Fig. (1).

Flexion and extension of the leading knee joint during ascent in the sagittal plane. Each curve represents a step (in the upward direction), where the first curve represents a normal step (step1), the second a high step (step 2), the third a low step (step 3) and finally a normal step (step 4): (a) knee flexion during a double limb support before ascending as V1; (b) maximum knee flexion in the air before the foot placing on the step, which is represented by P1; (c) knee extension when the foot rests on the step but non weight bearing (just before a single limb support) as V2; (d) knee flexion with weight bearing at just initial stage of the body weight transfer as P 2.