Editor’s note: This is an excerpt from the new VeloPress book, “Fast After 50,” by Joe Friel.

Research tells us that the performance decline we typically experience with aging has a lot to do with how active we are while growing older. For example, a paper released in 2000 examined the combined effects of age and activity level over time. The researchers reviewed 242 studies comparing aging and VO2 max involving 13,828 male subjects. Each of the subjects was assigned to one of three groups based on how active they were: sedentary, moderately active exerciser, or endurance-trained runner. Aerobic capacity was highest in the runners and lowest in the sedentary group. No surprises there. The aerobic capacity changes per decade of life were sedentary, 8.7 percent; active, 7.3 percent; runners, 6.8 percent. What this means is if, at age 30, a man had a VO2 max of 60 and then for the next 30 years didn’t exercise and lived a “normal” (sedentary) life, he could expect his aerobic capacity at age 60 to be around 46. If he was moderately active, it would be about 48. And if he trained as an endurance runner, it would be in the neighborhood of 49. Those are not significant numeric changes. But for normal folks who generally see VO2 max declines of 10 percent and greater for a 10-year period, these numbers are really high.

But regardless of the actual size of the change, here’s the main message: The study further reported that the subjects who were endurance-trained runners significantly decreased their volume (miles run per week) and training intensity as they got older. That’s a common practice with aging athletes. So maybe it’s not simply working out that maintains aerobic capacity and therefore, in part, race performances; instead, it is how much training you do and how intensely you do it.

Yes, I realize that I am repeating the same message from earlier chapters in “Fast After 50.” It’s a critical lesson for getting faster regardless of your age.

Here’s another. In 1987, Dr. Michael Pollock and his colleagues at the Mt. Sinai Medical Center in Milwaukee, Wisconsin, reported a watershed finding in the understanding of performance and age. Dr. Pollack reported an astonishing finding that lent further credibility to the idea that very strenuous exercise was an aerobic capacity preserver. Well-trained, competitive endurance runners with an average initial age of 52 were able to totally maintain VO2 max values over a 10-year period.

In the full group of 24 athletes, VO2 max went into a tailspin, with an average nine percent decline during the 10 years of the study. However, Pollock discovered that 11 of the 24 had continued to train vigorously and were still competitive a decade after the initial testing. When he categorized the results, he found that the more active athletes had absolutely maintained their average VO2 max at a steady 53 ml/kg/min (10 years earlier it had been 54) despite being now in their early 60s. The less-active subjects had seen their VO2 max values plummet by 12 percent. In Pollock’s paper, “more active” meant that athletes continued to do high-intensity workouts while maintaining their volume.

Just like Pollock’s study, other research on aging in experienced endurance athletes generally supports the notion that in order to reduce the decline in aerobic capacity with advancing age, training must be intense.

That typically means training anaerobically — at or above the lactate threshold. For experienced endurance athletes, an exercise regimen based solely on long, slow distance (LSD) will do little to improve or even maintain your aerobic fitness status over the years.

Of course, you may not be able to improve it, especially if you’ve been training intensely for many years. Even with such focused training, there is still the age-related performance decay that research has repeatedly shown us is inevitable. As explained earlier, your history of exercise type and consistency along with your current age have a lot to do with how great your gains may be in the future. If your training has been inconsistent and you are now in your 50s, you’re probably more physiologically capable of significantly reversing the downward performance spiral through training than if you have been training consistently or are in your 70s.

That doesn’t mean a 70-year-old can’t make performance gains after a few years of slacking off. It’s just somewhat harder to accomplish due to physiological changes that occur with advancing age, such as reduced hormone production. Hormones have a lot to do with the response to training, fat accumulation, and muscle loss. We take a deeper look at this in later chapters of “Fast After 50.”

There is also the matter of genetics. Some people apparently chose the right parents. They are endowed with a great capacity for hard workouts with little risk of breakdown. They can do high-intensity training and experience a quick and positive response. Others of the same age can do the same workouts over the same time and see little or no performance change. Unfortunately, life isn’t always fair.

There’s little doubt that intense training is risky. As workouts become more challenging, the chances of injury, illness, and overtraining increase. Intense training needs to be modulated in regard to your current level of fitness and personal age-related limits. Those limits may have become magnified by the absence of high-intensity training in recent years. If that’s the case, you need to be extra conservative with training changes as you ease into what I’m going to propose.

If high-intensity training is something you haven’t done for a long time or have never done, you must consider several things: the type of hard workouts, the frequency of hard workouts, your short-term recovery from hard workouts, and your nutrition relative to hard workouts. I cover these points in Chapters six, seven, and eight. But for now, let’s look at high-intensity training designed with one critical goal: aerobic capacity.

Guidelines for High-Intensity Training

The most effective and efficient use of your time and energy to increase training intensity is to do some type of interval training.

Important point: When you perform intervals, the absolute intensity, the duration of the repetitions, the number of repetitions, and the duration of recovery between intervals must be only slightly more challenging than your estimated current capacity for physical stress.

