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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

Review written by Tom Goom info

Key Points

  1. This systematic review examined patellofemoral joint reaction force (PFJRF) during various activities and exercises as well as following physical interventions.
  2. It included both healthy individuals and those with patellofemoral pain or OA but found no significant difference in PFJRF between these populations.
  3. Activities with greater knee flexion resulted in higher PFJRF, although results did vary between studies.


Patellofemoral joint reaction force (PFJRF) is created by the tension in the quadriceps and patella tendon which drives the patella against the trochlear surface of the femur. It can increase due to greater quadriceps muscle force or increased knee flexion. In theory an increase in joint forces may influence symptoms in patellofemoral pain (PFP) so it is helpful for us to understand which activities may influence PFJRF.

This systematic review sought to examine PFJRF in daily activities, exercises and interventions, and to compare healthy individuals with those with PFP or osteoarthritis (OA).

Patellofemoral joint reaction force can increase due to greater quadriceps muscle force or increased knee flexion.
We could aim to decrease patellofemoral joint reaction forces in running by increasing the step rate to reduce patellofemoral load.


  • A comprehensive search strategy was used to identify relevant studies in key electronic databases. This was conducted in January 2019 and updated in November 2020. The reference list of these papers was then used to identity further research.

  • Selection criteria were followed, and the review included cross-sectional studies reporting PFJRF during activities and/or therapeutic exercises, and interventional studies investigating the effects of various interventions (such as orthoses, bracing or taping) on PFJRF. The studies were required to include the target population - healthy individuals or those with PFP or OA.

  • Where feasible, data from the studies was pooled for meta-analysis. Study quality and risk of bias were also assessed.


  • A total of 71 articles passed the selection criteria and were included in this systematic review. The results in terms of approximate PFJRF for healthy individuals during various activities are summarized in Figure 1 (where BW = body weight).


  • Generally, those activities involving greater knee flexion or greater external load resulted in higher PFJRF.

  • The authors reported, “There were no discernible differences in peak PFJRF during everyday activities between healthy individuals and those with PFP/OA”.

  • In terms of the effects of interventions, few conclusions were made as it was not possible to pool results.


  • In some areas there was limited research and a meta-analysis could not be completed (such as comparison between healthy individuals and those with PFP/ OA).

  • All studies had a high risk of bias and sample sizes were generally small (and often sample size calculations were not reported).

  • Heterogenous approaches were used which may explain the considerable variety in PFJRF seen in Figure 1. For example, some studies used external loads or greater range of movement which will influence the results, hence it is important to see these as approximate values.


The review suggests that PFJRF is greater with activities that involve greater knee flexion. This may be further increased by greater external load e.g. a deep squat with heavy weight.

Increasing knee flexion is also thought to increase the contact area and therefore may reduce patellofemoral contact pressure. The authors of this paper comment that this would be the case in a ‘normally aligned’ knee, but it is thought certain movements during loading (such as increased hip adduction and/or internal rotation) may reduce contact area and increase contact pressure (1).

In theory, in a clinical setting if we wish to reduce PFJRF we have three options:

  1. Reduce knee flexion range during loading
  2. Reduce external load
  3. Reduce hip adduction/ internal rotation during loading

We could apply this to a squat – by not getting the patient to go as deep into flexion (e.g. up to 70 to 80 degrees), start with light weights and move with minimal hip adduction. We could apply this to running by increasing the step rate to reduce patellofemoral load (2) and pain (3). It may achieve this by reducing knee flexion and hip adduction during the stance phase, coupled with a reduction in peak quadriceps force (2).

Our aim isn’t always to reduce load! It might be appropriate when symptoms are present and irritable, but we often seek to gradually return to loading when able. Understanding the approximate load during various activities as reported in this review may help us to plan a progressive approach. Ultimately it will be patient symptoms that guide us, alongside the patient’s goals.

The exact link between PFJRF and pain is complex and the lack of difference between healthy individuals and those with PFP / OA highlights this. The authors also mention research which suggests underloading of the cartilage may be an issue. This review provides interesting information to guide our reasoning but, as ever, it will need to be adapted to meet individual needs.


Hart H, Patterson B, Crossley K, Culvenor A, Khan M, King M, Sritharan P (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. Br J Sports Med, 56(9), 521-530.


  1. Powers CM. The influence of abnormal hip mechanics on knee injury: a biomechanical perspective. J Orthop Sports Phys Ther. 2010 Feb;40(2):42-51.
  2. Lenhart RL, Thelen DG, Wille CM, Chumanov ES, Heiderscheit BC. Increasing running step rate reduces patellofemoral joint forces. Med Sci Sports Exerc. 2014 Mar;46(3):557-64.
  3. Bramah C, Preece SJ, Gill N, Herrington L. A 10% Increase in Step Rate Improves Running Kinematics and Clinical Outcomes in Runners With Patellofemoral Pain at 4 Weeks and 3 Months. Am J Sports Med. 2019 Dec;47(14):3406-3413.
May the force be… By Tom Goom