We've come to the time of year outdoor exercisers loathe. Days are short, and who wants to run in the dark? And the weather turns nasty, an ugly combination.
So, most of us bring our exercise indoors. For some, this can offer quite a surprise, depending upon which exercise you choose to do indoors.
The reason is exercise training is task specific. Train for running, and your body will make adjustments specific to running that do not necessarily transfer to other exercises.
The most startling transition would be for a runner who decides to come indoors and take up swimming. This is because the modes of exercise are so different, and the muscles involved differ as well.
Running challenges the muscles of the legs, whereas swimming is an upper-body exercise.
But, you also could get a surprise even if you use the same muscles from one mode of exercise to another. Shifting from jogging to half-court basketball, for example, still emphases exercising the legs. Even so, the transition will not be a smooth one.
Several factors are at play. Let's examine some of them.
Oxygen supply
Exercise that entails ongoing effort is aerobic in nature and requires lots of oxygen.
The working muscles must receive a generous and uninterrupted supply of oxygen in order to create the energy needed to keep contracting and producing movement.
This presents two challenges to the body. One is delivering the oxygen, and the other is using it once it has been delivered.
Delivery of oxygen is a "central" function of the body's physiology. It entails teamwork from the lungs, heart, blood and vessels.
The lungs process oxygen and pass it though to the bloodstream where it is picked up by the hemoglobin of red blood cells.
Once loaded with oxygen, the heart acts as a pump to push the blood to its destination — the peripheral areas of the body where the working muscles are located.
Let's assume you are a runner and that your heart does a great job of pumping lots of oxygen-rich blood out to your working leg muscles.
At the delivery sites, tiny blood vessels (capillaries) have been created to allow blood to get close to the working muscles for easy delivery.
What's more, because of your daily runs, each working muscle cell contains highly efficient internal metabolic machinery. The machinery is composed of structures loaded with enzymes that can take sugar and fat and with the help of oxygen convert it into usable energy.
Someone who has been trained as a runner will have a strong oxygen delivery system (a big powerful heart, for example), as well as copious capillaries and muscle cells equipped with the necessary metabolic machinery able to use the oxygen that is delivered. The central and peripheral components are linked.
But when the runner dives into the pool and starts swimming, the central and peripheral components are uncoupled.
The delivery system is still more or less intact, and the heart is still a powerful pump. But because the upper-body muscles have not been trained, there are no extra capillaries, and the metabolic machinery in the muscle cells is undeveloped.
Thus, even though oxygen-rich blood can be pushed toward the working upper-body muscles, the oxygen cannot easily make its way into the muscles, and even if it could, the muscles would not be equipped to use it.
The result is the runner would be exhausted quickly after just a few laps in the pool.
In time, of course, with training as a swimmer, the necessary peripheral changes will occur in the upper-body muscles, and the central and peripheral factors will be linked once again.
Thankfully, the fitness earned as a runner will provide a nice foundation for training as a swimmer, and you will make good progress. It certainly won't be like starting from scratch.
When the same muscles are used but in a different way, the results are similar, but less striking, and the transition phase will be shorter.
Shifting from running to cycling, for example, will require a transition phase to re-establish the link, even though the leg muscles are involved in both.
Other energy systems
There are actually three energy systems used by muscles that are exercising.
One system uses oxygen — the aerobic system — as described above. The other two systems do not use oxygen. They are anaerobic.
In the example of a runner who decides to shift to half-court basketball, a surprise may occur even though both require seemingly similar work by the legs.
Running is an aerobic exercise, whereas half-court basketball, with its quick moves, sprints, leaps and stops, has a large anaerobic component, as well as an aerobic component.
The runner would not have developed this anaerobic component and likely would fatigue rather quickly in a competitive half-court game. He would do better in a full-court game, because his aerobic capacity would contribute more during the runs up and down the court.
The bottom line
Your muscles work just like most other things in life. When you make a change, there will be a transition phase.
Therefore, expect it when you shift from one exercise mode to another, and don't get discouraged.
Bryant Stamford is an exercise physiologist and director of the Health Promotion and Wellness Center at the University of Louisville. If you have questions about sports injuries, health, exercise or fitness, write to Body Shop, Gannett News Service, care of The Courier-Journal, 525 W. Broadway, P.O. Box 740031, Louisville, KY 40201-7431, or e-mail bryant@louisville.edu.