Muscle force contributions to anterior cruciate ligament loading

Key points

1. 01Quadriceps force increases ACL load at knee flexion angles below 30-50 degrees by producing anterior tibial shear forces; this effect diminishes or reverses at flexion angles above 80 degrees
2. 02Hamstrings reduce ACL load by generating posterior tibial shear force, but only when the knee is flexed more than 20-30 degrees; near full extension the hamstrings have little mechanical advantage for this role
3. 03Gluteus medius consistently opposes knee valgus loading to a magnitude 8- to 13-fold greater than any knee-spanning muscle during sidestep cutting and single-leg landing
4. 04Gastrocnemius acts as an ACL antagonist across most knee flexion angles, inducing anterior shear forces up to 334 N during single-leg landing, comparable in magnitude to the quadriceps group in some tasks
5. 05Soleus (a non-knee-spanning muscle) contributes posterior shear force second only to the hamstrings during sidestep cutting and single-leg landing, largely through dynamic coupling to the ground reaction force

How it was conducted

Design

Narrative review using a retrospective, citation-based search strategy

Databases

PubMed and Google Scholar

Search terms

Terms related to ACL (anterior cruciate ligament, knee, tibiofemoral) and muscle or joint loading (strain, force, shear, translation, rotation, valgus, abduction, muscle force, muscle contributions, muscle induced)

Inclusion criteria

Peer-reviewed English-language articles that specifically determined the role of lower limb muscle forces on ACL force or strain, or surrogate markers of ACL loading, in humans

Methodological categories

In vitro (cadaveric), in silico (musculoskeletal and finite element modelling), and in vivo studies

Quality assessment

No formal methodological quality assessment was performed; this is a narrative not systematic review

What they found

• Quadriceps induced anterior shear forces up to 233 N during sidestep cutting and 342 N during single-leg landing in musculoskeletal modelling studies, more than any other muscle group
• During a drop-lateral jump simulation, the quadriceps contributed approximately 1070 N of anterior shear force at the time of peak ACL force
• Vastus lateralis produced 0.89 bodyweights of ACL force compared with approximately 0.17 bodyweights each for other quadriceps components in a stop-jump modelling study
• With quadriceps force simulated in a bilateral drop jump finite element model, ACL strain was 7.2% and ACL force was not stated; with hamstring force only simulated, ACL strain was 2.6%; with all muscles, ACL strain was 3.3%
• Hamstrings generated posterior shear forces up to 188 N (knee flexion 21-42 degrees) during weight acceptance in sidestep cutting, up to 469 N (knee flexion 15-70 degrees) during single-leg landing, up to an unstated value during bilateral drop landing (knee flexion 34-93 degrees), and approximately an unstated value during drop-lateral jump (knee flexion 42 degrees)
• Reduced hamstring strength after a fatiguing protocol was associated with increased estimated peak ACL forces (821 N vs 605 N pre-fatigue) during sidestep cutting (knee flexion 30 degrees)
• Gastrocnemius anterior shear force was up to 334 N during single-leg landing and approximately equal to the quadriceps group during drop-lateral jumps; during sidestep cutting the quadriceps contribution exceeded the gastrocnemius
• During walking, a higher gastrocnemius force condition (5.3 N per kg vs 2.8 N per kg) was associated with higher anterior tibial translation (3.9 mm vs 3.0 mm) and ACL force (3.4 N per kg vs 2.4 N per kg) at 50% of stance phase
• Soleus posterior shear force contributions were up to 173 N during sidestep cutting and 393 N during single-leg landing, second only to the hamstrings in both tasks
• Gluteus medius opposed knee valgus moment up to 32 Nm during sidestep cutting and 38 Nm during single-leg landing; this was 8-fold and 2.5-fold greater than any knee-spanning muscle in sidestep cutting and single-leg landing respectively
• Gluteus medius opposed knee valgus moment 6.4-fold and 2.3-fold more than medial hamstrings during sidestep cutting and single-leg landing respectively, and 13.2-fold and 2.6-fold more than medial gastrocnemius in the same tasks

Limitations

• The majority of evidence comes from cadaveric and musculoskeletal modelling studies; direct in vivo measurement of ACL loading during high-demand sporting tasks is not yet feasible
• In vitro methods typically apply static or low-magnitude muscle forces, do not account for whole-body kinematics, and can underestimate dynamic in vivo loading conditions
• Musculoskeletal modelling relies on numerous assumptions and lacks direct validation of muscle forces; different modelling practices (foot-ground contact models, muscle attachment assumptions) produce conflicting results across studies
• Simulation studies are based on motion data from healthy participants safely completing tasks; actual injury scenarios may involve movement mechanics that limit the protective capacity of muscles such as the hamstrings and gluteus medius

Why it matters

For patients

Athletes and physically active people can lower ACL injury risk by training the hamstrings, soleus, and gluteus medius, particularly ensuring the hamstrings are strong and active when the knee is near full extension where they are least protective.

For clinicians

ACL injury prevention programs should specifically target hamstring strength at low knee flexion angles, soleus activation, and gluteus medius function to oppose knee valgus; quadriceps-dominant movement patterns at low flexion angles represent the highest mechanical risk.

For readers

This review synthesises cadaveric, modelling, and in vivo evidence to show that ACL protection is a multi-muscle challenge spanning muscles that do not even cross the knee, challenging simple quadriceps-hamstrings co-contraction models of ACL stability.