Deformity results from the natural resting postures of a swollen joint. The natural resting posture of a swollen joint is the position of minimal intra-articular pressure, where the capsule and ligaments are loosest. Thus, movement in either direction will cause an increase in intra-articular pressure and an increase in the tension of the capsule and ligaments. 15 plantar flexion, between 30 to 45 flexion, and 30 flexion respectively. For example, arthritis or other joint pathologies that cause swelling within the hip joint result in a flexed posture while walking in an attempt to keep the hip at minimal intra-articular pressure.
Muscle weakness occurs secondary to the pain of joint swelling causing reduced activity.
Experimentally it has been shown that activation of the quadriceps becomes more difficult when distension of the knee with sterile plasma increased intra-articular pressure. Once the pressure was raised high enough to prevent all muscular activity in the quadriceps, anesthetizing the joint returned full quadriceps activity. This finding indicates that there is a feedback mechanism designed to protect the joint structures from excess pressure. The final effect in the patient is disuse atrophy. During gait analysis, the patient will have less strength and more protective posturing when the joints are swollen.