May The (Knee) Forces be With You

Picture from Hart et al, 2022

We were discussing the knee joint in our clinic this past week. Meaning ALL the conditions that can cause pain in the knee. Patellofemoral joint pain, patella tendinopathy, MCL, ACL injuries, fat pad irritation, torn meniscus etc. 

In order to understand knee joint injuries, we need to know what can increase load or amplify forces in the knee joint. An increase in joint forces can increase symptoms when one has patellofemoral pain (PFP) so it will be most helpful to know what activities may influence patellofemoral joint reaction forces (PFJRF).

Picture from Dr JT Andrish

So happened that I came across an article explaining how PFJRF compares across different activities and interventions.

intercondylar notch

PFJRF is created by tension (or forces) in the quadriceps and patella tendon which moves the patella into the intercondylar (pictured above) surface (of the femur). It can increase due to greater quadriceps muscle force or when there is an increase in knee flexion (or knee bending).

The article (systematic review) examined PFJFR in daily activities, exercises, interventions (treatment). It also compares healthy individuals to those with PFP or osteoarthritis (OA).

A total of 71 articles were included in the systematic review. Approximate PFJRF for healthy individuals during various activities are pictured below where BW = body weight. 

Knee joint forces in healthy individuals

As you can see, activities that involve greater knee flexion or greater external load resulted in higher PFJRF. For example a deep squat with a heavy weight.

An increase in knee flexion can increase the contact area (in the intercondylar area) and may therefore reduce patellofemoral contact pressure. The authors felt this would be the case in a knee that is "normally aligned". However, certain movements during loading such as increased hip adduction (pictured below) and/ or internal rotation may reduce contact area and increase contact pressure.

Increase in hip adduction in picture A

The authors reported that there were no discernible differences in peak PFJRF during daily activities between healthy individuals and those with PFP/ OA.

There are 3 options if you have knee pain and we need to reduce PFJRF. Reduce knee flexion/ bending during loading. Reduce external load. Reduce hip adduction/ internal rotation during loading.

So if you are weight training by doing a squat and your knee hurts. You can squat less (70-80 degrees) and / or using a lighter weight. Preferably with less hip adduction too. 

In running we can apply this by increasing step rate/ cadence to reduce patellofemoral load. Smaller, quicker steps reduces knee flexion and hip adduction during the stance phase of running. Or better still make your gluteus medius, hamstrings and calf muscle a lot stronger.

Note that the goal is to reduce load only when symptoms are present and the knee is irritated. You can gradually increase load again when able. For other clinicians and physiotherapists reading this, a progressive approach is needed and utimately it will be your patient symptoms and goals that will guide you.

Do note that the exact link between PFJRF and knee pain is complex and the lack of difference between those healthy inviduals without knee pain and those with PFP/ OA draws attention to this. 

The authors also mentioned that when articular cartilage is underloaded (not enough load), it may be an issue too. 

Reference

Hart HF, Patterson BE, Crossley KM et al (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. BJSM. 56: 521-530. DOI: 10.1136/bjsports-2021-104686