Three Characteristics of Muscles.
When you think about muscles, there are three characteristics that matter: strength, power, and endurance. Strength is how much force that muscles have. This can be improved by strength training. Larger muscles are stronger. Power is how fast muscles respond, and endurance is how long they can keep performing. Power and endurance can be improved by exercise, but there is a biological baseline for these that may not be able to be overcome.
The biological limit has to do with types of muscles – slow twitch and fast twitch muscle fibers. To understand the differences, think of a chicken. It has white meat and dark meat. Have you ever wondered what the differences are? Dark meat is laden with mitochondria and myoglobin, a protein that stores oxygen in the muscles. These are slow twitch muscles, which work aerobically, and you know this because a chicken spends its days walking – something that requires endurance, but not much power. A chicken’s breast muscles are white meat. These are fast twitch muscles, which are important during anaerobic activity. They have great power, being able to contract ten time faster than slow twitch. You know these are fast twitch muscles, because you know how they are used: to quickly take flight at a sign of danger.
Humans have both slow twitch and fast twitch muscles too. For most people, there is an even distribution of these muscle types. In elite athletes, however, this is not the case. World class sprinters, like Usain Bolt, who excel in power, have been shown to have 80% of their muscles as fast twitch. World class marathoners who excel in endurance may have as much as 80% of their muscles slow twitch. It is not clear whether training can change the distribution of fiber types within an individual.