Along these lines, something I should have pointed out is the role of the abdominal muscles in leg lift during the sprint stride. Charlie has mentioned this in a couple places (The Jane Project in particular comes to mind). The forward projection of the hips caused by the abdominal muscles in counter rotation to the shoulders contributes significantly to the lift of the swing leg (along with foot reaction off the ground). This forward hip projection reduces the need for the hip flexors to contribute to the leg lift. If the abdominals are weak or lack endurance the hip flexors can become overworked during sprinting, leading to fatigue and spasm.