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

ACL Injuries: How Fatigue Impacts Knee Muscle Activity

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Mira Elwood in Health & Wellness October 2026 • 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.

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The Growing ACL Burden

Traumatic knee injuries have been described as a global “sporting epidemic,” and ACL injuries and meniscal tears are among the most common knee injuries. The cited Scandinavian figures are 80 ACL injuries and 79 meniscal tears per 100,000 people. Gitnux reports about 250,000 ACL injuries annually in the United States, with higher risk among athletes and reported rates in female athletes ranging from two to nine times those of males in sports such as basketball and soccer.

Rehabilitation as a Multifaceted Process

Postoperative rehabilitation after ACL reconstruction uses a multifaceted approach intended to promote tissue healing, restore knee function, reduce pain, and support return to pre-injury activity, including sports. Reviews have compared conservative or traditional rehabilitation with accelerated rehabilitation, highlighting benefits and limitations for both approaches. Evidence-informed rehabilitation is presented as a way to improve outcomes, reduce reinjury risk, and support a safe return to physical activity, although commonly held clinical beliefs continue to be challenged by newer research.

An Evolving Surgical History

The history of ACL surgery has undergone continual evolution, marked by successive advances and enduring clinical ideas. A 2025 article in International Orthopaedics examines the history of knee arthroscopy and ACL surgery. Modern recovery resources present rehabilitation as a staged process from surgery to return to sport, including comparisons of UPMC, SANTI, and Oslo protocols.

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.
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Fatigue and Neuromuscular Change

Recent research describes lower-extremity fatigue as producing neuromuscular alterations that may increase knee injury or reinjury risk. In people with a history of knee injury, injury-related muscle weakness may lead them to reduce their knee flexion angle because greater flexion generates higher moments that muscles must counteract. These findings connect fatigue, muscle capacity, and landing or jumping biomechanics.

Why Reconstruction Can Fail

ACL reconstruction failure may be defined as abnormal knee function caused by graft insufficiency, abnormal laxity, or failure to recreate a functional knee consistent with the expected outcome. Traumatic ruptures have been reported as the most common reason for failure. Other reviews emphasize that failure is multifactorial and may involve associated knee injuries or unfavorable anatomical conditions, including tibial slope, so revision treatment requires a broad range of surgical options.

Rehabilitation First or Surgery

Management of non-acute ACL injury can involve non-surgical rehabilitation or surgical reconstruction. The cited Lancet study reports that surgical reconstruction was clinically superior and more cost-effective than rehabilitation management for patients with persistent symptoms of instability. Contemporary pathways also describe early ACL reconstruction followed by rehabilitation, or rehabilitation first with delayed reconstruction for patients who develop symptomatic instability.

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.

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From Fatigue to Function

A case-control study examined fatigue-related knee muscle electromyographic activity during a landing task in 12 healthy participants, 12 ACL-reconstructed participants, and 12 ACL-deficient participants. The participants were recreationally active male volunteers. This design directly compares muscle activity patterns across healthy, ACL-reconstructed, and ACL-deficient knees under a physically demanding task.

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.

How far the landing-task comparison can take the claim

Observational study (level 4 of 8)

The comparison involved recreationally active male volunteers in healthy, ACL-reconstructed, and ACL-deficient groups, so the observed patterns describe those participants rather than every person with an ACL injury.[1]

What the muscle-activity contrasts suggest

The post-fatigue pattern was not a single response shared across the ACL groups: reconstructed knees showed earlier activation in several muscles relative to controls, while deficient knees showed greater vastus medialis activity relative to both comparison groups.[1]

Together, these findings point to different muscle-activity adjustments across the groups during the landing task. They describe measured activation patterns, not proof that one pattern improves stability or prevents injury.[1]

Fatigue and knee-muscle activity during landing

Study designObservational study[1]
PopulationRecreationally active male volunteers: healthy, ACL reconstructed (ACLR), and ACL deficient (ACLD).[1]
Main outcomeKnee-muscle activity patterns assessed by surface electromyography during single-leg drop landing before and after fatigue[1]
Times cited11[1]

How later work frames fatigue and landing activity

That review also concluded that fatigue did not alter biomechanics and muscle-activity patterns differently in ACL-reconstructed participants than in controls, which contrasts with a simple claim that reconstruction consistently changes the fatigue response.[2]

A subsequent study reported that ACL-reconstructed athletes who had returned to sport could still show task-specific muscle-activation differences, suggesting the task used can affect whether alterations are detected.[3]

What remains unresolved about fatigue-related activation

  • Would the group differences observed during single-leg landing also appear during other movement tasks or under unpredictable landing conditions?[1] [2] [4]
  • Do these fatigue-related activity patterns persist beyond the immediate post-fatigue assessment or correspond to differences in knee function?[1]

Terms used to describe the measurements

Surface electromyography
A method used to assess muscle activity.[1]

Sources

  1. Effect of Fatigue on Electromyographic Activity Patterns of the Knee Joint Muscles in Anterior Cruciate Ligament Reconstructed and Deficient Patients during Landing Task (MDPI AG, 2018)
  2. Effect of fatigue on neuromuscular and biomechanical variables after anterior cruciate ligament reconstruction: a systematic review (The Journal of Sports Medicine and Physical Fitness, 2025)
  3. Persistent alterations in muscle activation patterns during landing tasks after anterior cruciate ligament reconstruction in return‐to‐sport cohorts: A systematic review and meta‐analysis of electromyography studies (Knee Surgery Sports Traumatology Arthroscopy, 2026)
  4. Abnormality in re-programing of preparatory muscle activity for landing following unpredictable events in patients with anterior cruciate ligament injury (The Knee, 2024)

About this Article -

Written with AI assistance from published research, and reviewed by the Mystum team. 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

1

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.

2

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.

3

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.

4

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.

5

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.

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