Surreal illustration of knee muscles during landing after a jump, emphasizing muscle activation in ACL recovery.

ACL Injuries: How Fatigue Impacts Knee Muscle Activity

Mira Elwood in Health & Wellness December 2025 • 4 min read.

"New research reveals how fatigue affects muscle activation patterns in ACL reconstructed and deficient knees, highlighting crucial insights for rehabilitation."

Anterior cruciate ligament (ACL) ruptures can have lasting effects, preventing many from returning to their previous activity levels and increasing the risk of osteoarthritis. Even with surgery or conservative treatment, individuals often struggle to regain full function, with altered neuromuscular patterns persisting years after the initial injury.

Research has shown that those with ACL reconstructions (ACLR) or ACL deficiencies (ACLD) tend to activate their leg muscles earlier than healthy individuals. However, the impact of fatigue on these muscle activation patterns has remained unclear – until now.

This article dives into a recent study investigating how fatigue affects knee muscle activity in ACLR and ACLD patients during landing tasks. Understanding these effects is crucial for designing more effective rehabilitation programs and reducing the risk of re-injury.

Decoding Muscle Activity: What the Study Revealed

Surreal illustration of knee muscles during landing after a jump, emphasizing muscle activation in ACL recovery.

The study compared muscle activity patterns in three groups: those with ACLR, those with ACLD, and a control group of healthy individuals. Participants performed repetitive double-leg squats and drop landings to induce fatigue, while researchers measured muscle activity using electromyography (EMG).

Key findings highlighted significant differences in how the ACLR and ACLD groups responded to fatigue:

Timing Matters for ACLR: After becoming fatigued, individuals with ACLR showed earlier activation of the medial hamstring, lateral hamstring, and vastus lateralis muscles compared to the control group. This suggests they rely on preemptive muscle activation to stabilize the knee joint.
Quadriceps Power in ACLD: Those with ACLD exhibited greater activity in the vastus medialis muscle compared to both the ACLR and control groups after fatigue. This indicates an increased reliance on the quadriceps to maintain knee stability.
Gastrocnemius Decline: Fatigue led to decreased activity in the lateral gastrocnemius muscle in both the ACLD and control groups, suggesting a potential shift in stabilization strategies.

These findings suggest that ACLR patients utilize a feed-forward strategy, relying on earlier muscle activation to prepare for landing impact. In contrast, ACLD patients appear to depend more on quadriceps strength to compensate for ligament deficiency, especially as fatigue sets in.

Rehabilitation Strategies: Tailoring Recovery to Injury Type

The study underscores the importance of personalized rehabilitation approaches for ACLR and ACLD patients. For those with ACLR, rehabilitation programs should focus on enhancing pre-landing muscle activation to promote knee joint stability and prevent re-injury.

In ACLD patients, prioritizing quadriceps strengthening and addressing gastrocnemius function could be key to improving knee stability and overall outcomes. Further research is needed to fully understand the long-term effects of fatigue on muscle activity in this population.

Ultimately, by understanding how fatigue affects muscle activation patterns, clinicians can develop more targeted and effective rehabilitation programs to help individuals with ACL injuries return to their pre-injury activity levels and reduce the risk of future complications.

About this Article -

This article was crafted using a human-AI hybrid and collaborative approach. AI assisted our team with initial drafting, research insights, identifying key questions, and image generation. Our human editors guided topic selection, defined the angle, structured the content, ensured factual accuracy and relevance, refined the tone, and conducted thorough editing to deliver helpful, high-quality information.See our About page for more information.

This article is based on research published under:

DOI-LINK: 10.3390/jfmk3020022, Alternate LINK

Title: Effect Of Fatigue On Electromyographic Activity Patterns Of The Knee Joint Muscles In Anterior Cruciate Ligament Reconstructed And Deficient Patients During Landing Task

Subject: Physical Therapy, Sports Therapy and Rehabilitation

Journal: Journal of Functional Morphology and Kinesiology

Publisher: MDPI AG

Authors: Komeil Dashti Rostami, Mohammad Alizadeh, Hooman Minoonejad, Hamidreza Yazdi, Abbey Thomas

Published: 2018-04-05

Everything You Need To Know

What are the general effects of ACL injuries?

Anterior cruciate ligament (ACL) injuries, whether treated with ACL reconstruction (ACLR) or managed conservatively with ACL deficiency (ACLD), can lead to altered muscle activation patterns. These changes affect how individuals control their knee joint, often persisting even after the initial injury. The study highlights that fatigue exacerbates these issues, impacting the timing and intensity of muscle activation during activities like landing.

How does fatigue specifically impact those with ACLR and ACLD?

Fatigue significantly influences the muscle activity in individuals with ACLR and ACLD. For those with ACLR, the study found an earlier activation of the medial hamstring, lateral hamstring, and vastus lateralis muscles after fatigue. This suggests an increased reliance on preemptive muscle activation to stabilize the knee. In contrast, those with ACLD showed increased activity in the vastus medialis muscle, indicating a greater dependence on the quadriceps to compensate for the ligament deficiency as fatigue sets in.

What are the different strategies employed by individuals with ACLR and ACLD when fatigued?

The study's findings highlight different strategies employed by individuals with ACLR and ACLD when fatigued. ACLR patients tend to use a feed-forward strategy, activating muscles earlier to prepare for impact. They depend on the medial hamstring, lateral hamstring, and vastus lateralis. Conversely, ACLD patients rely more on the quadriceps, particularly the vastus medialis, to maintain knee stability. Both groups experience a decrease in lateral gastrocnemius activity, suggesting shifts in their overall stabilization strategies.

Why is it important to tailor rehabilitation approaches for ACLR and ACLD patients?

Tailoring rehabilitation is critical because individuals with ACLR and ACLD respond differently to fatigue. Rehabilitation programs for ACLR patients should emphasize exercises that improve pre-landing muscle activation to enhance knee joint stability and reduce the risk of re-injury. This is crucial because earlier muscle activation helps prepare the knee for impact, reducing strain on the reconstructed ligament. For ACLD patients, rehabilitation might focus on strengthening the quadriceps muscle to compensate for the deficient ACL and maintain joint stability, especially under fatigued conditions.

How was muscle activity measured in the study?

Muscle activity is measured using electromyography (EMG). The study used EMG to record muscle activity in the medial hamstring, lateral hamstring, vastus lateralis, vastus medialis, and lateral gastrocnemius muscles during repetitive double-leg squats and drop landings. This allowed researchers to observe how these muscles were activated and coordinated under conditions of fatigue in ACLR, ACLD, and healthy control groups. By understanding these patterns, clinicians can design more effective and targeted rehabilitation programs to improve outcomes and prevent re-injury.