All healthy people can benefit from some form of interval training. They can pick up the pace for a few seconds while walking, running, cycling, swimming, skiing or skating, and then slow down when they feel the least discomfort.  Intervals for most people means to warm up by starting your exercise slowly, then pick up the pace for a short time until you start to feel the least burning in your muscles or the beginning of shortness of breath.  Slow down and keep going at a low level of intensity until you feel fresh, and then pick up the pace again. Keep on alternating these pick-ups and slow-downs until your muscles start to feel heavy, then cool down at a very slow pace for a few minutes and your workout is finished.

If you have done your interval workout properly, your legs are likely to feel a little sore on the next morning, so you should take a recovery day where you go very slowly.  Most people can use a 48-hour schedule with each interval day followed by a slow recovery day.  When your muscles should feel fresh again, you can repeat your interval workout.

The American College of Sports Medicine (ACSM) surveyed more than 4,000 fitness professionals and found that High Intensity Interval Training (HIIT) was the most popular trend in fitness for 2018 (ACSM Health & Fitness Journal, Dec 2017).

Benefits of Intervals
The interval training technique has been used in all endurance sports for many years, but it wasn’t until a couple of decades ago that George Brooks of the University of California at Berkeley showed how interval training can make anyone more fit and a better athlete (American Journal of Physiology-Endocrinology and Metabolism, June 2006). Further research has shown that interval training can also help to control blood sugar more effectively than continuous training, helping to prevent and treat obesity, diabetes, heart attacks and certain cancers  (Int J Sports Med, published online November 6, 2014).

Your muscles burn sugar, fat and protein for energy, but sugar is the most efficient fuel since it uses less oxygen to convert sugar to energy. Sugar is the major fuel for your muscles during intense exercise. The faster you move, the greater percentage of sugar your muscles use for energy.   The limiting factor to how fast you can move is the time it takes to move oxygen into muscles. In a series of chemical reactions, the sugar, glucose, is broken down step by step, with each step releasing energy. When enough oxygen is available, the glucose releases all of its energy until only water and carbon dioxide remain and the carbon dioxide is blown off through your lungs.

If your heart cannot pump enough oxygen to your muscles to meet their needs for energy, the chemical reactions stop at lactic acid which accumulates in the muscles and spills over into the bloodstream. Lactic acid makes muscles acidic, causing a burning feeling in muscles that makes you slow down. However, if you can get rid of the lactic acid and convert it to energy to power your muscles, the burning disappears and you feel better.  Since the cause of the muscle burning and shortness of breath is lack of oxygen, slowing down allows you to catch up on your oxygen debt, which in turn converts lactic acid all the way to carbon dioxide and water and supplies a tremendous amount of energy to power your muscles.  Interval training causes you temporarily to exceed your lactic acid threshold, so you can increase the rate that muscles can take in and use oxygen and you can exercise at a faster pace.

The exciting news is that lactic acid requires less oxygen even than sugar does to be converted to energy and that means that while you are clearing excess lactic acid from your bloodstream, you can go much faster. Anything that helps muscles to break down lactic acid faster will make you a better athlete because it will allow you to exercise more intensely to increase your endurance and allow you to move faster when you are tired (Sports Medicine, 2006;36).

Inside each muscle cell are hundreds of mitochondria, the little furnaces that burn fuel for energy. Lactic acid is burned for energy in the mitochondria, and anything that enlarges the mitochondria builds a bigger furnace and helps to increase endurance. Lactic acid is carried from the cells into the mitochondria by special proteins called lactate transporter molecules, so anything that increases these molecules will build endurance. An enzyme called lactic acid dehydrogenase is needed to start the reaction, so anything that increases this enzyme will also help. Interval training does all three: it enlarges the furnace (mitochondria), increases lactic acid transporter molecules, and increases the amount of lactic acid dehydrogenase.

How I Use Interval Training
I am 82 years old and have been exercising all my life.  I do interval training on my bike four days a week and long slower rides on the other three days. I warm up for a mile or two, then do standing 50-pedal-stroke intervals that make me short of breath and cause some muscle burning.  Each interval takes around 24 seconds.  I then go slowly until I recover my breath completely and my muscles feel fresh and then start my next interval.  I usually can do 21 of these intervals in a workout.  Near the end of my workout, my muscles start to feel stiff and heavy.

Intervals can increase your chances of injuring yourself, so I have learned from my many previous injuries when to take a day off.  My legs are always stiff and heavy when I get up in the morning.  I never plan to take a day off, but if my muscles do not feel fresh after a 5 to 10-minute slow warm up on my bike, I always take the day off.  I usually take off every 7th to 11th day. I have never had to take off from my long slow rides. This caution has protected me from being injured for the last six years. See Interval Training Helps Your Heart
Interval Training for Sports
Short Intervals are Best

CAUTION: People with blocked arteries leading to their hearts can suffer heart attacks with intense exercise. Check with your doctor before starting a new program of intense exercise or increasing the intensity of your current program.

Checked 11/11/22