It has been observed that knee extensor muscles cannot be fully activated during voluntary contractions following knee injuries. This muscle inhibition has an unknown origin and appears to hinder full rehabilitation of the affected joint. We have investigated muscle inhibition during and following knee injuries in non-athletic and athletic patients and compared their results to non-athletic, unaffected volunteer subjects. There appears to be a small amount of muscle inhibition in the knee extensors of normal subjects; this inhibition increases dramatically following knee injury, and appears to go back to normal levels following surgical intervention, aggressive physiotherapy, or a sufficient amount of time. Depending on the intervention, strength deficits of the affected compared to the unaffected knee extensor muscles may persist. Aggressive physiotherapy can eliminate strength deficits following knee injury through an increased ability to recruit the knee extensors in patients more completely compared to normal subjects.