When you work out at a high intensity, it’s hard to maintain the intensity for more than a short period of time. In contrast, when you exercise at a low to moderate intensity, depending on your fitness level, you can maintain a steady exercise pace for an hour or more without being limited by fatigue. An example is distance runners who run 6 or more miles without having to stop to rest or recover. That’s not true when you do high-intensity exercise. Why do you fatigue so quickly when you exercise at a high intensity?
High-Intensity Workouts and Fatigue
You fatigue quickly during high-intensity exercise because you rely more on anaerobic energy pathways, which operate without oxygen, to supply your muscles with ATP. During moderate-intensity exercise, your muscles use energy, in the form of ATP, made primarily from aerobic pathways that require oxygen and use fat as their primary fuel source. Aerobic pathways are the main way your muscles generate ATP when you’re working out at a sub-maximal intensity since oxygen delivery isn’t a limiting factor. In fact, during moderate-intensity exercise like a jog or slow run, up to 90% of your energy needs are met by aerobic energy production.
Kick up the intensity of your workout and energy production shifts towards anaerobic pathways. As exercise intensity increases, you engage more fast-twitch muscle fibers. These muscle fibers are built for generating strength and power, not for sustained activity. This muscle type relies heavily on anaerobic metabolism for producing ATP, using carbohydrates as their primary fuel source. Recruiting more fast-twitch muscle fibers increases the amount of power your muscles can generate but this power comes at a price – faster fatigue. Your muscles begin to burn and you’re forced to stop exercising or reduce the intensity.
Why do you experience muscle burning and fatigue so quickly during high-intensity workouts? When your muscles use anaerobic pathways to make ATP, a number of by-products build up. These by-products include lactate and hydrogen ions. At one time, experts believed lactate accumulation was the primary cause of muscle fatigue. This theory has since fallen out of favor. The build-up of hydrogen ions seems to be a bigger contributor to muscle fatigue. As hydrogen ions accumulate, pH drops and this drop in pH changes the function of enzymes involved in muscle contraction. It also alters calcium. Calcium is important for muscle contraction. It binds to proteins that allow the muscle myofilaments (actin and myosin) to slide past one another to produce a muscle contraction.
Another factor that causes your muscles to tire during an intense workout is something called “neural fatigue.” Muscle fibers are activated by motor nerves that carry input from the brain and release chemicals called neurotransmitters. Neurotransmitters are chemicals that tell the muscle fibers to contract during exercise. This system fatigues and muscle contraction becomes less efficient. The end result is muscle “wipeout.”
Are Women More Fatigue Resistant?
For periods of sustained moderate-intensity exercise, women may have an advantage over men. Women have a higher percentage of body fat they can tap into as a fuel source during long periods of steady-state exercise. Women have the ability to more efficiently use stored fat as fuel and conserve glycogen during long periods of exercise so they’re more resistant to fatigue. But what about high-intensity exercise? Are women more fatigue resistant than men?
Women may not have the same resistance to fatigue during high-intensity exercise. Muscle biopsies show enzymes muscles use to produce ATP anaerobically aren’t as active in women as they are in men. So, men may be more resistant to fatigue than women during short periods of high-intensity exercise whereas women may be more fatigue resistant during long periods of endurance exercise. Interestingly, research shows children can better resist fatigue during high-intensity exercise than adults. One reason is they have less total muscle mass and generate less power during high-intensity workouts. Therefore, they can go longer.
Benefits of High-Intensity Exercise
High-intensity interval training offers multiple fitness benefits. With HIIT training, you exercise at a high intensity during the active interval and allow your muscles to partially recover during the rest interval. Because of the rest intervals, you can work out at an intensity you wouldn’t normally be able to sustain for a significant period of time.
The benefits? Research suggests vigorous exercise offers greater cardiovascular benefits than exercising at a low or moderate intensity. Plus, your body has to expend more energy to recover from an intense workout. As a result, you burn more calories after a HIIT workout than you do after steady-state exercise. Research shows people burn more calories in the 24 hours after a high-intensity workout than after steady-state exercise. High-intensity workouts also maximally activate fat-burning hormones like adrenalin and growth hormone. They’re a real time saver too. When you increase the intensity of your workout, you can get fitness benefits with as little as 10 minutes of exercise.
High-intensity workouts offer flexibility too. You can structure active and rest intervals of a HIIT workout to target your aerobic or anaerobic energy systems. The most popular interval structure is a 1:2 work to rest interval. For example, 60 seconds active followed by 2 minutes of recovery, although there are many ways to structure an interval workout. The variables you can change to customize a HIIT workout are active interval length, recovery interval length, intensity and number of cycles. Interval workouts also offer variety since you can change the exercises you do during the active intervals to keep things interesting. The key is to push hard during the active intervals so you’re working out at a high intensity.
The Bottom Line?
Yes, HIIT training is fatiguing and now you know why. High-intensity exercise is also time expedient and flexible. Is it tiring? Yes! But it’s worth it.
References:
Sports Conditioning and Fatigue. Lance C. Dalleck, M.S. and Kravitz, PhD.
J Sports Med Phys Fitness. 2010 Sep;50(3):243-53.
Sports Medicine. December 2006, Volume 36. Issue 12. pp 1031-1065.
J Strength Cond Res. 2011 Apr;25(4):1104-12. doi: 10.1519/JSC.0b013e3181d09ec9.
IDEA Health and Fitness Association. “Interval Training: The New And Better Way To Train Your Clients?”
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