Your brain can "turn off" muscles before the muscles themselves become weak.
A patient may have perfectly healthy muscle tissue and intact peripheral nerves, yet still be unable to generate maximal force because the central nervous system is inhibiting muscle activation. This is known as arthrogenic muscle inhibition (AMI).
A classic example is after an ACL injury or knee effusion:
🔸️Even a small amount of swelling in the knee alters signals from joint mechanoreceptors.
🔸️The spinal cord and brain respond by reflexively inhibiting the quadriceps, particularly the vastus medialis.
🔸️The patient "tries" to contract the muscle, but the nervous system limits motor unit recruitment as a protective mechanism.
This means:
🔸️Muscle strengthening alone may not restore function.
🔸️Reducing joint effusion, controlling pain, using neuromuscular electrical stimulation (NMES), sensory stimulation, and task-specific motor retraining can be more effective in the early phase than simply prescribing more repetitions.
If a patient's quadriceps "won't fire" after knee surgery, don't immediately assume poor effort or severe atrophy. The nervous system may be protecting the joint by reducing voluntary activation.
Physiotherapists don't just rehabilitate muscles, we retrain the nervous system.
Tell me something I don’t know: Physiotherapy edition