ACL Basics: Full Knee Extension

September 16, 2019

The aim of this blog series is to provide some insight into methods of post-surgical ACL rehab and recovery. Each entry will detail the nuances of why and how I approach things a certain way, which will hopefully give athletes a bit context and knowledge surrounding the structure of their rehab. 

Priority Number 1: Full Knee Extension
Generally the top priority of rehab after ACL surgery should be to regain full knee extension range. Following surgery patients will consistently exhibit a deficit in their ability to extend the knee, and failure to regain this range  is associated with significantly poorer rehab outcomes.

Why do I want to get it to extend ASAP?
When the knee is unable to fully extend there will always be an inability of the quads to activate normally throughout their full range and during the activities of everyday life. Getting knee extension range back will allow quads to start working more normally in walking and standing, allowing the muscles to more quickly regain their strength and coordination.

If knee extension remains incomplete the quads will not be able to activate normally… will not get stronger or regain normal coordination as quickly..will not be as effective as a muscle pump to drain swelling…will continue to have persistent pain and limited ROM around the knee…will continue to inhibit quads function…you get the idea! This is the cycle we want to break by restoring quads function and knee extension range.

Why can’t I extend my knee properly after surgery?
This deficit is typically the result of increased muscle guarding of the hamstrings, coupled with inhibition of the quads, rather than any specific damage or limitation to the joint structure itself. In the literature this phenomenon is attributed to “arthrogenic muscle inhibition” or AMI which refers to a number of factors such as swelling, inflammation, pain, and pre-surgical joint laxity resulting in reflexive changes in the central nervous system to limit movement around the area. To get knee extension back as fast as possible we’ll have to retrain the nervous system to allow the range of motion again, while managing the local factors that contribute to AMI such as swelling and inflammation.

How do I retrain my nervous system to allow knee extension and what does this actually look like in practice?
The principles behind developing mobility quickly and safely in post-surgical knees is relatively similar to developing mobility anywhere else. The general formula that I would use to guide training is as follows:

1. Antagonist (hamstring and calf) relaxation under load at outer range (yielding isometric + eccentric)
E.g. resisted knee flexion + passive extension, single leg jefferson curls, straight leg heel drops

2. Agonist (quads) activation/strength at inner range
E.g. band resisted terminal knee extension, maximum activation of quads over fulcrum

3 . Frequent low intensity range of motion and agonist activation
Walking on an incline treadmill.

Wearing 0 drop shoes to make sure heels aren’t elevated.
Repeated quads activation in sitting, pressing the back of your knee down into a couch or bed with leg straight.
Repeated active range of motion when sitting like straight leg lifts.
Low intensity passive stretching like propping your foot up on a chair and letting the knee drop down towards the floor or lying on your stomach with the knee over the end of your bed and letting gravity pull it down.

Sample intensity and recovery training days:
Day 1 (High intensity day)
A1. Incline treadmill walk 15-20 minutes at 10 degree incline at 4km/h, hand assist if necessary

B1. Prone knee flexion isometric with knee over bed 5x (6x20s resist with hamstring /20s contract quads and end ROM)
B2. Open chain terminal knee extension over towel 5×10+10s hold

C1. Single leg jefferson curl 3x30s isometric hold at end range
C2. Band resisted terminal knee extension 3×10 + 10s hold
C3. Double leg straight leg heel drop off of step 3x60s hold

Day 2 (Active recovery day)
A1. Incline treadmill walk 15-20 minutes at 10 degree incline at 4km/h, hand assist if necessary

B1. 6x50m reverse sled drag (low intensity, slow, use light sled initially, focus on quads contraction)

C1. Static quads contraction 5-10 minute pulses on the hour throughout the day
C2. Towel assisted passive extension stretch 1-2 minutes on the hour throughout the day
C3. Passive prone knee extension over bed 1-2 minutes on the hour throughout the day

Wrap Up
Hopefully this blog gives a bit of insight into the importance of regaining knee extension post-injury and some tools to help you do just that. Remember for your own safety, please don’t attempt to perform your rehab without the guidance of a licensed physiotherapist.

Be sure to keep your eyes peeled for the next instalment of the series, and let us know if you have any questions about taking a systematic and step by step approach to your ACL rehab!