The best evidence supporting decreased hamstring strain re-injury occurrence is the incorporation of eccentric hamstring exercises. Researchers have determined through isokinetic testing (via high-tech research laboratory equipment) that a strength imbalance ratio of - 20% between eccentric hamstring force production and concentric quadriceps force production resulted in a 4X increase in risk of hamstring injury compared to a normal strength profile ratio! THIS IS HUGE FOLKS.
Many authors suggest that the hamstrings must be strong enough to eccentrically control and offset the strength of the concentric action of the quadriceps during late terminal swing phase such as when you are performing a kick or doing hurdles. You have two big groups of muscles (hamstrings and quadriceps) on opposite sides of the knee joint. When one muscle (quadriceps) fires and begins moving the knee into extension, what’s going to stop the knee from going into hyperextension? Muscle-wise, that would be your HAMSTRINGS. If your quadriceps are too strong—or hamstrings too weak, rather—when you forcefully move into extension, your hamstrings won’t be able to control the motion and will rip, resulting in a hamstring strain.
If you’re strong enough like @porto.ido here, the Harop Curl will be cake. If you’re like the rest of the mortals like me, try the second variation using your hands to help control the descent (eccentric part) and push yourself back up (concentric part).