What that means is that you must know or be able to sense your physical limits and not exceed them. It’s best to take a conservative approach to intervals if it’s been a while since you last did such a workout. Don’t try to get in shape in just a handful of these sessions. Too much, too soon will nearly always result in a breakdown of some sort, such as injury. Take a long-term approach — as in several weeks — to safely produce the results you want.

Because an interval session can be quite stressful, it should be preceded by a gradually progressive warm-up. This approach has also been shown to improve workout performance. Stop the workout when a reasonable workout goal is attained, when it is apparent that high-end performance is declining, or when the effort feels unusually high for the output (pace, speed, power). My advice to the athletes I’ve coached for the past 30-some years has always been the same: Stop when you know you can do only one more interval. The last interval is the one to be most wary of, so simply don’t do it.

Note that I am not going to describe specific interval workouts for each individual endurance sport. I am going to assume that as a lifelong athlete, you are familiar with interval training and that you either have a record of or remember the interval workouts for your sport that have worked for you in the past. If this is not the case, you will need to research the proper interval training for your sport.

I am also not going to prescribe specific methodology for measuring intensity. Again, I am going to assume that your years in your sport have given you a strong foundation in monitoring the intensity of your workouts. You can use a rating of perceived exertion (RPE), heart rate, lactate threshold heart rate, or functional threshold power if you are using a power meter and are familiar with that measurement. Go with whatever you are comfortable using and whatever is most repeatable for you.

I recommend that you keep track of your heart rate even if you do not use it to measure intensity. Heart rate is a useful component in determining the state of your fitness and the rate at which you are improving it, as you will see below.

If you have been diagnosed with coronary heart disease, have concerns about your heart health, or are taking statins or other medications that alter heart rate, then you should consult your doctor before starting an interval training program. Fortunately, the risk of heart attack among otherwise apparently healthy athletes as they age is quite low.

If you’ve previously done intervals throughout your sport career but have had a gap in recent years, then you know how to get started again. Just be conservative (as I mentioned before) with the progression of this workout. If LSD training has been your only training method, you may need some guidelines for getting started with interval training. Here is what I suggest for the first few sessions for someone who has not done interval training recently.

Step 1. Warm up 10–30 minutes gradually, ratcheting intensity up to a moderate effort. It’s common for older athletes to need more warm-up time than young athletes. Warm-up may also vary by sport. For example, it is typically longer for cycling than for running. Swimming generally falls between these two.

Step 2. Do 3 × 3-minute intervals with each interval at or slightly below lactate (anaerobic) threshold and 1 minute of light recovery between them. As I mentioned before, the gauge of intensity for an interval workout such as this may be based on heart rate, pace, speed, power, or perceived exertion. Some of these measurements are better than others, depending on the sport (Appendix C of Fast After 50 includes data). Heart rate–based intensity is perhaps the worst way to gauge how hard to work with intervals, especially short ones of a few minutes or less, as heart rate rises slowly during each interval. It may take several minutes to achieve lactate threshold, during which time you are left guessing how hard to work. Most athletes err on the side of starting intervals too fast to force heart rate up quickly, and then they slow down later as the goal heart rate is finally achieved. This is just the opposite of what should be done; instead, you want to finish each interval with a slightly higher intensity than when you started.

Step 3. Cool down with several minutes of easy exercise. As with the warmup, the duration of the cool down depends on the sport, with cycling typically long and running relatively short.

At first you should most likely do only one such interval workout in a seven-day period. Over time, however, you can increase the number of weekly sessions to two if you have the stamina and time. It’s not common for senior athletes to be capable of doing more than two of these in a week. I’ve coached some who could, but they are rare and more likely in their 50s than their 70s.

An indicator of improving fitness is that the speed or power of your intervals increases relative to your heart rate (this is where keeping track of your heart rate during interval training comes in, as mentioned before). That’s always a sure sign of improving aerobic fitness. To determine this, divide the combined average speed (run, swim, Nordic ski) or normalized power (bike or row) for all of the intervals by their combined average heart rate (see my book, “The Power Meter Handbook” for details on this). The increase in this ratio is the indicator that your aerobic fitness is improving. TrainingPeaks calls this ratio the “efficiency factor” (EF).

For example, let’s say you did 3 × 3-minute intervals running at just below lactate threshold, and the combined average pace of the intervals was 7:15 per mile. To convert pace to speed, divide 60 by 7.25, and you’ll find that the speed of the intervals was 8.3 mph. That’s a speed of 243 yards per minute (8.3 × 1,760 yards in a mile divided by 60, the number of minutes in an hour). Yards and minutes are easier to work with than miles and hours for our purpose. And let’s say the combined average heart rate of all of the intervals was 150. That means your EF for that workout was 1.62 (243 ÷ 150 = 1.62). As that number increases, your aerobic fitness is also improving.

