What are Type I muscle cells?
|
Type I fibers (sometimes called “slow-twitch” [ST] or “slow-oxidative” [SO] fibers) are better designed for prolonged exercise performed at a lower intensity. In comparison to Type II fibers, Type I fibers will have more mitochondria (See Muscle Cell Figure) and rely more heavily on the aerobic generation of ATP. The primary energy molecules used to generate ATP in these muscle cells will be fatty acids and glucose. Since ATP production in mitochondria requires oxygen, proper function of these muscle fibers is very dependent upon oxygen supply via the blood. Luckily, Type I muscle cells always seem to have many capillaries around them to deliver oxygen-endowed blood. In addition, Type I fibers contain a substance called myoglobin. Myoglobin is an iron-containing protein that binds oxygen and serves as an oxygen reserve for these cells during exercise. |
 |
What are Type II muscle cells?
Type II muscle fibers (sometimes called fast twitch [FT] or fast-glycolytic [FG] fibers) can execute a much faster speed of contraction than Type I muscle fibers. This is to say that Type II muscle fibers are designed to generate force more rapidly, thereby allowing them to be more powerful as they will allow a job to be performed in a shorter amount of time. Meanwhile, Type II muscle fibers are relatively limited in their ability to generate ATP by aerobic means. So, when these cells break down glucose to pyruvate and generate a couple ATP in the process, much of the pyruvate that is formed will then be converted to lactic acid (lactate). This is because these muscle cells have less mitochondria and receive less oxygen as they are served by fewer blood vessels (see Table 11.1.).
How does the brain know which type of muscle cells to use for different sports?
|
This is a no-brainer for the brain! This is because the brain will always call upon Type I muscle fibers first and then Type II. The major factor will be the required force to perform the exercise. For instance, when an exercise requires less force (e.g., jogging, fast walking, casual cycling) the brain will for the most part call upon Type I muscle fibers (see Muscle Fiber Recruitment Figure). However, as the necessary force to perform an exercise increases (e.g., run/cycling sprint, weightlifting), the brain will also call upon Type II muscle fibers to generate force to support the force generated by Type I fibers. |
 |
