It's terrible to see and even worse to experience: a serious hamstring tear. The Olympic athlete has trained for years to get to the standard where he can mix it with the world's best. He's competing in the heats of the 100 meters and he's going well, into the second round. Suddenly, at 60 meters his head is thrown back, he clutches the back of his leg, he stumbles, the other competitors hear him cry out and he ends up prostrate on the track. It's all gone, all that training, all that time, all that effort and commitment. He's torn a hamstring muscle big time and it will take weeks, perhaps months to recover.
Of course, hamstring tears or strains of much lesser consequence are a problem in all walks of life from weekend warriors to professional dancers to sedentary homebodies who just extend themselves a bit too much. I can't say why this group of muscles is so relatively delicate but it probably has something to do with the evolution of primates from all fours to standing and running upright on two legs.
I'll take a look at what's known about the causes of hamstring tears and what you can do to prevent hamstring injuries with strength training and other measures.
What Causes Hamstring Injury?
Surprisingly little is known for certain about the prevention, causes and best methods of rehabilitation of hamstring injuries. Poor quality studies and absence of randomized trials, which are the most valuable, don't help either. Firm conclusions are in a minority and theoretical approaches in the absence of strong experimental studies are the norm. Here is a sample of the possible causes of hamstring injuries discussed in sports medicine:
- Less than ideal lower back and core muscle strength and mobility
- Hamstring muscle inflexibility
- Quadriceps inflexibility
- Ankle inflexibility
- Greater quadriceps versus hamstring strength
- Less than ideal running mechanics
- Age -- older equals more susceptible
- Previous hamstring, knee or pubis injury
- Inadequate warmup
It's quite a list. Being older and having a previous hamstring injury seem to be consistent with injury but not much else, even hamstring inflexibility or quadriceps/hamstring strength imbalance, factors consistently touted as causes of hamstring injury.
How hamstrings fail. In running at least, it seems hamstrings are most susceptible to injury in the 'eccentric' transition phase of movement when the muscle is lengthening and being 'pulled up' for ground contact. Consider your leading leg when you are running at a fast pace. It stretches out before impacting the ground to push off and facilitate the other leg's movement forward. At a point before the leading leg hits the ground, called 'swing phase', the hamstrings pull that leg up under control so that it does not overextend the knee joint, which would be inefficient (and dangerous). It is at this point of muscle lengthening and sudden control that the hamstrings are most vulnerable to injury. Stretching injuries are slightly different.
Stretching hamstrings. Now this may seem like heresy to anyone who has been told to stretch their hamstrings to prevent hamstring injury, but there is no consistent evidence that a stretching program prevents hamstring injury or even that 'tight' hamstrings predispose to injury. Stretching may even make things worse in some circumstances and in any case stretching probably only works to lengthen muscle if you keep doing it consistently. Stretching may have a role in hamstring rehabilitation but there is only limited evidence for this as well.
Quadriceps imbalance. If the quadricep muscles at the front of your thigh are much stronger than the hamstrings at the rear of the thigh then you may be at increased risk of hamstring injury. A hamstring less than 60 percent as strong as the quad supposedly predisposes to hamstring injury. Some studies show this muscle imbalance to be associated with hamstring injuries and others do not. It is a favorite tenet of exercise scientists based on theoretical considerations but lacks really solid evidence in my view. At the same time, getting those hammies up over that 60 percent threshold seems to make sense and it may prevent anterior cruciate ligament injury (ACL) of the knee. One way or another you need strong hamstrings for running and similar physical activity.