Now, if you don’t have a pacing or power device, or even a heart rate monitor — or if using those tools seems like too much math and measurement — we can simplify the decision regarding when to bump your intervals up. If you are familiar with the RPE method mentioned earlier, you probably know it isn’t nearly as accurate as power meters, pacing devices, and heart rate monitors, but it’ll do in a pinch. Simply do the intervals at the same perceived effort as usual and make a conscious determination during the intervals whether you seem to be going faster than the last time you did them. If you are on a measured course, you can time your intervals. This effort will never be “easy.” But if you do the intervals at roughly the same effort each time, over time you will go faster as fitness improves. The RPE method is best on a measured course using a stopwatch. Old school, but it still works.

When you are going faster at the same level of effort, it’s time to make some changes. As the EF for your interval sessions rises (meaning you get faster at the same effort), you will need to increase the number of intervals. As these longer interval workouts become more tolerable with a rising EF, begin to gradually increase the duration of the individual intervals. The recovery time between them should be about one-third of the preceding interval time. For example, after a 3-minute interval, recover for 1 minute. When you can manage about 20 minutes of total combined interval time in a single workout (such as 4 intervals of 5 minutes each at lactate threshold with 1 minute 40 seconds of recovery between them), then you are ready to move on to more intense intervals.

The next stage in the progression for those just getting back into high intensity training is to move up to aerobic-capacity intervals. To do this, start with 5 to 10 intervals of 30 to 60 seconds each with recoveries between them of the same duration, for a total of 5 minutes of intervals in a single session (e.g., 10 × 30 seconds or 5 × 60 seconds). This time, however, do them above the lactate threshold. As a memory refresher: Lactate threshold is the intensity at which you first begin to experience labored breathing. You should feel as if you are working quite hard on these aerobic-capacity intervals, but they aren’t quite an all-out effort.

Aerobic-capacity speed or power is the highest average speed or power you can sustain for about 5 minutes. If you really want to know what that number is for you right now (it varies with changes in fitness), do an all-out 5-minute time trial on your own to find your average speed or power for that duration. Or you could have a VO2 max test done in a university lab, medical clinic, health club, or sporting goods store or by a coach. If you go the test route, be sure to ask the technician what your top-end or VO2 max speed or power was.

Rather than doing a time trial or lab test, you can get a rough estimate of your VO2 max output by multiplying your speed or power at lactate threshold (perhaps found in the round of intervals described above) by 1.05 and 1.2. That will give you a range to shoot for on each of these aerobic capacity intervals. Start at the low end of this range, and over time gradually increase the intensity to the upper end.

In Chapters five and six of “Fast After 50,” I offer the details of doing the advanced aerobic-capacity interval workout.

If you prefer to be somewhat less structured in your training, do these initial VO2 max intervals as “fartlek.” That’s a Swedish term for “speed play” that has been commonly used by athletes, especially runners, for the past 70 years to mean “unstructured intervals.” To do such a workout, start with a warm-up. Then go fast when you feel like it, go slow when you feel like it, and complete as many of these fast segments as you feel like doing. As explained earlier, you still must pay attention to how you feel throughout the workout, being cautious not to do so many fast segments that you can’t fully recover within three days.

After a few weeks, and once you are fully adapted to high-intensity training, your aerobic capacity should be increasing. You can measure that as before with the 5-minute time trial, or you can undergo a technician-administered test. You may also simply discover that you’re going faster in your workouts for the same effort. Depending on your fitness at the start of this program, your rate of workout progression, the number of such sessions completed, and your age, it would not be unusual to see a five percent improvement in VO2 max in about eight weeks. And by continuing such training in future years, your rate of loss of aerobic capacity should be no greater than about five percent per decade.

What if your personal limiter isn’t aerobic capacity but rather lactate threshold? Just because nearly all of the research has found VO2 max to be the aging athlete’s primary limiter, it doesn’t necessarily mean it’s that way for all serious senior athletes.

First let me say that this situation is rare. Nevertheless, it would likely be the case if your known VO2 max or 5-minute time trial speed has declined by less than about five percent per decade, or around half a percentage point in a year. If this describes your recent experience, then you may benefit from devoting a good portion of your training to sub-lactate-threshold sessions, as described earlier. That could mean fewer aerobic-capacity interval sessions. Note, however, that some research has found that doing aerobic capacity intervals is also an effective way to boost lactate threshold. So both types of intervals may improve your fitness even if your aerobic capacity is stable.

Joe Friel’s groundbreaking new book, “Fast After 50,” is for every endurance athlete who wants to stay fast for years to come. For runners, cyclists, triathletes, swimmers, and cross-country skiers, getting older doesn’t have to mean getting slower. Drawing from the most current research on aging and sports performance, Joe Friel — America’s leading endurance sports coach — shows how athletes can race strong and stay healthy well past age 50. In “Fast After 50,” Friel offers a smart approach for athletes to ward off the effects of age. Friel shows athletes how to extend their racing careers for decades — and race to win.