A researcher was investigating the composition of muscle tissue in the gastrocne
ID: 3477859 • Letter: A
Question
A researcher was investigating the composition of muscle tissue in the gastrocnemius muscles (in the calf of the leg) of athletes. A needle biopsy was taken from the muscle, and the concentration (or enzyme activity) of several substances was determined. Describe the major differences this researcher would see when comparing the muscles from athletes who performed in the following events: heavy weight lifting, and the marathon run (26.2 miles). Elements to consider: mitochondria, glycolytic enzymes, glycogen vs. fat storage, myoglobin, capillary content. Explain the physiological significance of these differences. What are some of the features of the muscles as related to speed of contraction and rates of fatigue?
Explanation / Answer
To understand the differences in the composition of Gastrocnemius muscle in marathon runners and weightlifters, it is essential to know a little about the types of muscle fibers in our body.
Skeletal muscles are made up of different proportions of three types of muscle fibers:
Type I - Also known as 'slow twitch' fibers, these use the 'aerobic'(with oxygen) or 'oxidative' metabolic cycle to generate low intensity energy for longer periods. They are resistant to fatigue but take time to fire up. The muscles that support the body's posture - the muscles of the neck, spine and legs are rich in Type I fibers.
Type II - Further divided into Type IIA and Type IIB fibers
Type IIB - Also known as 'fast twitch' fibers, these use the 'anaerobic' (without oxygen) metabolic cycle to generate energy that is short-living. Hence, though they fire up quickly (even when there is no oxygen), they fatigue rapidly. The muscles of the shoulder and arm typically contain many Type IIB fibers.
Type IIA - These are a hybrid variety containing both Type II and Type IIB fibers and can utilise both types of metabolic cycles to generate rapid strong energy strokes but fatigue easily.
Now it would be obvious that marathon runners would have more Type I fibers in their gastrocnemius muscle while weightlifters would have more Type IIB fibers. Moving on to the specifics:
Feature
Type I Muscle Fiber (In Marathon runners)
Type IIB Muscle Fiber (In Weight lifters)
Mitochondria
More
Less
Glycolytic enzymes
Low
High
Glycogen Vs Fat storage
Fat
Glycogen
Myoglobin
High
Low
Capillary content
High
Low
Speed of contraction
Slow
Fast
Rate of Fatigue
Slow
Rapid
Physiological significance of these differences:
Type I muscle fibers have a high myoglobin content (hence, red in colour) to bind and store more oxygen. Oxygen is important for aerobic metabolism. So, these fibers have a higher density of mitochondria to generate more ATP/energy. There are more number of capillaries to increase the blood supply which in turn increases the amount of oxygen reaching the muscle. These muscles store and use fat as its slow oxidation yields a great deal of ATP.
Type IIB muscle fibers have low myoglobin content (hence, white in colour) as they don't rely on oxygen to derive energy. They have high amount of glycolytic enzymes and high stores of glycogen which can be broken down quickly by anaerobic glycolysis to generate ATP. So there are fewer mitochondria and fewer capillary content in these fibers.
Hope this helps!
Feature
Type I Muscle Fiber (In Marathon runners)
Type IIB Muscle Fiber (In Weight lifters)
Mitochondria
More
Less
Glycolytic enzymes
Low
High
Glycogen Vs Fat storage
Fat
Glycogen
Myoglobin
High
Low
Capillary content
High
Low
Speed of contraction
Slow
Fast
Rate of Fatigue
Slow
Rapid
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