After knee injury, surgery and pathology, it’s common both as a patient and a clinician to see marked weakness of the quadriceps muscles at the front of the thigh, even following extended periods of rehabilitation. This is partly due to muscle wasting in some cases, but is also partly due to a phenomenon known as athrogenic muscle inhibition (AMI). AMI occurs in all cases of knee injury, regardless of the pathology. Why is important to minimize this? Because without good quadriceps strength and control, there is a decrease in lower leg control during walking and running, and an impairment in the normal stability and control of the knee during everyday activities and of course during recreational and sporting activities.
AMI seems to be worse immediately after knee injury or surgery, but it can actually persist for month, and sometime even years. What is even more interesting, is that studies have found that following knee injury or surgery, this inhibition of the quadriceps muscles occurs in the non-injured leg as well! Compared to the injured leg, there isn’t the same degree of muscle loss, but it does take just as long (sometime up to 4 years) to recede. What this means clinically, is that both patients and clinicians have to be careful when comparing strength in an injured to an un-injured leg, as both are likely to be impaired.
For a long time, it was thought that AMI was caused by pain and inflammation. It was a bit of a mystery why the quads weakness persisted long after the knee was back to feeling comfortable after an injury. It is now known that the presence of swelling in the knee, in the absence of pain and inflammation, can also cause AMI. In fact, the link between knee pain and quadriceps AMI hasn’t been clearly shown in studies. From clinical studies, it has however been found that just 10ml of fluid is enough to cause the AMI to be present. This is a very small amount indeed, and would probably not be able to be detected clinically. So we know, therefore, that the presence of knee swelling, even small amounts, can have a powerful effect on quadriceps weakness and inhibition after knee injury/surgery.
So the next important question is: what can we do therefore to minimize AMI? Again from clinical studies, it has been found that local anaesthetic or aspiration of fluid from the knee can help, but unfortunately these interventions don’t always work, nor are they practical from an everyday patient point-of-view. A really interesting piece of research has been carried out however by David Rice and colleagues at the Auckland University of Technology here in New Zealand. They found that using ice on the knee (they used bags of partially crushed ice wrapped around the knee for 20 minutes temporarily reduced the severity of AMI, thereby giving a ‘window of opportunity’ to work on strengthening. This is likely to lead to more effective and quicker strengthening of the main things to remember?
• Keep the swelling down in your knee as much as possible by following the well known principles of Rest, Ice, Compression and Elevation (RICE)
• Ice the knee for 20 mins (make sure it’s the knee you are icing, not the quads muscle itself) and then do your quadriceps strengthening exercises immediately following this
• Also work on knee stability and proprioception exercises (ask your physio about these), as they can also minimize AMI)
• In cases of severe persistent swelling, knee aspiration or anaesthetic may be required to further minimize AMI.
As always, consult your Physio or health professional for appropriate advice and exercises first.
Rice, D., McNair, P.J., Dalbeth, N. (2008) Effects of cryotherapy on arthrogenic muscle inhibition using an experimental model of knee swelling. Arthritis Care & Research, 61 (1), 78-83.
Rice, D., Mcnair, P.J. (2010). Quadriceps arthrogenic muscle inhibition: neural mechanisms and treatment perspectives. Seminars in Arhritis and Rheumatism. 40 (3), 250-266.
Rice, D. (2008) Cryotherapy reduces arthrogenic muscle inhibition following experimental knee joint infusion. Clinical Neurophysiology, 119, 90.