The by product buildup idea states that it is the buildup of certain by products or substances that cause fatigue at several different levels. The most common, although WRONG, example of this theory is the buildup of lactic acid. In essence, the buildup of certain products creates fatigue by impairing force output at any number of the different steps to get from muscle recruitment to contraction. Whether these products directly or indirectly cause fatigue is another subject that will be covered shortly.
There are many different products that can potentially cause fatigue. Many of the earlier ideas centered on the energetic model in which supplying ATP to the muscle for contraction is the main component. As is well known, the three basic energy systems are the immediate (phosphagen and Myokinase), glycolysis, and aerobic system. Each system requires a series of chemical reactions that ultimately result in ATP reformation. The belief is that the energy systems, mainly the glycolytic one, create by products that inhibit subsequent energy production. As the use of these systems increased, byproducts that cause fatigue increased to a degree that they interfered with total energy production from all the systems. In the energetic model the enzymes that catalyze the numerous chemical reactions become less active, thus directly slowing energy production.
Looking at it from a contraction standpoint, these by product increases can interfere with contraction via methods outside of energy production. Any number of the steps to get from sending the signal to actual muscle contraction can be impaired. The accumulation of such products as ammonia and potassium are prime examples, as these alter muscle fiber excitability. The impairment of muscle contraction can occur at any number of different sites, ranging from impairing the action potential to changes in calcium release or uptake which can delay contraction and relaxation (Hargreaces & Spriett 2006)
While lactate was initially seen as the culprit, hydrogen ions (H+) and the corresponding drop in pH are more compelling examples. As pH drops, the rate of ATP replenishment drops due to a reduction of two enzymes, PFK and ATPase, as well as an increase in the amount of Calcium needed during muscle contraction (Maglischo, 2003).