How Common is Muscle Inhibition after ACL Injury?

AMI is very common.

Three recent manuscript quotes on the topic:

Marked weakness and atrophy of the quadriceps muscle is often observed after knee injury, knee surgery or in patients with knee joint arthritis. - Rice et al., Arthritis Res Ther, 2014

Quadriceps weakness is a frequently observed barrier to effective rehabilitation following ACL injury and reconstruction. - Sonnery-Cottet, et al., BJSM, 2019

Arthrogenic muscle inhibition (AMI) is a common impairment in individuals who sustain an anterior cruciate ligament (ACL) injury. - Pietrosimone et al., JSR, 2022

Who gets AMI?

Everyone with joint injury experiences AMI. It is a reflex. Although our reflexes are tuned to different intensities (i.e., hyporeflexive or hyper-reflexive), everyone will suffer muscle inhibition to some degree in the presence of joint swelling and pain.

How long does it last?

We think from the moment of injury, through surgery, to approximately 3 months post-op - but this will vary. Remember this: as long as the sensory signal from the joint is disrupted, spinal reflexive inhibition is present.

Achieving a “metabolically quiet knee” is critically important.

Any swelling, pain, or active inflammation in the joint perpetuates muscle inhibition. The only way to relieve inhibition is achieving a healthy, non-swollen, joint.

But, this takes too long (approximately 3 months post-op, but sometimes longer). Until then, we must bridge function with disinhibitory treatments. When we don't, the muscle atrophies, fiber types change, and motor units lie dormant. This cascades into poor long-term strength and quadriceps muscle size, which lead to ACL re-injury and development of osteoarthritis.

We must bridge function with disinhibitory treatments.

Even if you haven't seen AMI, it has seen you.

Your patients with joint injury exhibit signs and symptoms of AMI, guaranteed. With how common this phenomenon is, its amazing to me that clinician's aren't taught how to treat it. It's time to change that.

Figure 1: The vicious cycle of muscle inhibition.

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