Researchers in the Monash University Centre for Biomedical Engineering are investigating a measure of the propensity for tearing of hamstring muscles. This article is intended to provide information on this project for lay readers. Further details of the theory and related work may be found here.
We have been interested for many years in how muscle responds to being stretched while actively generating tension. This is known as an eccentric or pliometric contraction, where contraction infers activation of the muscle, not shortening. A "contraction" that does involve shortening is known as a concentric contraction. Mechanically, a concentric contraction does work, or involves muscles acting as motors, while an eccentric contraction absorbs work, or involves muscle acting as brakes.
Exercise that involves repeated eccentric contractions is known as eccentric exercise. Such exercises include walking downhill, skiing and horse riding, but not swimming or cycling, where muscle act exclusively as motors. One characteristic of unaccustomed eccentric exercise is that it leads to tenderness and stiffness, typically beginning the next day, peaking over 1 to 2 days, and then subsiding over the next week or so. This is known as Delayed Onset Muscle Soreness, or DOMS, and is a very common experience. Microscopic examination of DOMS muscles shows small areas of damage, initially as small as a few microns (thousandths of a millimetre) in extent, but then extending along the muscle fibres.
An additional factor that we have shown to be important is the length range over which the muscle is stretched. Muscles have an optimum length at which they produce most tension, with tension capability falling at longer and shorter lengths. DOMS is now known to occur mainly when the muscle is stretched beyond the optimum length.
DOMS is relatively easily prevented by eccentric exercise training. Climb down a mountain this weekend and you will get sore. Repeat the exercise next weekend and you will be much less sore. We have shown that this training effect consists of changing the muscle so that its optimum length increases. Hence the training effect is explained as avoiding muscle lengths beyond optimum by shifting the optimum.
Those interested can read about our theory to explain these things at a structural level, including why DOMS occurs beyond optimum length, and how the optimum length can change. Much evidence has been produced to support it.
We are proposing that the microscopic damaged areas typically seen in DOMS may be the initiating event for a gross muscle tear. While this is not proven, and indeed very difficult to prove, there is evidence to support it. Torn hamstring muscles most commonly occur when the muscle is at long length (fully extended knee and flexed hip) and acting as a brake to slow down the forward motion of the leg after a kick, or while sprinting or hurdling. If our proposal is correct, then individuals who operate their muscles beyond optimum length should be more likely to suffer tears. Furthermore, training to reduce DOMS should also help prevent gross muscle tears.
Because of the possibility of eccentric training causing or exacerbating the injury that we are trying to provide protection against, we are proceeding cautiously, and encourage others to be similarly cautious.
The first step was to show that eccentric training of uninjured subjects could shift the optimum length of hamstring muscles. This has been published in the journal Medicine and Science in Sports and Exercise.
The second step is to show that previously injured muscles, known to be especially prone to injury, are working further beyond optimum length than uninjured muscles.
If this is successful, the third stage will involve suggesting exercises for injured muscles.
We are now keen to find people with injuries that have recovered beyond the acute stage, so that we can test where their optimum length occurs. This involves a single visit to Monash Clayton for about 30 mins, to undertake concentric isokinetic contractions on a Biodex human dynamometer. This will provide the individual with a measure of their potential for re-injury, as well as advancing the study.
People interested in contributing to this study, and in assessing their state of "eccentric exercise fitness", are invited to contact Paul Percival at 040 759 8833 or David Morgan at 9905 3483. Electronic mail to paul.percival@eng.monash.edu.au also works well.
