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June 2022

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

Hart H, Patterson B, Crossley K et al.

La Trobe Sports and Exercise Medicine Research Centre, La Trobe University, Bundoora, Victoria, Australia; Department of Physical Therapy, Western University, London, Ontario; 3Motion Analysis and Biofeedback Laboratory, The University of British Columbia, Vancouver, British Columbia, Canada

British Journal of Sports Medicine 2022; 56: 521-30.

The patellofemoral joint plays an important role in knee function, particularly during activities involving increased knee flexion (1). Pain arising from the patellofemoral articulation impacts more than 20% of adolescents and adults (1). The patella is attached to the quadriceps tendon proximally and the patellar tendon distally, increasing the mechanical advantage of the quadriceps muscle via increasing its moment arm (3). When the quadriceps muscle and patella tendon are tensioned, it results in the patella increasing its contact against the surface of the distal femur, creating a reaction force, termed the patellofemoral joint reaction force. This force becomes larger when there is an increase in the generated quadriceps muscle force and greater degrees of knee flexion (4). During everyday activities and exercises, this force can be modified based on the demand of the quadriceps muscle, such as doing a deep squat compared to a shallow squat (5).

The patellofemoral joint contact force is determined by the muscle force and contact area. Studies have suggested that excessive patellofemoral joint contact pressure between the patella and femur may contribute to the development and progression of patellofemoral pain and knee osteoarthritis (6, 7). However, in normal alignment, as knee flexion increases, the contact between the patella and femur increases. As a result, activities involving increased knee flexion don’t appear to translate into excessive patellofemoral joint contact pressure (4). Understanding the patellofemoral joint reaction force during movements may provide important insights into understanding the biomechanical changes associated with patellofemoral pain and/or the development of osteoarthritic changes.

The aim of this systematic review is to evaluate the patellofemoral joint reaction forces during everyday activities, therapeutic exercises and physical interventions in healthy individuals. This will provide a summary of the forces produced during a range of activities, as well as assisting clinicians to select and prescribe exercises to offload and/or progressively load the patellofemoral joint, depending on the clinical situation. This study also assessed if the magnitude of force is elevated in the presence of patellofemoral pain and/or patellofemoral osteoarthritis.

After full-text screening, 71 articles were included, with the majority assessing healthy individuals. For risk of bias, most studies scored positively on the reporting domain, all studies indicated high risk of bias for the assessment of external validity, most studies didn’t report sample size calculations, most studies had low risk of reporting bias and detecting bias, and the scores for selection bias were mixed.

Patellofemoral joint reaction force during walking, stair ambulation and running:
Eleven studies reported peak patellofemoral joint reaction forces during walking in healthy individuals and the pooled reaction force was 0.9 +/- 0.4 BW (BW = body weight). Three studies reported peak patellofemoral joint reaction forces during walking in patellofemoral pain individuals which was 0.8 +/- 0.2 BW.

Nine studies reported patellofemoral joint reaction forces during stair ambulation. During stair ascent, pooled patellofemoral joint reaction forces in healthy individuals was 3.2 +/- 0.7 BW, 2.5 +/- 0.5 BW in those with patellofemoral pain, and 1.6 +/- 0.4 BW in patients with patellofemoral osteoarthritis. Editor’s note: the lower joint reaction forces may suggest that those with pain or OA may adapt their movement strategies to offload the joint somehow? During stair descent patellofemoral joint reaction forces in healthy individuals was 2.8 +/- 0.5 BW, 2.6 +/- 0.8 BW in patellofemoral pain patients, and 1.0 +/- 0.5 BW in patellofemoral knee osteoarthritis patients.

Twenty-seven studies reported patellofemoral joint reaction forces during running. Peak patellofemoral joint reaction forces in healthy individuals was 5.2 +/- 1.2 BW and 4.1 +/- 0.9 BW in patellofemoral pain patients.

Patellofemoral joint reaction force during everyday activities, therapeutic exercises and physical interventions:
Studies reported patellofemoral joint reaction forces during squatting, lunging, cycling, jumping, hopping, and stepping with variations and physical interventions in healthy individuals and in those with patellofemoral pain. It was not possible to pool these studies due to heterogeneity, however.

