Introduction

Patellofemoral joint reaction forces arise by the pull of the quadriceps muscle during knee flexion which drives the patella against the joint surface on the femur. These forces increase with greater quadriceps force and with increasing knee flexion angles. This way, certain activities will cause greater loads on the patellofemoral joint than others. In this light, studying how the patellofemoral joint is loaded is very informative to guide exercise prescription and rehabilitation. Therefore, this review aimed to investigate the patellofemoral joint reaction forces across different everyday activities and aimed to compare healthy subjects to individuals with patellofemoral pain. It is often thought that patellofemoral pain precedes patellofemoral osteoarthritis and therefore, this condition was compared to healthy subjects as well.

Methods

To retrieve as many articles as possible to include in this systematic review, a comprehensive search strategy was used to include cross-sectional or interventional studies on patellofemoral joint reaction forces during everyday activities or therapeutic exercises. Patellofemoral joint reaction force was defined as “the magnitude of the net reaction force between the patella and femur due to the action of the quadriceps muscle and patellar tendon on the patella at any given knee flexion angle.” The forces were expressed in body weight (BW) to allow for comparison of different studies.

Results

Seventy-one studies were included in the systematic review: 63 within-subject studies, 1 RCT and 7 cross-sectional studies. Peak patellofemoral joint reaction forces during walking were pooled from 9 studies in healthy subjects, 3 in subjects with patellofemoral pain and 1 study reported on patellofemoral joint reaction forces in subjects with patellofemoral articular cartilage defect. These pooled peak forces were 0.9 ± 0.4 BW in healthy subjects (walking speed ranged between 1.33 – 1.50 m/s) and 0.8 ± 0.2 (walking speed between 1.32-1.36 m/s) in those with patellofemoral pain. In one study in subjects with patellofemoral articular cartilage defect, the patellofemoral joint loads were 1.3 ± 0.5 BW at a walking speed of 1.55 m/s.

During stair ascent, pooled patellofemoral joint reaction forces were 3.2 ± 0.7 BW in healthy subjects and 2.5 ± 0.5 BW in subjects. With stair descent, pooled patellofemoral joint reaction forces were 2.8 ± 0.5 BW in healthy subjects and 2.6 ± 0.8 BW in subjects with patellofemoral pain.  In one study with individuals with patellofemoral OA, these forces were 1.6 ± 0.4 BW during stair ascent and 1.0 ± 0.5 BW with descending stairs.

When looking at the forces during running, pooled data revealed 5.2 ± 1.2 BW and 4.1 ± 0.9 BW in healthy subjects with patellofemoral pain, respectively. The speed of running ranged between 2.33-4.47 m/s in the first and 2.77-4.00 m/s in the latter.

Unfortunately, pooling during other everyday activities, therapeutic exercises and physical interventions was not possible. The same was true for the comparison of patellofemoral joint reaction forces between healthy individuals and those with patellofemoral pain/OA. Generally, when comparing both, it appears that patellofemoral joint reaction forces are lower in patients with patellofemoral pain.

Based on individual studies, peak patellofemoral joint reaction forces ranged from approximately 1 to 18xBW for therapeutic exercises. During squatting, peak forces range from 1 to 18xBW. Lunges had peak patellofemoral joint reaction forces ranging between 3 and 6 times BW. Cycling was reported to range between 1 and 7 times BW. Jumping caused higher patellofemoral joint reaction forces between 9 and 11 times BW. Some therapeutic exercises expose the patellofemoral joint to higher patellofemoral joint reaction forces than others. For example, higher peak patellofemoral joint reaction forces were reported during lunge with stride than lunge without stride, and during squat with knees beyond toes than squat with knees behind toes.

Questions and thoughts

Had you expected that patellofemoral joint reaction forces would be larger in subjects with patellofemoral complaints? Sometimes, clinicians prescribe strengthening exercises of the quadriceps as they want to unload the patellofemoral joint. Yet, the tension in the quadriceps muscle and patella tendon drives the patella against the patellofemoral joint, which creates a reaction force. So strengthening the quads merely increases the stress in the joint. In those with patellofemoral complaints however, you shouldn’t try to unload the joint. Rather try to increase the load tolerance of the patellofemoral joint. Increasing knee flexion angles also increases the patellofemoral joint reaction force, but as the contact area gets larger with knee flexion as the patella makes more contact with the femur, the forces are more evenly distributed. Therefore it would be interesting to ascertain that the patella tracks properly in the femoral groove. Correctable alignment issues should thus be addressed as maltracking may make certain parts of the joint not used to greater loads susceptible to injury. When these features are non-correctable, the only solution is to build up the load tolerance of the patellofemoral joint, and therefore, loads should be gradually increased throughout rehab. This review sheds a light on possible activities that can help increase the loads progressively.

Talk nerdy to me

This review excluded non-English papers and included studies which were all of high risk of bias for external validity. Another limitation of this study lies in the fact that many of the included studies used cadaveric data to calculate the joint reaction forces, like the effective moment arm of the quadriceps around the knee. Yet it was prespecified that cadaveric studies would be excluded as they might not fully reflect true kinematics, but it was necessary to use data collected from cadaveric investigation. Also, almost all studies used two dimensional models to report about 3D movements.

Unfortunately, for therapeutic exercises it was impossible to pool data, and the reported results from individual studies showed large differences in peak forces. While this was not due to shortcomings in the methodology of this study, it should be taken into account that there is a wide variation in measurement methods and exercise execution across the included studies. By normalizing the joint forces to body weight, the authors tried to make the results of different included studies more comparable. Another important aspect to note is that the pooled data did not directly compare healthy participants to subjects with patellofemoral pain, but this was the result from separate analyses which were then compared against each other.
Walking speed differed across studies and may have had a spell on the differences observed, as gait variables are sensitive to change with alterations in walking speed. However, this could have been the other way around as well, as walking speed can be determined by patellofemoral pain.

Take home messages

This review sheds light on the joint reaction forces across the patellofemoral joint, which were generally lower or similar in subjects with patellofemoral pain compared to healthy subjects. As with any joint, the patellofemoral joint needs to be loaded to remain healthy. It is thus important that kinematics, range of motion and strength are optimized to ascertain healthy joint functioning. Optimize quadriceps force and improve muscular coordination and movement patterns throughout the whole range of motion of the knee joint. This review may help to find activities to progressively adjust loads throughout the rehabilitation.

Reference

Hart HF, Patterson BE, Crossley KM, Culvenor AG, Khan MCM, King MG, Sritharan P. 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. Br J Sports Med. 2022 May;56(9):521-530. doi: 10.1136/bjsports-2021-104686. Epub 2022 Feb 3. PMID: 35115309. 

Learn more

Watch:

Listen:

https://www.physiotutors.com/podcasts/episode-037-patellofemoral-pain-with-claire-robertson/