by Gabe Mirkin, MD
Athletes use interval training to make themselves faster and stronger, and everyone with a healthy heart can benefit from this technique. A typical interval workout for non-competitive exercisers would be a session of jogging, walking or cycling in which they:
• Warm up by moving slowly for about 10 minutes,
• Pick up the pace until they feel a slight burning in their muscles (this usually takes 10-20 seconds),
• Slow down as soon as they feel this muscle burning, and go slowly until the burning is gone and breathing is back to normal.
• Alternate picking up the pace for 10-20 seconds and slowing down until they have recovered, then cool down and stop the workout whenever their muscles start to feel tight or tired.
A fit person should be able to do between 10 and 20 repetitions of these 10-20 second bursts in a single workout. For non-athletes, these intervals do not have to be done at 100 percent effort; 60 to 80 percent effort is sufficient. People in good shape can often work up to 50 or more of these short intervals before their legs tire.
The burning can come from acidity caused by a buildup of lactic acid in muscles, and accumulating lactic acid in your muscles makes you more fit, can help to prevent diseases and may even prolong your life.
Intervals to Increase Endurance
When you exercise, your muscles burn sugar, fat or protein in the presence of oxygen to produce energy. If you exercise so intensely that you become very short of breath and your muscles can’t get enough oxygen, lactic acid accumulates in your muscle fibers. An older disproved theory was that lactic acid interferes with a muscle’s ability to contract, so you feel tired. However, more recent research shows that muscles contract more efficiently when lactic acid accumulates in them (Science, August 26, 2004).
Electric currents cause muscles to contract. This electricity is generated by cell membranes causing potassium to move inside cells and chloride ions to stay outside. With vigorous exercise, potassium ions accumulate outside cells. When large amounts of potassium ions accumulate outside cells, electricity is not generated and the cells cannot contract. Another ion called chloride accumulates outside cells and prevents potassium from getting back inside cells. Lactic acid removes the chloride, so it is easier for potassium to get back inside cells. Lactic acid increases the ratio of potassium inside cells to the amount outside, and this helps the muscle contract with more efficiency.
Interval training teaches your muscles and liver to use lactate for energy much faster than just doing continuous training (Am J Physiol, 1983;244:E83-E92). The faster you can use up lactic acid,
• the more quickly you relieve the acid burning in muscles that slows you down, and
• the faster you can go because lactate requires less oxygen than even sugar does.
Interval training has been used in all endurance sports since the 1920s. George Brooks of the University of California at Berkeley showed why interval training can increase endurance (Am J Physiology-Endocrinology and Metabolism, June 2006). A major fuel for your muscles during exercise is glucose (a sugar). In a series of chemical reactions, 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 carbon dioxide and water remain; these are blown off through your lungs. However, if you exercise so intensely that you can’t get all the oxygen you need, the chemical reactions stop at lactic acid which accumulates in the muscles and spills over into the bloodstream. Lactic acid makes muscles acidic and causes a burning feeling that forces you to slow down. Thus lactic acid helps to prevent severe muscle damage by slowing you down when you run low on oxygen.
When you slow down after each intense interval, you catch up on your oxygen debt, and your body uses lactic acid as its most efficient source of energy for muscles. Muscles require less oxygen to turn lactic acid into energy. When your muscles produce lots of lactic acid, they can use this chemical for energy. This allows you to move faster and stronger for longer periods of time (Sports Medicine, 2006;36(4):279-91). Anything that helps muscles to break down lactic acid faster will increase your endurance and allow you to move faster when you are tired (Fed Proc, 1986;45:2924-2929). Lactic acid can also be used by your liver to make even more sugar to feed your muscles during exercise.
Intervals to Make You Stronger
You cannot enlarge a muscle and make it stronger unless you damage muscle fibers at their Z-lines with vigorous exercise. When the muscles heal, they become larger and stronger. Interval training allows you to exercise more intensely than continuous training and therefore gives you a stronger training effect by causing more muscle fiber Z-line damage that results in increased strength and greater oxygen debts that give you greater endurance. You can tell that you are damaging muscle fibers because the muscle feels sore the next day (delayed onset muscle soreness or DOMS).
How Intervals Help to Prevent Disease and Prolong Your Life
A regular exercise program helps to prevent diabetes, heart attacks, and cancers. Interval training can make your exercise program even more effective because it helps to lower high blood sugar better than continuous training by making cells far more sensitive to insulin (Int J Sports Med, 2015;36:209-14).
