Lombards Paradox is irrelevant here. This is a simple issue of the knee flexing (along with the hip) to allow ground clearance for the foot out of the blocks. Watch youtube, watch yourself…
The knee angles become more acute (flexion) as the feet are brought forward out of the blocks to place on the ground ahead! Look at Usains back leg angle as it comes forward, the other leg will do similar. Only at the end of the swing phase does the knee/hip extend again.
Ok both these posts effectively say the same thing but there you go.