Most studies suggest that hamstring strains occur during the later part of the swing phase when the player is sprinting. In this phase the hamstring muscles are working to decelerate knee extension - that is, the muscle develops tension while lengthening. This means that the muscles change from functioning eccentrically to concentrically. We hypothesize that it is during this rapid change from eccentric to concentric function that the muscles are most vulnerable to injuries, and that the injuries can be prevented by increasing the eccentric muscle strength in the hamstring.