To begin to understand hamstring strains you need to understand a concept first expressed by noted Physical Therapist Shirley Sahrmann. Sahrmann stated that "when a muscle is injured look for a weak or inhibited synergist". The big key to understanding hamstring strains is realizing that the hamstrings are probably not the primary problem. Most often a hamstring strain will be the result of either poor progression ( not obeying the 10% rule in conditioning or sprinting) or poor glute strength.
Priority one in hamstring rehab is to attack the glute.
Bridges, hip lifts, squats, deadlifts etc are actually the keys to hamstring rehab. It is important to make the primary hip extensor the dominant muscle. Remember, the hamstring is a secondary hip extensor. In fact, the hamstring group is actually the third most powerful hip extensor behind the glute max and adductor magnus.
If the glutes are not up to par the hamstrings will be forced to overwork and will strain. Short-sighted rehab programs will focus on the hamstrings with foolish open chain exercises like leg curls. The end result of this type of rehab will usually be another strain.
To properly rehab the hamstrings you need to understand the muscles three functions. As mentioned above, the hamstring works in concert with the glute max and adductor magnus to extend the hip. In order to address the extensor function we need to prioritize exercises like the One Leg Straight Leg Deadlift.
In addition to performing hip extension the hamstrings also act eccentrically to resist leg extension in sprinting. To visualize this function think of the muscle acting as a brake to control leg extension while sprinting. Without the proper eccentric / isometric function of the hamstrings we would run like a drum major with our legs flying into extension. I had always viewed this part of the hamstring action as the eccentric control phase. However noted Dutch author Frans Bosch has begun to postulate that this phase is in fact isometric in nature vs eccentric. I'm not sure yet if I agree or, disagree but in either case I still like slideboard leg curls
The slideboard leg curl:
and eventually the glute ham raise:
Slideboard leg curls do an excellent job of addressing the eccentric strength of the hamstring muscle without overwhelming it. Recently Nordic variations have become popular but, my feeling is that "Nordics" as we often see them done are too intense for most athletes.
It is important to emphasize that prone leg curls and slideboard leg curls have very little in common. In my mind the lying or prone leg curl is a useless non-functional exercise.
In contrast the slideboard leg curl forces the hamstring to work in concert with the glute in its role as a secondary hip extensor while also allowing the muscle to work both eccentrically to control leg extension and concentrically to create knee flexion.
The concentric knee flexor role of the slideboard leg curl might be the least useful of the three roles of the hamstring but is the one most therapists and trainers concentrate on. The slideboard leg curl can fulfills this knee flexor role but is a closed chain exercise that incorporates an element of ground reaction force, something lacking in the prone version.
The most important teaching and coaching point in the slideboard leg curl or stability ball leg curl is that any element of hip flexion is a failure to properly perform the exercise. Hip flexion means that glutes are no longer involved and, this may be the cardinal failure point. The glute must provide a constant isometric contraction while the hamstring must be forced to perform all three roles ( eccentric, concentric, hip extension) to maximize benefit from the exercise.
Last and certainly not least, look at your running/speed program. It is important to closely monitor the volume of work in both sprinting and in conditioning. Repeatedly increasing the volume of either by more than twenty percent is another potential cause of hamstring strains.
Even strong extensors can fail if exposed to poor training programs. Keep sprints short initially ( 10- 20 yds) and volume low ( 3-4 sprints). Gradually expose the extensors to greater distances while keeping the volume roughly the same. This can be done by adding flying sprint variations like a Flying 10. We do 3-4 short sprints twice a week, gradually increasing the intensity and, gradually increasing the length of of the fly-in. For us the biggest change here has been to time all these sprints. Timing insures that sprints are in the 90-100% range on a consistent basis
The same applies to conditioning. Although here volume increases over the summer, be sure to track your volume and look at percentage of increase in total yardage each week. Ten percent increases are ideal but, never more than twenty percent.