May the force be with you: understanding how patellofemoral joint reaction force compares across different activities and physical interventions: a systematic review and meta-analysis

Key points

1. 01The patellofemoral joint reaction force is the compressive force pressing the kneecap against the thigh bone, and it rises with greater quadriceps force and deeper knee flexion.
2. 02In healthy adults, peak force averaged about 0.9 body weights when walking but 3.2 climbing stairs, 2.8 descending, and 5.2 running.
3. 03Forces during everyday activities were not clearly higher in people with patellofemoral pain; if anything, direct comparisons showed similar or slightly lower forces in those with pain.
4. 04Therapeutic exercises spanned a wide range, with squats producing roughly 1 to 18 body weights depending on depth, load, and modelling method.
5. 05Included studies were mostly small with high risk of bias for external validity, and considerable variability prevented pooling for exercises and interventions.

How it was conducted

Design

Systematic review with partial meta-analysis of observational and interventional biomechanics studies (PROSPERO CRD42016033552)

Search

Medline, Embase, Scopus, CINAHL, SportDiscus, Cochrane Library; 71 studies included

Participants

Healthy individuals and those with patellofemoral pain or osteoarthritis; most studies had fewer than 30 participants

Outcomes

Peak and average PFJRF (in body weights) during walking, stairs, running, exercises, and physical interventions

Analysis

Weighted average of means and SD; modified Downs and Black quality appraisal; no formal certainty (GRADE) assessment

What they found

• Healthy adults, pooled peak PFJRF: walking 0.9 plus or minus 0.4 BW (9 studies), stair ascent 3.2 plus or minus 0.7 BW (6 studies), stair descent 2.8 plus or minus 0.5 BW (4 studies), running 5.2 plus or minus 1.2 BW (17 studies).
• Patellofemoral pain, pooled peak PFJRF: walking 0.8 plus or minus 0.2 BW, stair ascent 2.5 plus or minus 0.5 BW, stair descent 2.6 plus or minus 0.8 BW, running 4.1 plus or minus 0.9 BW.
• Single studies in patellofemoral osteoarthritis or cartilage defects: walking 1.3 plus or minus 0.5 BW, stair ascent 1.6 plus or minus 0.4 BW, stair descent 1.0 plus or minus 0.5 BW.
• Walking PFJRF in healthy people averaged 0.90 BW (95% CI 0.84 to 0.96); running averaged 5.2 BW (95% CI 5.1 to 5.3).
• Therapeutic-exercise forces (unpooled) ranged from about 1 to 18 BW for squats, 3 to 6 BW for lunges, 1 to 7 BW for cycling, and 9 to 11 BW for jumping.
• Direct cross-sectional comparisons mostly showed lower or no-different peak force in people with patellofemoral pain or osteoarthritis versus healthy controls, based on only one or two studies per activity.

Limitations

• This review describes joint loading and does not test the effectiveness of any treatment or link force to clinical outcomes.
• Most everyday-activity comparisons between healthy and patellofemoral groups rest on weighted-average means, not direct statistical comparisons, and direct comparisons came from only one or two studies.
• Considerable methodological variability (87% used 2D planar models, mostly from cadaveric source data) precluded pooling for exercises and interventions and limits accuracy in living people with pathology.
• Sample sizes were small (most under 30), only English-language studies were included, and no certainty (GRADE) assessment was performed.

Why it matters

For patients

Deeper knee-bending activities like squats and stairs put much more force on your kneecap joint than walking, and carrying excess body weight raises that load.

For clinicians

Use these force values to pick and grade exercises, choosing variations that load the patellofemoral joint less at a given knee flexion angle when offloading is the goal.

For readers

People with patellofemoral pain do not appear to have higher kneecap joint forces during everyday activities, so symptoms may relate more to contact area and load location than to raw force.