STUDENT INVESTIGATION 3

Background

Remember from a previous section that all muscles have varying percentages of slow twitch (Type I) and fast twitch (Type II) muscle fibers. Remember also that fast twitch fibers can deliver extreme amounts of power for short periods of time, while, on the other hand, slow twitch fibers provide endurance, delivering prolonged strength of contraction over much longer periods of time. For this exercise, you will be answering a few questions and performing a simple demonstration activity that, together, are designed to help you understand which muscle fiber type you may be particularly endowed with. Everyone is different. And a person who has more Type I muscle fibers cannot be considered "better" than a person who has more Type II fibers. This is simply an exercise for you to determine which fiber type is likely dominant in your upper leg muscle group, and to understand the different kinds of activities that you are usually involved in so that you can predict what muscle types dominate certain areas of your muscular make-up. Before we begin, let's go into a bit more detail about the difference between Type I and Type II muscle fibers.

Since we have reviewed the different energy systems that provide "fuel" to the muscles, you now have the background you need to explain why the two types of muscle fibers provide different kinds of movement capabilities. The following facts should clarify the differences for you:

1. Fast twitch fibers are about two times as large in diameter as slow twitch fibers.
2. The enzymes (chemicals that direct and accelerate chemical reactions in the body) that promote rapid release of energy from the phosphagen and glycogen-lactic acid energy systems are two to three times as abundant in fast twitch fibers as in slow twitch fibers, thus making the maximal power that can be achieved by fast twitch fibers as great as two times that of slow twitch fibers.

3. Slow twitch fibers are mainly organized for endurance, especially for generation of aerobic energy. They also contain considerably more myoglobin (a hemoglobin-like protein that combines with oxygen within the muscle fiber), and even more importantly, myoglobin increases the rate of diffusion of oxygen throughout the fiber by shuttling oxygen from one molecule of myoglobin to the next.
4. The number of capillaries per mass of fibers is greater in the vicinity of slow twitch fibers than in the vicinity of fast twitch fibers.
5. These facts indicate that:
   • oxygen delivery and, therefore, the aerobic energy systems, dominate in the slow twitch fibers, and
   • the phosphagen and glycogen-lactic acid energy systems dominate in the fast twitch fibbers.

Partly, it depends on the kinds of muscle fibers that you have either been endowed with through genetic expression (your parents would have a lot to do with that) or that you have developed due to the kinds of activities that you normally participate in. For instance, those of you who play basketball probably have a high percentage of Type I (slow twitch) fibers to allow for extended use of your muscles but also enough Type II (fast twitch) fibers to allow you to move quickly and powerfully to make the big score of the game. On the other hand, those of you who are sprinters in a track meet probably have a lower percentage of Type I (slow twitch) and more Type II fibers to allow for the quick bursts of power it takes to run fast over a short

No matter what kind of muscle fiber types you have, they all function by contracting, or shortening. Contractions that result in muscle fiber shortening are called isotonic contractions (iso = same; tonic = strength). An example of this is when you lift an object with your arm, your biceps muscles will shorten, allowing your arm to bend. In this case, you are matching and, in fact, increasing the force of your muscle contractions beyond the force exerted by the weight of the object, thereby lifting it. You may have experienced a situation, however, where you have tried to lift an object with your arm and you were not strong enough to lift it. Your biceps muscle could not produce enough force to match or overcome the pulling force of the object. In this case, your biceps were not able to contract and shorten the fibers in your muscles. This is called isometric contractions (iso = same; metric = length). For isometric contractions, the muscle fibers do not actually shorten, they are just trying to shorten. The simple activity that you will be asked to perform for this exercise is actually one that involves an isometric contraction.

With the knowledge of how the fiber types are different and why each of us develops different kinds of fiber types, let's begin our personal examination of what fiber types likely dominate our particular muscular makeup.

Materials
For this exercise, you will need only two things:

1. a stopwatch, and paper to record the results of the experiment.
2. paper to record the results of the experiment.

Procedure
You should break into small groups, or your teacher can select a handful of students to serve as "demonstration subjects" for this exercise. In either case, each student will perform the same activity. The activity was chosen to demonstrate the endurance capability of your quadricep muscles in your upper leg by performing an isometric contraction of the muscles for a period of time. The longer a student can hold the isometric contraction, the more likely that the student has a high percentage of Type I slow twitch fibers. If a student cannot hold the position for very long, it is more likely that this student has a higher percentage of Type II fast twitch fibers. Before the activity is started, think about what kinds of activities or sports each student is normally involved with and try to predict who would be more likely to withstand the activity the longest. These predictions will serve as the group's hypotheses.

This is how the activity should be carried out. Each student should stand with his/her back against a flat wall and, while the back is still touching the wall, each should slowly lower himself to a sitting position. At this point, the student's thighs should be parallel to the floor and the back should be flat against the wall. Now, start the stopwatch and time how long each can maintain the sitting position. Each student should try to hold the position as long as possible, but stop when you cannot tolerate the burning sensation is felt in the thighs. Once the student reaches a point where he/she cannot continue, have two students ready to pull the subject forward to relieve the isometric contraction. Repeat this exercise for each student.

Discussion
This demonstration will reveal some general characteristics about each student's upper leg muscles. The longer the student can hold his/her isometric contraction, the more likely he or she is to have a higher percentage of Type I muscle fibers. Here is the approximate breakdown related to the time each student is able to hold the position. If the student can hold the position for:

• less than 30 seconds, the student probably has more Type II muscle fibers in their upper leg;
• more than 30 seconds but less than 1 minute, the student probably has at least half of Type I muscle fibers in their upper leg (the closer the student comes to reaching one minute, the higher percentage of Type I fibers); and
• more than 1 minute indicates that the student's upper leg consists primarily of Type I muscle fibers

The results of this exercise should be compared with your earlier activity survey for each student to answer the following questions:

1. Does it appear that students who were able to last longer during this exercise also participate in sports or other physical activities that require greater endurance?
2. Does it appear that students who were not able to last very long during this exercise are those who normally participate in activities that require bursts of power?
3. Were your hypotheses supported or refuted by your data and observations?
4. Let's move on to review Dr. Baldwin's actual space flight experiment!