A biomechanical analysis of normal walking assessed the mechanical work cost in joules per unit mass and distance walked. For 21 walking trials--seven subjects at slow, natural, and fast cadences--these work costs (min = .73 J/kg.m, max = 1.65 J/kg.m) were correlated with maximum knee flexion during stance (min=6 degrees, max=33 degrees). The results were contrary to the predictions of previous researchers who claimed that the energy cost would increase as the knee became more rigid during stance. This study showed a significant positive correlation between work cost and maximum knee flexion. The implications of these findings and the predicted increase of bone-on-bone forces as knee flexion increases are discussed relative to the gait training of certain patient populations.