STUDENT INVESTIGATION 1

Background

In an earlier section in this chapter, it was stated that about 50% of our bodies is composed of muscles. Of course, this percentage will vary depending on how large our muscles are. In any case, the muscles are the major energy-consuming components of our bodies. What is so unique about muscle energy metabolism is that chemical energy (in the form of ATP) is transformed into mechanical energy (movement). Remember that there are three muscle energy systems that our bodies call on in order to perform various kinds of activities. Table 2 lists those systems, but we are going to review them briefly once again. After reviewing these systems, you will be asked to predict which of the energy systems are used in the performance of a selected list of sports activities. Let's first review the muscular energy systems and how they are used by the body.

Table 2. Muscle energy systems used for various magnitudes of activities.
Muscle Metabolic Energy Systems	Duration of Maximal Muscle Activity	Activity Types
Phosphagen System	10-15 seconds	Power surges
Glycogen - Lactic acid System	30-40 seconds more	Intermediate athletic activities
Aerobic System	Unlimited time (as long as nutrients last)	Prolonged athletic activities

The chemical energy that fuels our muscular activities is adenosine triphosphate (ATP). For the first five or six seconds of muscle power, we can depend on the ATP that is already present in the muscle cells. Beyond this time, however, new amounts of ATP must be formed to enable the activation of muscular contractions needed to support longer and more vigorous physical activities. For activities that require a quick "burst" of energy that cannot be supplied by the ATP present in the muscle cells, the next 10-15 seconds of muscle power can be provided through our bodies use of the phosphagen system, which utilizes a substance called creatine phosphate to recycle ADP into ATP.

For longer and more intense periods of physical activity, however, our bodies must rely on systems that break down the sugars (glucose) in our bodies to produce ATP. The complete breakdown of glucose occurs in two ways: through anaerobic respiration (oxygen is not used), and through aerobic respiration (which occurs in the presence of oxygen).

The anaerobic utilization of glucose to form ATP occurs as our body increases its muscle use beyond the capability of the phosphagen system to supply energy. In particular, the glycogen-lactic acid system, through its anaerobic breakdown of glucose, provides about 30 - 40 seconds more of maximal muscle activity. For this system, each glucose molecule is split into two pyruvic acid molecules, and energy is released to form several ATP molecules providing the extra energy. The pyruvic acid will then partly break down further to produce lactic acid. If the lactic acid is allowed to accumulate in the muscle, you will experience muscle fatigue. At this point, the aerobic system must kick in.

The aerobic system in the body is utilized for those sports that require an extensive and enduring expenditure of energy such as a marathon race. That is, endurance sports absolutely require aerobic energy. Lots of ATP must be provided to your muscles in order to sustain the muscle power that you need to perform such events without excessive production of lactic acid. This can only be accomplished when oxygen in the body is used to break down the pyruvic acid (that was produced anaerobically) into carbon dioxide (C02), water, and energy by way of a very complex series of reactions known as the citric acid cycle. This cycle supports muscle utilization for as long as the nutrients in the body last. The breakdown of pyruvic acid requires oxygen, and slows or eliminates the accumulation of lactic acid.

In summary, the three different muscle metabolic systems that we have at our disposal to supply the energy required for various activities are: (1) the phosphagen system (for 10-15 second "bursts" of energy), (2) the glycogen-lactic acid system (for another 30-40 seconds of energy), and (3) the aerobic system (providing a great deal of energy that is only limited by your body's ability to supply oxygen and other important nutrients). Many sports require the utilization of a combination of these metabolic systems. By considering the vigor of a sports activity and its duration, one can estimate very closely which of the energy systems are used for each activity.

Now that we have reviewed the three different muscle metabolic systems, we are going to predict which systems are used to play certain sports. Let's get started.

Procedure
After reviewing the three muscle energy systems, predict which system is used to supply the muscular energy that is required to perform each sport listed in Table 3. Choose from the list of "Energy Systems" provided. There may be some hints in an earlier section of this chapter that dealt with "Muscle Energetics." Your teacher will provide you with a separate handout and each student will do this exercise independently. The entire class can discuss the answers together.

Energy Systems

1. Phosphagen system, almost entirely
2. Phosphagen and glycogen-lactic acid systems
3. Glycogen-lactic acid system, mainly
4. Glycogen-lactic acid and aerobic systems
5. Aerobic system