation patterns changed with running duration within participants.
Results
Individuals with ACLR showed a shift toward slower frequency content in the quadriceps and medial gastrocnemius, as indicated by lower principal component scores for RF, VM, and MG.
Frequency analysis was conducted using principal component (PC) analysis applied to the frequency characteristics of EMG signals.
PC scores for RF, VM, and MG were significantly lower in individuals with ACLR than in healthy controls (all p < 0.05).
Lower PC scores indicate a shift toward recruiting slower frequency contents in these muscles.
This finding reflects differences not just in amplitude but in the frequency domain characteristics of muscle activation.
Results
The PC score for semitendinosus decreased from the 5th to the 25th minute of running in the overall sample.
The ST PC score was significantly lower at the 25th minute compared to the 5th minute (p = 0.02).
This suggests a shift toward slower frequency content in the semitendinosus as running duration increased.
This was a time effect observed across participants rather than a group difference specific to the ACLR cohort.
Background
Prior ACLR running studies have primarily used short running protocols and examined only discrete amplitude outcomes of EMG signals.
The authors identified this as a gap in the existing literature that their study aimed to address.
This study used a 30-minute run to examine changes over time, contrasting with shorter protocols used previously.
The study examined both amplitude and frequency characteristics across entire EMG signals rather than discrete amplitude outcomes.
Six lower extremity muscles were analyzed during the stance phase to provide a comprehensive examination.
What This Means
This research suggests that people who have undergone anterior cruciate ligament reconstruction (ACLR) — surgery to repair a torn ACL in the knee — activate their leg muscles differently during running compared to people without this history. Specifically, the ACLR group used less muscle activation in the early and middle parts of each foot strike but more activation at the end of the stance phase. Additionally, the muscles involved in knee extension (quadriceps) and the calf muscle showed signs of recruiting slower-firing muscle fibers, which may reflect compensatory movement strategies adopted after surgery.
The study also found that running for 30 minutes caused changes in muscle activation patterns over time in both groups, with most muscles increasing in activation from the 5th to the 25th minute, while the rectus femoris and biceps femoris decreased in certain parts of the stride. This type of fatigue-related change in the hamstring frequency content was also observed across the full group. Importantly, this study went beyond many previous studies by analyzing the full shape of muscle activation signals over time — rather than single summary numbers — and by using a longer running duration.
This research suggests that returning to running after ACL reconstruction may involve ongoing differences in how the leg muscles are coordinated and fatigued, even after individuals have been cleared to run. These findings could inform rehabilitation programs by highlighting the importance of assessing muscle activation timing patterns and muscle fiber recruitment quality — not just overall strength — during running recovery after ACL surgery.
Check Your Own Numbers
Upload your bloodwork. We'll cross-reference your results against this study and 4,700 others.
Lee J, Lee K, Lee Y, Kim H, Park S, Park J. (2026). Lower extremity muscle activation patterns during running in individuals with and without anterior cruciate ligament reconstruction.. Clinical biomechanics (Bristol, Avon). https://doi.org/10.1016/j.clinbiomech.2026.106878