Question

What are the muscle fiber types, contraction types, and the energy sources utilized by muscle fibers?

Answer 1

SKELETAL MUSCLE FIBER TYPES

Classically, skeletal muscle fibers can be categorized according to their speed of contraction and their resistance to fatigue. These classifications are in the process of being revised, but the basic types include:

- Slow twitch oxidative (type I) muscle fibers,

- Fast-twitch oxidative-glycolytic (Type IIA) muscle fibers, and

- Fast-twitch glycolytic (Type IIX) fibers

1) Concentric Contractions

A concentric contraction is a type of muscle contraction when the length of the muscles shorten while undergoing tension.

For example, when you pick up a curl bar and perform a biceps curl, the length of your biceps muscles shorten. Your hands start down by your sides, and ends with your hands up by your shoulders. The biceps muscles shorten during this motion.

Another example would be if to perform a couple of leg curls on a hamstring machine. As knee is flexing, foot is approaching buttocks, and hamstring muscles shorten in the process.

Any muscle activity where the strength of the muscle can overcome the resistance of an object forcing the muscle’s length to shorten, is considered a concentric contraction.

Therefore, most of the exercises that you would typically do at a gym by using the various machines and/or dumbbells, etc. involve concentric contractions.

Eccentric Contractions

An eccentric contraction is a type of muscle contraction when the length of the muscle elongates, or lengthens while undergoing tension.

Isometric Contractions

An isometric contraction is a type of muscle contraction where the length of the muscle doesn’t change while undergoing tension. For example, when a person take a 20 pound weight and perform a biceps curl and hold a position halfway between the repetition for 10 seconds. The length of biceps muscle doesn’t change during this time, alyough a force is still being applied.

Another example would be if a person was to push against a wall for 10 seconds. The wall doesn’t move and neither does the length of the muscles in arms pushing against it. Again, a force is still being applied.

Isotonic contraction :

It is the contraction of the muscle in which the length of muscle changes but the tension applied on the muscle is constant.

Example: Bicep curling exercise with a constant weight.

Energy sources:

1. Creatine Phosphate: Once the cytosolic stores of ATP are depleted, the cell calls upon another rapid energy source, Creatine Phosphate. Creatine phosphate is a high energy compound that can rapidly transfer its phosphate to a molecule of ADP to quickly replenish ATP without the use of oxygen. This transfer requires the enzyme creatine kinase, an enzyme that is located on the M-line of the sarcomere. Creatine phosphate can replenish the ATP pool several times, enough to extend muscle contraction up to about 10 seconds. Creatine Phosphate is the most widely used supplement by weight lifters.

2. Glycolysis: Glycolysis is the breakdown of glucose. The primary source of glucose for this process is from glycogen that is stored in the muscle. Glycolysis can function in the absence of oxygen and as such, is the major source of ATP production during anaerobic activity. This series of chemical reactions will be a major focus in the next unit. Although glycolysis is very quick and can supply energy for intensive muscular activity, it can only be sustained for about a minute before the muscles begin to fatigue.

3. Aerobic or Oxidative Respiration: The mechanisms listed above can supply ATP for maybe a little over a minute before fatigue sets in. Obviously, we engage in muscle activity that lasts much longer than a minute (things like walking or jogging or riding a bicycle). These activities require a constant supply of ATP. When continuous supplies of ATP are required, the cells employ metabolic mechanisms housed in the mitochondria that utilize oxygen. We normally refer to these processes as aerobic metabolism or oxidative metabolism. Using these aerobic processes, the mitochondria can supply sufficient ATP to power the muscle cells for hours. The down side of aerobic metabolism is that it is slower than anaerobic mechanisms and is not fast enough for intense activity. However, for moderate levels of activity, it works great. Although glucose can also be utilized in aerobic metabolism, the nutrient of choice is fatty acid.