Whereas strength training provides variety as you move from machine to machine and work different muscle groups, endurance exercise sessions require a lot of time doing the same movements and maintaining a sustained effort level. Basically nothing changes from the beginning to the end of the aerobic activity period. I've noticed that most regular endurance exercisers don't even look at the video displays that provide performance feedback during the workout. Instead, they cover the display with their newspaper, magazine or towel, so that they can't see how much time they have yet to exercise. Generally, they are doing the exercise because they know it is physically beneficial, but they do not find it mentally interesting.

Although there are occasions where a relatively steady pace is essential, such as running a 5K or 10K race, there are more interesting and effective means for conducting your exercise sessions. My preferred approach is known as interval training, and it provides almost unlimited variations in workout designs.

Basically, interval training involves interspersing more demanding and less demanding exercise segments within the workout. The training variables include (1) how hard to make the more demanding intervals, (2) how easy to make the less demanding intervals, (3) how long to make the more demanding intervals, (4) how long to make the less demanding intervals, and (5) how many intervals to perform.

For example, instead of treadmill walking at a steady 3.5 mph pace for 30 minutes, you may try 5 intervals of 6 minutes each. Let's say you begin with 6 minutes of slower-paced walking at 3.0 mph to warm up. You then perform 6 minutes of faster-paced walking at 4.0 mph. Because this is a more demanding pace than you normally do, you follow-up with 6 minutes at the more relaxed 3.0 mph pace. As this permits plenty of recovery time, you can now complete another 6 minutes at the faster 4.0 mph pace. Finally, you cool-down with another 6-minute segment at the slower 3.0 mph pace.

Overall, you have accomplished about the same workout as usual, namely, 30 minutes of treadmill walking averaging about 3.5 mph. However, the 2 faster-paced segments actually enhanced your physical development by requiring a higher work effort than your body normally produces, even for a total of only 12 minutes.

Perhaps just as important, you should find the interval training workout more mentally stimulating due to the variations in walking pace. Many people prefer interval training to steady state exercise because it places a different perspective on the workout duration. For example, when you keep the same pace for 30 minutes, it may seem that the time passes very slowly. However, when you perform higher-effort intervals, you may find that the slower-paced recovery intervals pass almost too quickly and that the total workout time seems considerably shorter than usual.

In the previous treadmill walking example, the harder and easier intervals were equal in duration, 6 minutes each. However, as you become better conditioned, you may make the higher-effort intervals longer and the lower-effort intervals shorter. Consider the following interval training approach to a stationary cycling workout in which the more demanding segments are twice as long as the less demanding segments:

1. 4 minutes warmup	75 watts resistance
2. 8 minutes higher effort	125 watts resistance
3. 4 minutes recovery	75 watts resistance
4. 8 minutes higher effort	125 watts resistance
5. 4 minutes cooldown	75 watts resistance

28 minutes total training time

Another means for enhancing the exercise effort within a similar half-hour time frame is to perform more intervals. Because each interval is relatively brief, the training intensity can be increased for the harder workout segments. For example, an interval treadmill running session could be programmed as follows for people with relatively high levels of fitness:

28 minutes total training time

If exercise time is a concern, interval training can provide a pretty challenging workout in a rather brief duration. Shortening the hard intervals to 2 minutes each increases the anaerobic energy component and requires less total workout time to experience an excellent training effect. The following sample exercise program for stationary cycling intersperses 2-minute intervals of high and low training effort.

20 minutes total training time

Interval training can be a very effective and efficient means for achieving higher levels of cardiovascular fitness. It presents almost unlimited variations with respect to the exercise protocol, and provides a more interesting workout than standard steady-pace training. Because interval training can be more physically demanding than even-paced exercise, be sure to check with your physician before giving it a try. You may also want to consult with a personal trainer or fitness instructor to design an appropriate interval training program for your present level of cardiovascular fitness.

Wayne L. Westcott, Ph.D., is fitness research director at the South Shore YMCA in Quincy, MA., and author of several fitness books including the new releases, Building Strength and Stamina and Strength Training Past 50.