Key Points
- The patella exerts force on the femur when you bend your knee or use your quadriceps muscle, which causes patellofemoral joint reaction force (PFJRF) and puts load on the patellofemoral joint.
- PFJRF is an important factor to consider when choosing appropriate activities— it allows us to select exercises that put varying loads on the joint, enabling us to craft a personalised exercise plan according to your symptoms and goals.
Anatomy of the knee and pain in the front of the knee
The knee consists of the femur (thigh bone), tibia (shin bone), and patella (kneecap), as well as cartilage and ligaments. The patella is a flat, triangular bone at the front of the knee that is attached to the quadriceps muscle. It acts as a lever to enhance the movement of the quadriceps muscle, and is subjected to a lot of force.
Anatomy of the knee
When doing exercises involving the knee, we often consider the patellofemoral joint reaction force (PFJRF). PFJRF is a force created when you bend your knee or use your quadriceps muscle, causing patella to exert force on the femur; these forces may then affect symptoms and elicit pain in the front of the knee, commonly known as runner’s knee or patellofemoral pain syndrome.
Patellofemoral joint reaction forces
Why is it important to be aware of the loads on the patellofemoral joint?
Well, by understanding the load on the joint during different activities, we are able to plan exercises that best suit your recovery stage to help you achieve your goals! For example, we may reduce load when you are experiencing more pain, but will likely gradually load the knee back up once symptoms subside. By understanding this, you will also better appreciate the exercises for your knee pain!
In February 2022, an article in the British Journal of Sports Medicine sought to analyse and compare PFJRF in different activities and physical interventions by reviewing results from 71 articles. The table below shows the approximate PFJRF in healthy individuals across various activities:
| Activity | Patellofemoral Joint Reaction Force |
| Walking | 0.90 x BW |
| Stair descent | 2.8 x BW |
| Stair ascent | 3.2 x BW |
| Running | 5.3 x BW |
| Squat | 1 to 18 x BW |
| Lunge | 3 to 6 x BW |
| Cycling | 1 to 7 x BW |
| Jumping | 9 to 11. x BW |
The review indicates that the PFJRF increases when the knee bends more, external load is applied (e.g. adding weights), or when the hip is abducted (e.g. leg moved outwards/away from body). When choosing exercises, it would be useful to consider PFJRF in modifying certain activities to increase or decrease the load on the patellofemoral joint. For example, if you feel a lot of pain, we may start off with shallower squats instead of deeper ones that require more flexion or bending of the knee. As the pain reduces, we might consider increasing the load that you’re carrying as well.
Classicially, you might feel pain in the front of your knee when coming down the stairs or pain when jogging. This would indicate an increase of the PFJRF and hence, the need to select exercises to reduce the load and thereby reduce pain in the front of your knee.
In this manner, we are able to plan and prescribe therapeutic exercises and physical interventions based on the magnitude of PFJRF, and choose the appropriate type of exercise to load or offload the joint. This allows for a more individualised approach for your specific symptoms to help you relieve the pain in the front of your knee.
What type of exercises are appropriate for your knee pain?
Your Physiotherapists at PhysioX can help to design an exercise program that is well suited to your condition after assessing and identifying potential barriers of exercise. They will ensure that the exercises are adequate for you and be certain to ensure that you are able to perform them safely! Your Physiotherapist will take the time to assess and understand the factors leading to the increase in the PFJRF and work with you to first reduce the load and to reload the joint so that you will be able to return to your activities comfortably and efficiently!
Let’s get moving! Don’t let the pain in your knee slow you down!
Book now or give us a call to schedule an appointment to see our Physiotherapist who will be able to partner you on your recovery and get you back!
Reference:
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 HF, Patterson BE, Crossley KM, et al. Br J Sports Med 2022;56:521–530.
