Low heel recovery

In sprinting, the starting action is critical. The heel keeps close to the ground level, in order to have another stride as fast as possible (i.e. recover). Some professional athletes like Usain Bolt, Asafa Powell and Christian Coleman can maintain their heels low during the start. Nonetheless, most of the professional sprinters cannot perform this action, and seem no women use this strategy.
How to do low heel recovery?
The second moving leg/ back leg on the back starting block has to push away from ground in quite an explosive power. Then, it is maintained in straight state (i.e. extend the whole leg). It is quite usual to discover that the leg heel will touch the ground. The main goal is to shorten the time for generating the stride afterward. Also, the stride length can be lengthened by performing this. This action requires one to generate an explosive power to extend the leg as much as possible, which directs the person to the 45°forward.
Advantages
As said, low heel recovery aims at generating the next stride as fast as possible. That is to increase the stride frequency. Meanwhile, the power generated and the full extension of the leg can lengthen the stride length more for that specific step. As a result, the acceleration is enhanced, which quickly translates from the static mode to maximum velocity later on.
Disadvantages
To perform a perfect low heel recovery, much practice is needed to familiarize. This action actually differs from the traditional starting action which the runner takes up the lower leg high. Therefore, the sprinter has to practise this new action more to erase the old muscle memory. If not, the sprinter will start even worse than the traditional way. Meanwhile, strong muscle power is needed to perform low heel recovery. The pushing-out action of the leg requires excellent fast-twitch muscles which contract quickly. If the sprinter has weaker muscle groups for pushing out, it wastes more time on this. The resulting stride length is not as long as expected.