Quadriceps inhibition is a problem that may occur when the knee is very painful, and a reflex inhibits the muscle from contracting.
Atrophy or inhibition?
It is important to differentiate between muscle inhibition, where the muscle itself is not damaged, and muscle atrophy, where the muscle cells have broken down.
Arthrogenic quadriceps inhibition and rehabilitation of patients with extensive traumatic knee injuries. Hurley MV, Jones DW and Newham DJ. Clin Sci (Lond). 1994 Mar;86(3):305-10.