Vigorous exercise also increases the number, size and efficiency of mitochondria in your muscle cells (Sci Rep, Jun 23, 2017;7(1):4191). All of the cells in your body (except mature red blood cells) have anywhere from a few to thousands of organelles, called mitochondria, that turn the food that you eat into energy. Muscle cells need a lot of energy so they have lots of mitochondria. Nerves don’t need a lot of energy to transmit messages so they need only a few mitochondria. When you exercise so intensely that you can’t get all the oxygen you need and you become short of breath, you increase the number, size and efficiency of mitochondria in cells everywhere in your body (J Physiol, 2010 Mar 15; 588(Pt 6): 1011-1022). Accumulating evidence shows that this helps to prevent obesity, diabetes, heart attacks and certain cancers. It helps to explain why exercise may increase memory and nerve function. Exercise also helps to reduce the loss of mitochondria in cells that occurs naturally with aging (J Gerontol A Biol Sci Med Sci, 2006;61(6)534-540; Exercise and Sport Sciences Reviews, April 2007). See More Mitochondria for Better Athletes
If you have not been exercising regularly, spend several weeks exercising at a casual pace. Try to exercise every day and exercise until your legs start to feel heavy or hurt and then stop for the day. You may go for five minutes on one day, and have to take the next day off because your muscles feel sore. You may have a progression of five minutes on one day, then zero on the next day, then 10 minutes, then three minutes. Gradually you should be able to work up to being able to exercise casually for 30 minutes every day and not feel sore. Then you should be able to start your interval workouts. Start out with just a few of the 10-20 second pick-ups and gradually work up to ten or more repetitions.
Do not try to do interval workouts more than two or three times a week, and not on consecutive days. Each time you do interval training intense enough to damage the Z-lines of muscle fibers, you will probably develop a soreness 6-24 hours after you finish exercising. Physiologists call this Delayed Onset Muscle Soreness (DOMS). If you exercise intensely when you feel DOMS, you are at increased risk for injuring yourself. Skip a planned interval workout if your legs feel heavy or hurt from a previous workout.
Caution: People with blocked arteries leading to the heart can get a heart attack from intense exercise. Check with your doctor before starting a new exercise program or making a sudden change in the intensity or duration of your program.
Geoff Hazel says
I’d like to know how durable the effects of exercise as detailed above are. If I was to ebark on a solid exercise program, ramping up over a 3 month period and then maintain rigorous regular exercise for another 3 months, improving in every way, and then just stopped cold, how long would it be for my body to revert back to where I was when I started?
Thinking, for example, of working out with a goal of completing a bucket-list event like an Ironman. Work out like crazy up to the event, and then once it’s over, you get that time back to do other things. Because working out for a big event takes a lot of time.
Jim Langley says
Hi Geoff, Coach John Hughes recently wrote a column about how fast you lose fitness. Here’s a link:
Hope that answers your question – and good luck in the Ironman!
Lukasz Muniga says
Damaging of anything doesn’t sound healthy. How come damaging of Z -lines is?
William Wightman says
Its the old “you rest, you rust” problem. We are constantly swapping out cells. The damage simply tells the muscle that it needs more cells at a certain location. On the flip-side, if you lie on a hospital bed (or couch) too long you lose muscle mass (bone strength, mitochondrial density…). Damage and repair are ubiquitous in the body.
Richard Goldfinger says
What heart rate level does “muscle burn “ correspond to? I can easily adjust my exercise level to a percent of my maximum heart rate with a monitor, but I find it too vague to be trying too get to a level of “muscle burn.” Specifically:is this 85, 90,95 percent of MHR?
Jim Langley says
I didn’t write the article, but I train the way Dr. Mirkin describes. I would say that his intense intervals are around 85 percent of your MHR. The perceived exertion level is very hard to sustain the pace – no way would you be able to maintain a conversation. It’s a very hard but not all-out effort. In an all-out effort, you would not be able to repeat it if it was truly all-out. So these intense intervals are as intense as you can go and then do it again and again.
I hope that’s helpful,
Jeff DH says
I hardly ever get DOMS – I either feel the soreness already by the end of the workout, or not at all. What does that say about my workout habits, if anything?
Jim Langley says
In my experience it means that you are training the same muscles the same way (same load). In order to get/feel DOMS we have to either push our muscles harder than they are used to or we have to recruit and overload new muscles somehow. So, for example, if a cyclist rode only flat roads and then took a trip to hill country and did a lot of steep climbs, they’d probably feel fine after the ride. But the next day waking up the hotel room their legs would probably have the DOM. You might just try one day doing something completely different to test the theory.
Mark Edwards says
Additionally, it’s worth noting that more muscle damage (DOMS) is done during the eccentric phase of exercise (lengthening the muscle under load) than during the concentric phase (contracting the muscle against load). With an exercise like running or weight lifting, the passive (eccentric) portion of movement does more damage than the concentric phase.
Cycling doesn’t include eccentric muscle loading (fixed gear bikes being the exception), so it’s considerably harder for moderately or higher trained athletes to generate the muscle damage so common in running and weight lifting.
So, while you may not get as sore doing intervals on a bike compared to running, the benefits Dr. Mirkin discusses are just as valuable from high intensity intervals on the bike as from other forms of exercise.