As a runner, understanding injuries and injury prevention is critical to your longevity in the sport. A few years ago, Michael Muller, a professor of physical therapy at Washington University, presented a new theory of tissue adaptation to physical stress that provides a helpful conceptual framework for runners seeking to stay healthy without sacrificing performance.
Physical Stress Theory
Physical stress theory, as it is called, is based on the simple premise that body tissues adapt in a predictable way in response to changes in the relative level of physical stress they are exposed to. When tissues are exposed to an accustomed level of stress, they maintain their current structure and function at a state that is often referred to as homeostasis.
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When these same tissues are exposed to a slight or gradual increase in stress, they modify their structure and function—after an initial period of breakdown—to become more tolerant of that type of stress. They achieve a new homeostasis at a greater level of durability.
For example, a recent animal study found that when rats were exposed to a running program, fingerlike branches of new tissue grew in the attachment between the tendons and muscles of their legs, strengthening these important junctions. But if a stress is increased too quickly or abruptly, the tissues never recover from the initial period of breakdown. They lose their homeostatic balance and progressively degenerate. All running overuse injuries follow this pattern.
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On the other hand, if the level of stress is reduced, the tissues adapt in the other direction, finding a new homeostasis at a lower level of durability and function.
Running as Injury Prevention?
The physical stress theory reminds us that running does not only cause overuse injuries—it also protects us against them. When you increase your running volume at a sensible rate and then maintain your mileage at a sensible upper limit, the tissues of your lower body become significantly better able to tolerate the stress of running without losing homeostasis.
Several studies have shown that experienced runners who have been training for several years are significantly less likely to become injured than beginning runners. This may also be the case in part because some of the most injury-prone novice runners quit, leaving less injury-prone runners to become more experienced. But it is also a consequence of the durability-increasing tissue adaptations that more experienced runners have earned.
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Another factor is the much greater relative increases in physical stress that beginning runners typically experience relative to veteran runners. The bones, muscles and joint tissues of the person who's going from zero to 20 miles a week in training for his or her first 5K are more likely to lose their homeostatic balance than those of a veteran runner who's working from 40 to 60 miles per week in training to set a 10K PR.
It seems to be a Catch-22: You need to run more to increase the injury resistance of your legs, but the relative increase in physical stress that comes with running more is often greater than the resulting increase in durability, so you wind up getting injured more often.
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