The hamstrings are more vulnerable to pain and injury in heel strike running on account of greater stretch force on the hamstrings as a result of over-stride mechanics and a higher collisional force generated at heel strike. This force is transmitted to the hamstrings while the hamstrings are in an over-stretched state. In contrast, hamstring injury is less of an issue in forefoot running because a smaller over-stride angle results at touchdown which perfectly prevents the hamstrings from stretching beyond their maximum tolerable limit. Read more here! 
Is Heel Strike Running Bad for the Hamstings?
