|
The Physiologic Benefits of Long-Term Exercise on the Body
Following up last month's newsletter, where we explored the benefits of an active summer vacation as a starting point or upgrade for a lifetime of fitness, this month's newsletter will explore what happens to your body at the systems level when you step up your exercise.
I was speaking with some friends recently, and inquired about their son -- a 24-year-old who took a hiatus from his career in April to hike the Appalachian Trail. He's just completed the 2,200 mile trek from Springer Mountain, Georgia to Mount Katahdin, Maine. They told me that no matter how much he ate, he could not consume enough calories in a day to maintain his weight over the five-month trek.
I've heard the same story from many people who participate in endurance activities ranging from training for a marathon to cycling across the United States, and experienced this first-hand while training for bicycle races.
The same training effect can be achieved with consistent, long-term exercise. Many people become frustrated when they do not see immediate results from their exercise program. The advice I would give is to 'stick with it.'
What are the physiologic effects of long-term exercise on the body? Here's an overview of the major systems. (For a more detailed explanation of the physiologic, metabolic, and immunologic effects of long-term exercise, follow the link in the reference portion of the newsletter.)
The process of movement requires activation of the musculoskeletal system, while the cardiovascular and respiratory systems provide the ability to sustain this movement over extended periods. Consistent exercise (several times a week or more) causes these systems to adapt to stress and increase the body's efficiency and capacity for movement. The amount of change depends largely on the intensity and duration of the training sessions and the initial level of fitness. Conversely, discontinuing exercise results in loss of the efficiency and capacity that was gained through exercising. This loss is a process called detraining (www.cdc.gov)
Cardiovascular and Respiratory Systems
The primary functions of the cardiovascular and respiratory systems are to provide the body with  oxygen (O2) and nutrients, to rid the body of carbon dioxide (CO2) and metabolic waste products. In order to be effective, the cardiovascular system should be able to respond to increases in muscle activity.
The cardiovascular system, composed of the heart, blood vessels, and blood, responds predictably to exercise. The normal cardiovascular response to exercise is directly proportional to the skeletal muscle oxygen demands for any given rate of work, and oxygen uptake (VO2 or volume of oxygen uptake) increases linearly with increasing rates of work. The higher the physical demand, the harder the cardiovascular system works. The pattern of blood flow also changes when going from resting to exercising. At rest, the skin and skeletal muscles receive about 20 percent of the cardiac output. At maximal rates of exercise, about 80 percent of the cardiac output goes to active skeletal muscles and skin (Rowell 1986).
Skeletal Muscle
The main function of the musculoskeletal system is to move the body. In order to be effective,  muscles must adapt to change in function and demand. To accomplish this, the musculoskeletal system changes its ability to extract oxygen, choose its energy source, and rid itself of waste products.
Skeletal muscle is composed of two basic types of muscle fibers, distinguished by their speed of contraction: slow-twitch and fast-twitch. Slow-twitch fibers have relatively slow contractile speed, are able to extract oxygen from the blood with ease, contain a large number of mitochondria (the cells' powerhouse) and are slow to fatigue (Terjung 1995). Fast-twitch muscle fibers have a fast contractile speed, are less efficient at extracting oxygen from the blood and fatigue rapidly.
There is a direct relationship between predominant fiber type and performance in certain sports. For example, in most marathon runners, slow-twitch fibers account for up to 90 percent of the total fibers in the leg muscles. On the other hand, the leg muscles in sprinters are often more than 80 percent composed of fast-twitch fibers. There is some evidence for genetic predisposition to the percentage of fiber type and to one's ability to change fiber type with specificity of exercise.
Adaptations of Skeletal Muscle and Bone
Endurance training also increases the number of capillaries in trained skeletal muscle, thereby allowing a greater capacity for blood flow in the active muscle (Terjung 1995). The musculoskeletal system cannot function without connective tissue linking bones to bones (ligaments) and muscles to bones (tendons). Extensive animal studies indicate that ligaments and tendons become stronger with prolonged and high-intensity exercise. These structures also become weaker and smaller with several weeks of immobilization (Tipton and Vailas 1990). Inactivity results in muscle atrophy and loss of bone mineral and mass. People who exercise generally have more bone mass than those who are sedentary.
Metabolic Adaptations
Fat is a substance found in all humans and is a source of energy. Glycogen is the main form of carbohydrate storage and is found primarily in the liver and muscle tissue. Glycogen is converted to glucose by the body to satisfy its energy needs. With regular exercise, over a long period of time, the muscles are able to use fats for energy more efficiently. Aerobic exercise increases the efficiency with which the body uses fats and glycogen, using fats for energy and allowing carbohydrates (glycogen) to be preserved. Aerobic exercise essentially trains your body to use fats to supply energy for the working muscles.
Going back to the young man who was having difficulty maintaining his weight while hiking the Appalachian Trail, we are now able to answer the question why. Endurance training increases the capacity of the muscles to store glycogen (Kiens et al. 1993). The ability of trained muscles to use fat as an energy source is also improved, and this greater reliance on fat spares glycogen stores (Kiens et al. 1993). Our hiker was efficiently burning fat, not glycogen, to supply energy to his muscles.
Detraining
With complete cessation of exercise training, a significant reduction in VO
2
max (the maximum amount of oxygen the lungs are able take in - considered the best estimate of a person's cardiorespiratory fitness) occurs within 2 weeks, and all prior functional gains are dissipated within 2 to 8 months, even if routine low- to moderate-intensity physical activity has taken the place of training (Shephard 1994). Muscular strength and power are reduced at a much slower rate than VO
2
max. After 12 months, almost half of the strength gained might still be retained if the athlete remains moderately active (Wilmore and Costill 1994).
Conclusions
- Physical activity has numerous beneficial physiologic effects on the cardiovascular and musculoskeletal systems.
- Many of the beneficial effects of exercise training from both endurance and resistance activities diminish within 2 weeks if physical activity is substantially reduced, and effects disappear within 2 to 8 months if physical activity is not resumed.
- Consistency of exercise does make a difference. Choose your exercise, and stick with it!
Share your summer adventure photos & stories (or a caption) with me or with our Montclair Physical Therapy community at:
tweet to @MPTNYNJ
|
