As one fatigues, the force exerted by the muscle decreases. What physiological processes explain the decline in strength ?

Question

As one fatigues, the force exerted by the muscle decreases. What physiological processes explain the decline in strength?

Answer 1

Your question is a good one and probably multi-factorial (hopefully I am understanding what you are asking and can answer appropriately). For the short answer - it is likely the build up of metabolic products (Cl-, K+, Pi, H+, its even been suggested that ADP may have an inhibitory role according to pubmed) and perhaps the lack of availability of energy sources ATP.

Consider what is happening metabolically in terms of energy supply. In a muscle we initially use stored ATP (1-4 s), then utilize the creatine phosphate system to generate ATP (4-10 s). These two mechanisms give rapid access to ATP, which meanwhile, allows for the body to begin anaerobic glycolysis followed by aerobic glycolysis (somewhat slower mechanisms that eventually develop ATP). During the later two mechanisms, the energy is primarily derived from the metabolism of glycogen, lactic acid (contributes to increased H+), and even fatty acids. Eventually, prolonged exertion results in energy demands that outpace energy production. When this happens, you don't get the myosin powerstroke and cycling that is creates the force generated from muscle.

Regarding the metabolites, (to give the shortest answer) the majority of these metabolites can ultimately interfere with the release of calcium (another important aspect needed for muscle contraction).

Answer 2

There is a good TED-Ed video on this topic: https://www.youtube.com/watch?v=rLsimrBoYXc&feature=youtu.be

To summarize, there can be:

1. depletion of the ions necessary to control muscle contractions including sodium, potassium, and calcium
2. depletion of ATP
3. depletion of of the neurotransmitter acetylcholine