A23 Repetitive Overload

Topic

An athlete's tissue generally undergoes repetitive overload when the cumulative load placed on it over a specific period exceeds its capacity for repair and adaptation. This type of repetitive overload differs from an acute injury—such as a fracture—precisely because it does not stem from a single, isolated event where the load exceeds capacity; rather, it results from the progressive accumulation of microscopic damage to the tissue across many successive events.

In contrast, Dye’s load-capacity model describes the balance between the load applied to the tissue and the tissue's tolerance to that load: as long as the cumulative load remains within the tissue's zone of homeostasis, the tissue can repair and adapt without sustaining net damage; however, once the cumulative load exceeds this zone, the tissue enters a zone of overload that ultimately leads to injury. The further the cumulative load exceeds the zone of homeostasis, the more likely it becomes that the tissue will suffer injury.

The internal load borne by a runner's leg can be estimated as the product of the impact force of each step and the total number of steps taken during a typical training session—a figure that is substantial even during a standard workout. If the runner takes an even greater number of steps during the session, the resulting internal load on the leg increases accordingly. This progressive accumulation of microscopic damage in the runner's tissue follows the same Wöhler fatigue curve—previously established in this model for bone stress fractures—but now applied more generally to any biological tissue in the organism; the curve defines the relationship between the amplitude of cyclic stress borne by the tissue and the total number of load cycles it can tolerate before failure. Consequently, if the amplitude of cyclic stress on the tissue increases, the number of load cycles the tissue can tolerate before failing decreases further, exactly as dictated by the Wöhler fatigue curve.

Therefore, given the initial parameters of accumulated load and repair capacity, an excessive increase in the athlete's training load from one week to the next constitutes the most well-documented risk factor for this type of repetitive overload injury. Such an excessive increase pushes the accumulated load on the athlete's tissue out of the homeostatic zone defined by the model and into the overload zone—leading to tissue injury—before the tissue's repair and adaptation mechanisms can adjust to the higher load. The more pronounced this week-to-week increase in training load, the greater the resulting risk of repetitive overload injury for the athlete.

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