This systematic review aimed to evaluate patellofemoral joint reaction forces during everyday activities, exercise and physical interventions. Healthy individuals and individuals with patellofemoral pain don’t appear to have any discernable differences in peak patellofemoral joint reaction forces during walking, running and stair ambulation. In healthy individuals, peak patellofemoral joint reaction forces were, on average, 0.90 x BW (body weight) during walking, 3.2 x BW during stair ascent, 2.8 x BW during stair descent and 5.2 x BW during running. Based on individual studies, the magnitude of peak patellofemoral joint reaction forces ranged for therapeutic exercises such as 1 to 18 x BW for squat, 3 to 6 x BW for lunges, 1 to 7 x BW for cycling, and 9 to 11 x BW during jumping. The findings confirm that activities involving larger knee flexion create a higher magnitude of patellofemoral joint reaction force. Some exercise variations expose the patellofemoral joint to higher reaction forces, such as during a squat with knees beyond the toes than a squat with knees behind the toes.

More studies are required but a few cross-sectional studies that weren’t pooled suggest individuals with patellofemoral pain compared to healthy individuals have lower patellofemoral joint reaction forces during walking and stair ambulation, and studies have found inconsistent results for running. Only one single cross-sectional study assessed patellofemoral joint reaction forces in healthy individuals compared to osteoarthritis individuals and found no difference in peak patellofemoral joint reaction forces during walking and stair ascent, but lower peak patellofemoral joint reaction forces during stair descent in osteoarthritis patients compared with healthy individuals. As a result, when comparing healthy individuals to those with patellofemoral pain or osteoarthritis, this review found that there may not be a difference in the patellofemoral joint reaction forces produced.

There is a biomechanical theory that an increase in the patellofemoral joint pressure is influenced by both reaction forces and contact area, which could contribute to degenerative changes in articular cartilage (8). In this analysis, peak patellofemoral joint reaction forces were similar to healthy individuals, and as a result, elevated patellofemoral joint contact pressure may instead be driven by a smaller patellofemoral joint contact area rather than the force. This effect could be caused by altered frontal plane alignment, movement patterns and muscle coordination that is observed in patellofemoral pain patients (9-11). It is also a possibility that altered movement patterns lead to a lower patellofemoral joint reaction force as a strategy to reduce pain or fear of movement.

Another theory proposes that changes in the location of the applied load, in addition to the magnitude of joint loading, contributes to altered joint mechanobiology (12). Altered kinematics seen with patellofemoral pain may shift the contact pressure locations within the patellofemoral joint, leading to abnormally elevated contact pressures in regions of cartilage that are not accustomed to high pressures and diminished pressures in regions that are accustomed to higher pressures. Both may contribute to the development and progression of patellofemoral osteoarthritis. Studies assessing the development and progression of knee osteoarthritis following knee trauma and ACL reconstruction suggest that osteoarthritis may occur due to “underloading”.

The information in this paper can be used by clinicians when selecting and prescribing exercises based on the magnitude of the patellofemoral joint reaction force. Exercises can be selected that result in lower forces such as those with less knee flexion (forward vs side lunge), gait retraining and physical interventions such as orthotics. Larger studies with more consistent methods and longitudinal data to understand the role of the patellofemoral joint reaction force in the development and progression of patellofemoral pain and osteoarthritis is important.

Everyday activities and exercises involving higher knee flexion expose the patellofemoral joint to greater reaction forces. This review indicated that individuals with patellofemoral pain do not appear to have elevated peak reaction force during every day activities. However, a lower force applied to a smaller joint contact area can still produce higher patellofemoral joint pressure. Also, the absence in significant differences between healthy individuals and those with patellofemoral pain or osteoarthritis may reflect the modified movement patterns or strategies employed by the latter two groups to somehow ‘offload’ a sore knee.

A literature search was conducted on the following data bases: Medline, Embase, Scopus, CINAHL, SportDiscus, Cochrane Library. Key concepts covering patellofemoral, force/load and activity were searched.

Independent reviewers assessed titles and abstracts for eligibility and screened full text articles.

There was no restriction on age, sex and method of recruitment. Cadaver and non-human studies were excluded and studies with a sample size of less than 10 and non-English studies were excluded.

Data Extraction:
Data relating to the participant characteristics, interventions, activities completed, patellofemoral joint reaction force and units of analysis were extracted. Meta-analyses were completed when possible.

Risk of Bias Assessment:
The modified Downs and Black checklist (13) was used which consisted of 27 questions across five domains: reporting, external validity, internal validity and power. All items were scored as “yes” or “no” or “unable to determine”.