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Author(s): Joanne R. Lupton, PhD
Showing Results for: astronauts Return to Presentation

Human Muscle Response Under Different Conditions of Unloading

This figure illustrates what happens in human muscle under different conditions of unloading, or a state of non-weight bearing, especially during space flight. It summarizes information that has been gathered from human astronauts living in space, as well as human subjects exposed to unique conditions on Earth that stimulate protein breakdown, as discussed in the previous slide. These studies involve: 1) Bed Rest; 2) Limb Immobilization (IM), in which the lower limb is placed in a cast and the individual is not allow to ambulate (this is a common practice when one breaks a bone in the leg); and 3) Unilateral Lower Limb Suspension (ULLS), in which the individual wears a shoe with a very thick sole so that the opposing leg cannot make contact with the ground (the opposing leg is placed in a sling and the subjects can move about only by using crutches). This latter paradigm demonstrates that when we can no longer perform normal weight bearing activity, as in standing and walking etc., the muscle undergoes an atrophy response. The data focus on different components of one’s calf muscles—a bulky group of muscle in the back of the lower leg.

In the graph above, the vertical axis presents the decrease in the size, or girth, of the muscle, while the horizontal axis depicts the length of time that the muscle is unloaded. The findings clearly suggest that 1) all types of unloading paradigms result in similar degrees of loss in muscle fiber size; and 2) the longer the duration of exposure, the greater the loss in muscle fiber size. Note that all the different points on the graph are clustered together. This tells us that muscle atrophy is not necessarily linked to space flight. Any condition that creates a state where the muscles cannot perform their primary function of generating force and creating movements under loading conditions will cause muscles to atrophy.

What do you think the effects will be on muscle strength if one lives a lifestyle described as a “couch potato?”

Suggested Reading:
Adams, G. R, Caiozzo, V. J., & Baldwin, K. M. (2003). Skeletal muscle unweighting: spaceflight and ground based models. J. Appl. Physiol. (Invited Review) 95: 2185-2201.

What we have here are data plots for leg muscles under a variety of conditions. You see ground-based analogs that we call bed rest. All one needs to do is put individuals in bed and not let them get out for many days. We have kept individuals in a bed rest state for as much as 90 days. While we all like to sleep and take things nice and easy, it is not easy for the subjects to participate in this type of an experiment. We can also illustrate some interesting phenomena by immobilizing limbs. If you have ever broken your leg, and the leg is placed in a cast, the muscle is basically immobilized. Usually, the muscle is not allowed to bear weight. Just by not letting the limb come in contact with the ground, we can see that these muscles are affected. Another type of a situation illustrated in the red figures is called ULLS, or Unilateral Limb Suspension. This does not involve any kind of injury to the muscle. All we do is have the subject wear a large shoe that has a lot of padding to it so that the opposite leg cannot come in contact with the ground. The individual walks around on crutches, and the opposing limb is put into a sling to help the individual support the leg without it coming in contact to the ground. This is a unique experiment in that it also shows that there is a tremendous impact on the muscle by not being able to make contact with the ground, or to assume any ground reaction forces. The longer we prevent the limb from weight bearing or being able to bear force and so forth, the greater the decease in muscle size. Whether an individual is in space, in bed rest, or incapacitated in some way, the end result is a similar response. The muscles will wither away.