It is not uncommon to read that inflexible (tight) leg muscles can put strain on your back and create other problems. Many doctors and physical therapists bring out that runners who come them with back pain usually have tight hamstrings. But what are tight hamstrings? This is never defined or explained — at least as far as I have been able to ascertain.
I have worked with many male runners who when they bend over from the hips can only go approximately 30 to 40 degrees while still maintaining the normal spinal curvature, i.e. a slight arch in the lumbar spine. This range of motion is determined by the flexibility in the hip joint, which is determined mainly by the amount allowed by the hamstrings.
Women runners on the other hand can go up to approximately a 90 degree angle in the hip joint while still maintaining an arch in the lower back. They demonstrate much greater hamstring flexibility. No one is appears is capable of explaining why there is such a discrepancy between the average male and female.
Regardless of the amount of hamstring flexibility, they do not experience injuries or back pain nor is there any interference with their running. Thus when someone says they have tight hamstrings what does it mean? How far should you be able to bend over from the hips while maintaining normal spinal curvature?
Notice I have not used the standing toe touch as an example of hamstring flexibility. The reason for this is that the standing toe touch, and even the seated toe touch, stretch more of the lower back ligaments rather than the hamstrings. Thus until we get an answer to what constitutes a tight hamstring or even a tight muscle, we should not use the term tight hamstrings so loosely. In fact, studies have shown that tighter hamstrings are better for high performing athletes!
Doctors and physical therapists who maintain that tight hamstrings limit motion in the pelvis and cause lumbar strain and back spasms, never explain how this occurs. For example, if you had tight hamstrings how would it limit motion in the pelvis when the hamstrings mainly limit bringing a straight leg forward?
In running, your knee is bent when the thigh swings through so that the knee joint flexion gives slack to the hamstring and allows you to bring it forward without any problem. Even with tight hamstrings, the pelvis will tilt forward during the push-off allowing for you to push off with the leg well behind the body. In fact, a tight hamstring will enhance this action as it holds the leg and pelvic girdle in position as the pelvis rotates forward.
We need much more explanation of terms that are used loosely and have no significant meaning. Perhaps if we more closely examined running technique and explained what some of the common terms such as tight muscles are, we would have a better understanding of what takes place in running and as a result, be able to truly help runners with their problems.
For more information see Explosive Running, 2nd edition.