What causes arthrogenic muscle inhibition (AMI)?

• A. Pain and joint effusion after surgery or injury
• B. Pain alone
• C. Joint effusion only
• D. Muscle fatigue from overuse

Answer: (A)

Arthrogenic muscle inhibition is a protective reflex that reduces activation of the muscles around a damaged joint. After surgery or injury, the joint is often painful and swollen with effusion. The painful input and the increased intra-articular pressure from the effusion change the signals coming from the joint’s receptors, which alters spinal and supraspinal control of the surrounding muscles. This shift lowers the level of voluntary drive to the muscles (especially the quadriceps) and dampens their ability to produce force. The combination of pain and joint effusion is the strongest trigger because both nociceptive and mechanoreceptive signals contribute to the reflex inhibition. Pain alone can cause some inhibition, and effusion alone can provoke it, but together they produce the characteristic AMI. Muscle fatigue from overuse does not produce this reflex-mediated inhibition pattern.