STUDENT INVESTIGATION 2

Background

Our bodies are the vehicles that we drive to produce our physical movement. Of course, we as humans are much more than a moving machine, but for this activity we will be looking only at the physical aspects that provide for our mobility. We should always remember, however, that it is the intangible parts (the things we can't touch), more than the physical parts of ourselves that make us human. Nevertheless, for this section, let's describe our ability to move around in terms of this "vehicle" analogy. As you will see, this analogy does make some sense since the movement of a vehicle requires the interaction of many different parts, just like the body! After this Background section, you will be asked to design and carry out certain physical activities during a "mini field day" to demonstrate how various muscle energy systems contribute to the expression of muscle strength, power, and endurance. Of course, the strength, power, and endurance of a vehicle depend on its "make, model, and year," how well it is cared for, and what kind of fuel is used. All of this is true of our bodies as well!

Our "vehicles" come equipped with "standard equipment" including, in part, our cardiovascular system, pulmonary system, and muscles. The "computer" that controls the system lies primarily in our brain, and the "electrical system" that allows our computer to control the standard equipment and create movement is primarily the nerve conduction system. Our "navigational equipment" is composed of the various parts of our sensory and balance system (eyes, inner ear, muscular sensors, and touch sensors). And our vehicles can't do without the "fuel" provided by the utilization of the three different muscular energy systems.

In athletic events, our vehicles are expected to take us farther or faster than they do when we are performing normal day-to-day functions. Therefore, for athletic events, it is necessary to "change gears" to force our bodies to perform for us at a faster and more vigorous level. Changing gears often means changing the kind of muscle energy system that we must rely on to produce the muscle contractions that are appropriate for certain movements. We have already discussed under what circumstances our bodies utilize each of the three different muscle energy systems. Table X summarizes those systems. Changing gears also involves utilizing certain features within our bodies that, with extra effort, can even be "upgraded." These features are strength, power, and endurance.

Strength, power, and endurance are the special built-in features that we can find within our vehicles, or bodies, if we look for them. Each of these features can be expressed in a physical sense and an emotional sense. Let's talk about them only in terms of our skeletal muscles. The strength of a muscle is determined mainly by its size, with a maximum contractile force between 2.5 kg and 3.5 kg per cm of muscle cross-sectional area. That means that for every cubic centimeter of muscle mass that a person has, that person can create a force that is necessary to pull a 2.5 to 3.5 kg mass. That may not seem like much but if you look at your biceps muscle in your arm, you can see that it is composed of many cubic centimeters of muscle tissue (even if your muscles are small). So, the bigger the muscle, the stronger the muscle (Figure 11). As an example, a world-class weight lifter might have a quadriceps muscle (in the thigh) with a cross-sectional area as large as 150 cm . This would translate into a maximum contractile strength of 525 kg (1155 lb). Of course, on Earth, most of the tension would fall upon his patellar tendon in the knee, which could cause this tendon to be ruptured or actually torn from its insertion point below the knee. Yet, to make matters worse, the holding strength of muscles is about 40% greater than the contractile strength. That is, if a muscle is already contracted and a force then attempts to stretch out the muscle by increasing the time that the contraction must be held, this produces about 40% more force on the muscle. Therefore, the force of 525 kg calculated previously for the patellar tendon becomes 735 kg (1617 lb). This obviously further compounds the problems of the tendons, joints, and ligaments. It can also lead to internal tearing in the muscle itself.

The power of muscle contraction is different from muscle strength, because power is a measure of the amount of work that the muscle can perform in a given period of time. This is determined not only by the strength of the muscle contraction but also by its velocity of contraction (how fast it contracts) and the rate of contraction (the number of times that it contracts each minute). Muscle power is generally measured in kilogram-meters (kg-m) per minute. That is, a muscle that can lift a kilogram weight to a height of 1 meter or that can pull some object horizontally against a force of 1 kg for a distance of a meter in 1 minute is said to have a power of 1 kg-m/minute. The maximum power achievable by all the muscles in the body of a highly trained athlete with all the muscles working together is approximately the following:

Thus, it is clear that a person has the capability for an extreme power surge for a short period of time, such as during a 100-meter race that can be completed entirely within the first 10 seconds. On the other hand, for long-term endurance events, the power output of the muscles is only one-fourth as great as during the initial power surge. Keep in mind that this does not mean that one's athletic performance is four times as great during the initial power surge as it is for the next half hour. The fact is that, during rapid bursts of activity, the muscle power output is less efficient. In contrast, for slower sustained activities, the efficiency for muscle power output is higher.

The final measure of muscle performance is endurance, or how long one can last while performing physical activities. This, to a great extent, depends on the nutritive support for the muscle. More than anything else, endurance depends on the amount of glycogen that has been stored in the muscle prior to the period of exercise. A person on a high carbohydrate diet stores far more glycogen in muscles than a person on either a mixed diet or a high fat diet. Therefore, endurance is greatly enhanced by a high carbohydrate diet. The corresponding amounts of glycogen stored in the muscle are approximately the following:

To return to our "vehicle" analogy above, it is safe to say that a high carbohydrate diet is similar to "high octane fuel."

Now, why did we go into so much detail about the three characteristics of muscle capabilities? Well, you are going to design a set of activities that will demonstrate various combinations of physical strength, power, and endurance for the class to participate in. That is, your class will "test drive" each of your vehicles in various activities that you will define, and then you will get together to evaluate and rate the particular physical movements that were involved in each activity. So let's get started!

Procedure
Our main focus for this Student Investigation involves designing a mini field day that includes activities that demonstrate various levels of strength, power, and endurance. Each activity will be evaluated to determine which of the muscle energy systems is used in the body to perform such activities. The following steps will guide you in this exercise:

Step 1.
Each student will make a list of simple activities that could be done by the class during a "mini field day." These activities should require:

• different levels of strength, and
• different levels of power.

Step 2.
After each student makes an independent list of activities, discuss as a class the various options that were presented by the students and come up with a final list of activities that the class will participate in together. Once a list of activities is developed, design a chart (similar to Table 4, which will be provided by your teacher) to rate the activities in terms of the strength, power, and endurance needed to perform them. Try to select activities that represent different combinations of strength, power, and endurance. Then predict the kind of muscle energy systems each activity would probably utilize. These predictions will be your hypotheses. Use your experience with the previous exercise (Student Investigation 5.1) and Table 2 as a reference for this part of the discussion.

Step 3.
Now the fun begins! As the field day is held, evaluate the movements of each activity to see if your predictions were right. After performing the activities, the class will discuss which of the muscle energy systems were utilized for each activity by eval uating the physical movements that it took to perform each one. The field day is meant to offer the class an opportunity to understand how the different metabolic systems in the body contribute to a variety of physical movements. Enjoy yourselves!

Step 4.
As a final discussion point, look at each activity and evaluate how easy or how difficult it would be to perform them in the microgravity environment of space. How, then, might our body's energy systems change in space? Don't be afraid to use your imagination.

Table 4. Rate your list of activities based on their strength, power, and endurance characteristics. Predict which muscle energy systems will be used to perform the activities.
Endurance Ratings
S t r e n g t h R a t i n g s	LOW MEDIUM HIGH L O W Activity Power Level Energy Systems Utilized M E D I U M H I G H