Muscle cells<unk> ork by detecting a flow of electrical impulses from the brain,<unk> hich signals them to contract through the release of calcium by the sarcoplasmic reticulum. Fatigue (reduced ability to generate force) may occur due to the nerve, or<unk> ithin the muscle cells themselves. New research from scientists at Columbia University suggests that muscle fatigue is caused by calcium leaking out of the muscle cell. This makes less calcium available for the muscle cell. In addition, the Columbia researchers propose that an enzyme activated by this released calcium eats away at muscle fibers. Substrates<unk> ithin the muscle generally serve to power muscular contractions. They include molecules such as adenosine triphosphate (ATP), glycogen and creatine phosphate. ATP binds to the myosin head and causes the ‘ratchetting’ that results in contraction according to the sliding filament model. Creatine phosphate stores energy so ATP can be rapidly regenerated<unk> ithin the muscle cells from adenosine diphosphate (ADP) and inorganic phosphate ions, allowing for sustained powerful contractions that last between 5–7 seconds. Glycogen is the intramuscular storage form of glucose, used to generate energy quickly once intramuscular creatine stores are exhausted, producing lactic acid as a metabolic byproduct. Contrary to common belief, lactic acid accumulation doesn't actually cause the burning sensation felt<unk> hen people exhaust their oxygen and oxidative metabolism, but in actuality, lactic acid in presence of oxygen recycles to produce pyruvate in the liver,<unk> hich is known as the Cori cycle. Substrates produce metabolic fatigue by being depleted during exercise, resulting in a lack of intracellular energy sources to fuel contractions. In essence, the muscle stops contracting because it lacks the energy to do so.
Weakness
Baricity
Spasticity
Pain out of proportion
Waxy flexibility
Hyperintensity
Whispered pectoriloquy
Lusitropy