The magnitude of physical strength, often referred to as just strength, determines the ability of a person or animal to exert force on physical objects using muscles. Increasing physical strength is the goal of strength training.

Strength can be divided into two categories. Short-term endurance and long-term endurance. In humans, there are three primary muscle fiber classifications: Slow Twitch (Type I or ST), Fast Twitch A (also known as Type IIa or FT-A), and Fast Twitch B (Type IIb or FT-B).

Comparison chart

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	Slow Twitch	Fast Twitch A	Fast Twitch B
Endurance	Long term	Short-term, upper limit	Short-term, lower limit
Contraction/Reaction	Slow	Fast	Very fast
Motor neuron size	Small	Large	Very large
Glycolytic capacity	Low	High	High
Mitochondrial density	High	High	Low
Capillary density	High	Average	Low
Oxidative capacity	High	High	Low
Resistance to fatigue	High	Average	Low
Activity used for	Aerobic	Extended anaerobic	Anaerobic
Major power source(s)	Triglycerides	CP(Creatine phosphate), oxygen	CP, oxygen

Notes

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While most people have more or less equal amounts of the fibers, some people are genetically-gifted with a disproportionate composition of them, resulting in excellent performance in a relevant sport. For example, a person with high concentrations of fast twitch B fibers would make an excellent sprinter. Conversely, this means that the same person would perform terribly in a long-distance run.

Hypertrophy of the muscles usually results from any sort of sufficient physical activity, though to varying degrees of size, dependent upon the physical activity itself. Hyperplasia, the introduction of additional muscle fiber, has only been observed to occur in animals.

Miscellaneous

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Many studies show that training one rigorously harms the other - eg. large muscles are good for maximal power during short time periods, but they consume great amounts of oxygen which muscles need during long periods of exercise.

Many athletes depend mostly on physical strength, even though coordination and dexterity also play an important role.

Identification

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One can determine the type of muscle that is present in a meat food product, by looking at it after it has been cooked. White meat is fast-twitch fiber, while red meat is slow-twitch fiber. The difference is due to the greater presence of myoglobin in slow twitch muscle, which gives it its red hue. Red meat from an animal would indicate that it came from body component that is expected to operate for extended periods of time, such as the legs. While white meat conversely comes from body parts that operate for extremely short periods, such as the muscles involved in chicken flight, an activity which doesn't last long.

Human muscle composition testing requires a biopsy.

Muscular system | Exercise physiology