Subtitles section Play video Print subtitles Muscles are the driving force of all movements in the body, varying from lifting, running, walking, and even organ function. The muscular system consists of three major muscle types: Cardiac muscles, which are responsible for the function of your heart; Smooth muscles, which are responsible for the function of all your other organs; And Skeletal muscles, which are responsible for, as the name suggests, the movement of your bones. Both smooth and cardiac muscles function involuntarily, meaning they operate by themselves. Skeletal muscles, on the other hand, function voluntarily and are under our conscious control. There are three different skeletal muscle fiber types, known as Type I, Type IIa, and Type IIx fibers. The difference of size, color, contractual speed, contractual force, and energy source classifies each fiber. Type I fibers, also known as slow twitch fibers, are the smallest fiber types with a darkish red color. It has a fairly slow twitch speed and produces a relatively small amount of force when contracted. It has high amounts of mitochondria, which are organelles within each cell that uses oxygen to produce energy. Although the force generated is fairly small, type I fibers are highly fatigue resistant, allowing it to be active for long periods of time. They are the primary fibers used during low-intensity activities with steady oxygen consumption, such as walking, jogging, or aerobics. Type IIa fibers, aka moderate fast-twitch fibers, are also red but intermediate in size. These larger fibers typically use a combination of oxygen and glucose as sources of energy. This combination allows for quicker contracting speed and higher force output compared to type I fibers. However, fatigue resistance isn't as high. These fibers are typically activated during anaerobic activities that are moderate in duration, such as a mile run, swimming, and short-distance cycling. Type IIx fibers, aka fast-twitch fibers, are white in color due to the low oxygen capacity and by far the largest fiber type. It makes up for the lack of oxidative capacity by having extremely high levels of glucose in its stored form of glycogen, producing the fastest twitch speeds and the most force. The downside, though, is that the fiber fatigues quickly, burning out after 15 to 30 seconds. High-impact, heavy resistance activities such as lifting weights and sprinting, will activate Type IIx fibers the most. For any activity, your muscles follow a certain recruiting order. The slow-twitch, low-force, fatigue-resistant Type I fibers are always activated first. When Type I fibers are maxed out, Type IIa fibers are activated, and then after those are maxed out, Type IIx fibers are then activated. This order, known as the Henneman's Size Principle, helps minimize muscle fatigue and allows precise motor control by using no more than the force necessary to complete a movement. Everyone has a genetically determined amount of each muscle fiber type. Some people are born with a predominant amount of a certain muscle fiber, making them effective with activities that favor those fibers. There have been some findings that suggests that Type IIx fibers can change into Type IIa fibers with proper training. However, this might simply be due to Type IIx fibers showing higher oxygen capacity through physical adaptation. They are, ultimately, still Type IIx fibers. Based on your exercises, which muscle fibers do you train the most? Leave your answers in the comment section below!