A certain decline in physical capacity may be inevitable as we age, but what can we do today to slow the process? To some extent, the aging process is physiologically predetermined: we lose about 1 percent of our physical capacity each year starting at about age 35 due to decreased cardiovascular, respiratory, musculoskeletal, and metabolic function. It happens to Olympic athletes and weekend warriors alike but staying fit can minimize the loss.

The decline in strength and speed tends to concern us most because we can see and feel the difference as we get older. Strength and speed peak in men between the ages of 20 and 25 and in women at around 18—unless you train. With training, you can keep improving until your early thirties. After that point, strength will decline slightly and then remain at the same level for maybe 20 years (if you stay active).

Unfortunately, you'll never again be as fast as you are in your 30s; speed tends to decline steadily from there. Speed and strength deterioration are primarily caused by the loss of lean muscle mass at a rate of about 1 percent per year, as well as the lack of recruitment of fast-twitch muscle fibers. In addition to a dwindling number of fast-twitch fibers, we also lose the ability to recruit those fibers as we age, especially if we don't work out hard. However, the more active you are, the more muscle mass you start with, and the slower you lose it.

Because lean muscle mass decreases largely due to lack of stimulation, high-intensity speed and power workouts become more important as you age. Obviously, weight training is instrumental in maintaining and building muscle, but it's most effective when you make complete muscle fatigue your goal during training sessions. This will ensure the most muscle-building bang for your buck and target those elusive fast-twitch fibers.

Here's how it works: If you usually feel fatigued after curling 60 lbs. 12 times, think again. You haven't really totally fatigued the muscle; you just can't lift 60 lbs. through the full range of motion anymore. You might still be able to lift 40 or 50 lbs., though. When you can no longer do a repetition with 60 lbs. on the bar, take off some weight and try to do a few more reps with 40 lbs. and so on until you can't lift any more weight. Remember, the goal is to totally fatigue the muscle, which means it can no longer contract at any level. Be sure to always lift and lower through a full range of motion; this will work the muscle completely and fatigue it faster.

How much time can you buy yourself by doing this diligently? Every person is different, but it's not uncommon for an avid exerciser to maintain the strength he or she had at age 30 for another 20 to 30 years.