Effects of and response to mechanical loading on the knee

Key points

1. 01Each knee tissue (ligament, tendon, meniscus, articular cartilage, subchondral bone) has a distinct stress-strain profile and adapts differently to mechanical stimuli via mechanotransduction.
2. 02Cutting, pivoting, and jumping sports impose the highest injury risk to the knee; ACL injury most commonly results from non-contact mechanisms involving excessive quadriceps force, knee valgus, and tibial internal rotation at low knee flexion angles.
3. 03High chronic training loads are associated with lower injury risk, whereas rapid increases in acute training load increase risk; the acute:chronic workload ratio is a useful monitoring tool.
4. 04Immobilization after knee injury is detrimental to all tissue types, causing atrophy, reduced stiffness, and impaired healing; early controlled loading and weight-bearing promote better outcomes.
5. 05Clinicians should monitor joint reactivity (pain, effusion, reduced ROM) to guide load progression, and soreness rules help determine when to maintain versus advance the rehabilitation program.

How it was conducted

Design

Narrative review

Scope

Effects of mechanical loading on knee ligament, tendon, meniscus, articular cartilage, and subchondral bone; rehabilitation loading prescriptions for each tissue

Topics covered

Mechanotransduction, tissue-specific load responses, workload-injury model, acute:chronic workload ratio, load monitoring tools, exercise prescription progressions for ACL/PCL/collateral ligaments, patellar tendon, meniscus, cartilage, and bone

Publication

Sports Medicine, 2022; 52:201-235

Authors

Logerstedt DS, Ebert JR, MacLeod TD, Heiderscheit BC, Gabbett TJ, Eckenrode BJ

What they found

• Peak ACL strain during walking is 5% at midstance and 12% at heel strike (measured by MRI and high-speed biplanar radiography at 1.3 m/s).
• Single-leg landing during deceleration produces approximately 1300 N of ACL force at knee flexion angles between 25 and 30 degrees.
• ACL peak strain during NWB activities peaks at 396 N but may be dependent on external load to the knee joint.
• Squatting through 90 degrees of knee flexion produced ACL strain values of 3.6-4.0% occurring at 10 degrees of knee flexion.
• ACL strain to failure was found to be 15.3% in mechanical simulation; therapeutic exercises and rehabilitation activities do not approach these values.
• Trunk flexion to approximately 30 degrees decreased ACL tensile forces by up to 24% and ACL strain by 16%, while hamstring force increased by 35%.
• Maximum PCL forces during squat and leg press (12 repetition maximum, through 90 degrees of knee flexion) were between 1500 and 2000 N, occurring at 60-90 degrees of knee flexion.
• MCL peak strain to failure was 17.1%; LCL peak strain to failure was also 17.1%. MCL resting length is 100 mm, LCL resting length is 64 mm; failure forces are 799 N and 392 N respectively.
• Eccentric loading is the frequent standard for tendinopathy rehabilitation, despite up to 45% of individuals not responding to the approach.
• After 4 weeks of a loading program at 80% maximum, patellar tendon pain reduction was similar whether isometric or isotonic loads were employed.
• Tibiofemoral compressive forces range from 0.9-2.5 times body weight during walking, 2.4-3.3 times body weight during stair climbing, and up to 5-6.3 times body weight during kneeling.
• A 6% and 5% reduction in patellar cartilage volume was demonstrated after 50 knee bends, with 90 minutes of non-weight-bearing required to restore pre-activity cartilage volume.

Limitations

• As a narrative review, the evidence synthesis is not systematic, and selection bias in citing primary literature may be present.
• Much of the mechanobiological evidence is derived from animal and in vitro models, limiting direct translation to human rehabilitation.
• The review covers a very broad range of tissues and conditions, which limits the depth of evidence presented for any single tissue or intervention.
• Quantitative load thresholds for optimal tissue healing are largely unknown in vivo; recommendations are largely based on indirect or surrogate markers.

Why it matters

For patients

Patients recovering from knee injury or surgery should expect rehabilitation to involve carefully graded loading rather than prolonged rest, as controlled movement promotes tissue healing while avoiding re-injury.

For clinicians

Clinicians should match exercise selection and load parameters to the specific injured tissue's healing stage, use joint reactivity monitoring (pain, effusion, ROM) to guide progression, and apply acute:chronic workload ratio principles to reduce re-injury risk.

For readers

This review provides a practical framework for understanding how each knee tissue responds to mechanical stress and offers evidence-based guidance for structuring loading progressions throughout rehabilitation.