SPACE PHYSIOLOGY
Muscles can change their character based on the kinds of stresses that they are exposed to. Most of you know this from personal experience. Normally, for day-to-day activities, your body movements take place within a certain personal comfort region. That means that you usually use the same muscle groups over and over in a repetitive habitual pattern. However, nearly everyone, at one time or another, has participated in sports or other physical activities that require muscle groups that have not been utilized in some time! Usually the next day, or at least by the second day afterward, you feel a soreness whenever you move; this soreness reminds you that you have moved out of your comfort region and into quite a different region! In order to move beyond our comfortable level of physical activity, we must activate muscle cells that we don't normally use heavily. That is, we change the pattern of usage of our muscles to fit the physical needs we are facing. In a similar way, we can stop using certain muscle groups frequently because we become ill or because our activity habits change. In this case, you've probably noticed that those muscles become weaker. In either case, we are actually changing the kinds of muscles that we have.How can this happen? Well, muscles are constantly being built and rebuilt. In only 7 to 14 days, half of the protein in human muscle cells is broken down, discarded and replaced. It's as if you had a contractor constantly tearing down the rooms in your house and rebuilding them with new materials. After a month, you've got a brand new house. Next month you'll have another brand new one. If the number and strength of your muscle contractions change (that is, if you drastically change the level of activity that you are involved in), your muscles respond by changing their character.
The point is that the muscles adapt to new situations. It seems that essentially all body systems are capable of adapting in response to rapid changes in the environment. When we need our muscles, we can activate them almost immediately. When we don't need certain muscles, they can go into what could be called a "hibernation" mode. This hibernation mode, however, can cause the muscle fibers to actually change their type. Just think of someone you have known who has worn a cast on their leg for a while. When the cast is removed, the leg appears thinner and it is definitely weaker for a while until this person can rebuild his or her leg muscle strength. In this and other situations where a person does not use his or her muscles for a period of time, the muscles themselves begin to waste away, or "atrophy." For instance, people who are confined to bed during an illness or even astronauts who, while in space, do not require the use of their "anti-gravity" muscles both experience this natural atrophy. Such muscle atrophy can cause problems for people here on Earth and for astronauts who fly in space.
Muscle change can begin quickly. Dr. Baldwin and his colleagues carried out ground-based experiments with rats that are actually involved in activities that simulate liking weights. They are not standing up curling weights, obviously, but their feet push against an object. They participate in about two minutes of weight training a day and, within two days, changes in their muscle fiber types can be observed.
| Figure 10. Gravity experiment with rats. |
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A group of Dr. Baldwin's rats also went up in space to determine just what happens to their muscles when gravity is removed. On their return, Dr. Baldwin discovered that being in micro gravity for 14 days had converted a large portion of their muscle fibers from Type I to TypeII. This is because, while in space, the rats no longer needed their legs to balance and control their bodies against the force of gravity. They floated around from one location to another (Figure 10). Therefore, their muscles actually began to change their character during space flight. Similarly, it is believed that astronaut's muscles weaken while in space because they do not have to use them as they normally would on Earth. When the astronauts return home, they experience gravity as much more of a force to reckon with than they had ever noticed before.
The space flight investigation that was developed by Dr. Baldwin and his research team was designed to carefully examine rat muscle changes that take place in space in order to try to understand how human muscles also change. In general, his experiment looked at the effects of space flight on:
- muscle mass and functional properties of skeletal muscles;
- running endurance capacity;
- the muscle cell's ability to use oxygen to convert nutrients into energy.
Hypothesis 1
In microgravity, tension is reduced on muscles that support the body
against gravity, resulting in a loss of muscle mass and an accompanying
loss of muscle strength.
Hypothesis 2
Exposure to micro gravity will cause a reduction in the endurance capacity
of skeletal muscle.
Hypothesis 3
The loss of endurance capacity will be due to a change in the muscle
cell's ability to convert nutrients into energy.
Remember to keep these hypotheses in mind as we review some of the important measurement sets that were actually carried out during Dr. Baldwin's experiment in space. We will return to them at the conclusion of our examination of Dr. Baldwin's experiment.
Before we begin looking at Dr. Baldwin's space flight results, we will participate in a set of activities (Student Investigations) that were designed to help you understand more clearly how muscles work.