- The purpose of the study was to describe the electrical activity of the vastus medialis, rectus femoris, and vastus lateralis muscles in subjects with normal knees and acute, severely effused knees when contracting maximally at 30 degrees and 0 degrees of knee flexion. Two groups identified were 1) an effused group (n=8) with subjects having one effused knee and one normal knee and 2) a normal group (n = 8) with subjects having two normal knees. The results were evaluated on two occasions by an observer who has unfamiliar with the groups to which the subjects belonged. The repeated evaluation yielded a high reliability coefficient (Spearman Rho r = .91). Analysis revealed a marked decrease in electrical activity at 0 degrees knee flexion compared to 30 degrees knee flexion in all muscles monitored in the effused knees, while the electrical activity was about equal in both joint positions for the normal knees in both groups. A hypothesis was formed that suggests the decreased electrical activity at 0 degrees in the effused knees results from reflex inhibition caused by joint capsule distension and intra-articular pressures that change with joint position.