Asafa Powell - Extensor's Paradox - 3D kinematic visual
This is a 3D kinematic model of Asafa Powell running. The most critical element of the video is that the quad muscles are inactive (blue color) when the leg is in optimal position to attempt a push-off (red signifies the 'active' muscles). i.e. the supposedly active muscles are in fact inactive. Thus 'push off' is a visual illusion. This is the best visualization of the extensor's paradox of an actual runner that we've seen.