The leg mechanics at the end of a stride.
At the end of a stride, should you pull with the hamstring first or pull with the hip flexor first?
Contracting the hamstring to curl the leg will generate horizontal force that pulls the body forward and that also throws the foot forward. My assumption is that because the knee is bending, the horizontal force is divided between propelling the body forward, and propelling the foot forward. Both are beneficial. Curling the leg also begins to decrease its radius before the hip flexor begins rotating it forward. The shorter radius allows the hip flexor to move the leg forward faster and with less effort. By completely curling the leg and making contact with the heel and glutes, or between the hamstring and calf, the center of mass of the leg will have a shorter radius than that of a partially curled leg. The fully curled leg also allows the thigh to rotate to almost 90 degrees from vertical with minimal leverage from the tight foot and lower leg. It is important to rotate the thigh to this point because the glutes can be used most effectively if they are stretched. This position allows the thigh to have a longer range of motion through which to accelerate the leg down and back before the foot contacts the ground. The foot does not need to be forward when the glutes are activated, and it can extend with the lower leg as the glutes rotate the thigh downward.
If the hip flexor is activated before the hamstring to bring the leg forward, then the initial effort by the hip flexor will be to move the leg when it is at its full length. This will be slower and require more effort by the hip flexor. This could result in the leg rotating forward too slowly. If it is too slow, the thigh will not rotate to the position described above, and would not allow the glutes to accelerate the leg and foot before contacting the ground. With a very slow forward rotation, the leg would only reach its forward position in time for the foot to contact the ground as it is still moving forward, working against the forward movement of the body.