The similar comment was given by Bosch&Klomp in their book (Running - Biomechanics…). If I remember correctly they stated that during the Vmax the rear leg if fully extended AFTER it leaves the ground and EMG activity is almost none during this priod. Trying to push far back with foot can yield ‘long axis rotation’ which demand further correction in next stride (neutralizing twist momentum of the body) which demands overstriding, thus longer ground contact and greater deceleration forces and thus lower maximal speed. This apply for vmax phase not to acceleration phase. For further detail I would suggest taking a look at this book. Great biomechanical analysis, but poor strength training exercise choices IMO.