Photo: Doing what I love to do at age 65. Deep in the Escalante Grand Staircase wilderness, Utah; on a eight day, eighty mile off-trail through hike. May 2019. Photo Credit Paul Gardner

In my previous What I Learned newsletter,  I defined interval training, how it works and the benefits of incorporating interval training into your fitness program. Interval training is not just for competitive athletes. As we age, our physical and athletic performance changes: our aerobic capacity declines, body fat increases, and we lose muscle mass. By incorporating interval training into our workout program, we can reverse some of these aging declines, maintain an active lifestyle and our athletic performance into our 80s.

Sport science has been telling us for twenty years the most effective way to improve or maintain aerobic capacity is by high intensity interval training. Research shows moderate intensity interval training is an effective and time efficient way to improve the body’s ability to burn fat. Studies also show that incorporating intervals in strength training improves muscle capacity by building strong, aerobically active muscles.

In this article, I explain how to use interval training to address aging and improve athletic performance.

Defining Intervals

Interval training is comprised of periods stress and recovery. There are several ways to gauge intensity: speed, pace, heart rate, power and rate of perceived exertion (RPE). I use heart rate and RPE because they are convenient. For cycling I prefer power. Heart rate training is based off your maximum heart rate (220 minus your age is the easy, but not very accurate formula for maximum heart rate). RPE is a 1 to 10 scale where 1 is very light activity and 10 is max effort activity. Remember heart rate lags well behind the effort produced. In very short intervals you may not reach your target heart rate.

The first training interval is Aerobic Threshold which is the upper limit of exercise where you are maximizing the oxidation of fat for energy. In other words, it is the level of exercise your body can maintain for long periods of time. Depending on the individual’s age and conditioning, your aerobic threshold is between 60 percent and 70 percent of your maximum heart rate. Using the Rate of Perceived Exertion (RPE) scale, your aerobic threshold would range from 3 to 5. Your aerobic threshold heart rate is generally 20 to 30 beats per minute lower than your anaerobic threshold.

The next training interval is Anaerobic Threshold: the point where your body switches to completely burning carbohydrates for fuel. The byproduct of burning carbs, lactate, is produced faster than the body can absorb it. Your breathing becomes labored and muscles fatigued. You can generally maintain an anaerobic threshold pace for twenty minutes. Again, depending on the individual age and fitness, your anaerobic threshold is between 80 and 85 percent of your maximum heart rate with an RPE range of 6 to 7. Your anaerobic threshold heart rate is generally 10 to 20 beats per minute lower than your aerobic capacity.

The final training interval is Aerobic Capacity: this is your VO2 max and your maximum sustainable heart rate. You can maintain this pace for 30 seconds to two minutes. Training at this level you are at 85 to 95 percent of your maximum heart rate and an RPE of 8 to 10.

Boosting your Ability to Burn Fat: Aerobic Threshold Intervals

There are two primary reasons for doing aerobic threshold intervals: to lower your body fat percentage and to train your body to rely more on fat as a fuel and spare your limited glycogen stores. Most of us carry extra weight in the form of body fat. As I detailed in my What I Learned newsletter, , added fat tissue is hard to lose.

To burn fat as a fuel, the body needs to move fat cells to the liver, and through a process known as gluconeogenesis, convert fat to glucose. At rest, our metabolism is low and we burn mostly fat for energy. As the intensity of exercise increases, the body defaults to burning more glucose or blood sugar for energy. We can teach the body to be a better fat burner by doing intervals below, and up to, our aerobic threshold. By repeatedly stressing the body through exercise at our aerobic threshold, the body adapts and our aerobic threshold moves higher. Becoming a better fat burner helps in losing unwanted body fat and the endurance athlete tap into a virtually unlimited supply of energy—fat.

Go Faster For Longer: Anaerobic Threshold Intervals

Your lactate threshold essentially defines the upper limit of your sustainable efforts in training and competition. Once you cross over and rely on your glycolytic system for energy, you’re moving on borrowed time.  While not closely tied with aging declines, anaerobic threshold training is important for long duration events: two hours or longer.

Long intervals combined with shorter recoveries are best for anaerobic threshold training. Long intervals cause a slight rise in blood lactate levels, increasing the acidity of the working muscles and reducing power output. Short recoveries ensure that lactate is not completely removed by the next interval. Over the course of a number of intervals up to your lactate threshold, the body adapts to this stress and you become better at tolerating the lactate and more effective at buffering it.

Improving VO2 max: Aerobic Capacity Intervals

Study after study shows intensive, vigorous training improves or arrest declines in your average VO2 max. Remember, VO2 max is an indicator or aerobic fitness; and aerobic fitness is the best predictor of overall health. In a 2001 study the average VO2max of a fit male cyclist in their 50s was 63, declining to 44 in their 60s; a drop of 30 percent. By comparison, the average VO2 max for males in their 60s is 35 and females 26. Train hard with high intensity intervals and you slow the high rate of decline in your cardio vascular fitness to 1 or 2 percent per year.

When you do aerobic capacity intervals, intensity, duration, number of repetitions, and recovery time must be only slightly greater than your estimated capacity. In other words, don’t exceed your physical limits. Stop your workout when it is obvious high end performance is declining. When you feel you can only do one more interval, just say no to that last interval.


Recovery is the key to avoid injury and fatigue from over training. If you are new to intervals start once a week and work up to three times per week. Plan on cycles of ten days to two weeks after which you dial back your training for three to five days and take a couple extra rest days. This periodization is called supercompensation. Over a week or ten days, even with a few easy workouts the body is breaking down. This is called catabolism. Taking time off allows the body to restructure tissues and organs. This is called anabolism and is how you gain the capacity to train harder in the next period.  Make sure you give yourself adequate time to recover from hard runs, cycling, and strength training. If you feel unusually fatigued or dropping in in efficiency or effectiveness—slow down and/or rest. You must respect fatigue if you are to gain fitness.

Parting Thought

There are lots of interval training articles on the internet. There are many gyms that offer interval training through boot camp classes and cross fit training classes. These are generally one-size-fits-all programs. I know from experience the size offered is not for the fifty plus age group. Interval training, especially high intensity interval training (HIIT) is not without risk.  Each one of us is different: in age, fitness, and goals. My advice is to go slow, seek advice and listen to your body.

Steven Markusen

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