Workout Intensity: It Matters

With the many demands of modern life, it sometimes feels that there simply isn’t enough time for exercise.

Furthermore, when you add up foam rolling, stretching, and warm-up, all practices that help us get the results we want, the time it takes to get a work out in becomes even less manageable.

Making matters worse, many people will spend hours in the gym, treadmilling to the edge of the universe, without having major positive outcomes to show for it. Does this spell doom for the rest of us?

The Holy Grail of Exercise: High Intensity

Fortunately, there is a solution for those of us short on time and hungry for results.

Exercise intensity is shown to predict positive fat loss outcomes in an exercise program. Lucky for us, this also means that less can be more, as long as we’re willing to push ourselves.

Let’s look at two powerful reasons why the science supports high-intensity exercise:

Increase in resting metabolic rate

In terms of fat-loss, high-intensity exercise interspersed with short rests trumps steady state, continuous exercise. This finding is widely supported in the scientific literature (1-3). Let’s look at one such study: At Laval University in Canada, a stationary bike-based experiment was conducted on 27 individuals (13 men and 14 women) with no regular involvement in exercise (3). Two experimental groups were constructed, one that performed both continuous, moderate-intensity cycling and high-intensity interval training (HIIT), and one group that exclusively performed continuous steady state cardio at moderate-intensity. The HIIT experimental group started out performing mainly moderate-intensity exercise so that they could establish a basic level of fitness before switching to mainly high-intensity, interval training. The HIIT group went at it for 15 weeks total, while the steady state group followed a 20-week program. The experimenters kept track of the average overall energy output of each group in the course of the program, based on initial measurements of their individual rates of oxygen consumption and carbon dioxide production while cycling at various intensities.

Despite following a shorter program and expending a little less than half the energy that the steady state group did, the HIIT group experienced 4.5 times more fat loss than did the steady state cardio group. When adjusted for total energy expenditure over the course of the programs, high-intensity interval training resulted in 9 times more fat loss than moderate-intensity, continuous training.

How can this be true? They burnt half the calories of the other group in ¾ the time, yet lost significantly more fat mass.

The reason is that high-intensity exercise results in dramatic changes in resting/basal metabolic rate, or the rate at which your body uses energy while you sprawl on the living room couch (1-3).

(For those interested in further reading, look up “excess post-exercise oxygen consumption).

The study mentioned above found that, in particular, enzymes involved in skeletal muscle metabolism were highly activated at rest in the subjects that performed high-intensity exercise. Thus, the greater demand on muscle recovery seems to be part of the reason that high intensity exercise triggers a long-term increase in metabolic rate.

Overall, it seems that the primary impact of exercise is not the calories you burn in 45 minutes, but rather the long-term adaptations that are relevant for the other 23 hours of the day, when you’re unlikely to even think about exercise. Low-intensity exercise offers no significant benefit in this respect.

Improved appetite regulation

Higher intensity exercise has been repeatedly shown to suppress appetite, more so than lower-intensity exercise protocols (4-6). This supports the outcome of greater fat loss in response to exercise.

What is the reason for this counterintuitive outcome? It seems that the human body would want more food if the demand of the exercise is greater.

Improved appetite regulation appears to be a result of the hormonal changes that result from a single bout of high-intensity exercise. For example, ghrelin, a hormone that triggers the hunger response, is suppressed following high-intensity exercise (4).

More generally, high-intensity exercise triggers activity of the sympathetic nervous system (AKA the “fight-or-flight” mechanism of our bodies), and one general feature of this system is suppressed appetite. In contrast, the parasympathetic nervous system, the state of the body defined by deepened breathing and a sense of relaxation (AKA the “rest-and-digest” mechanism of our bodies), is also responsible for increasing appetite. Thus, by pushing the envelope with exercise intensity, we can suppress appetite and break our bodies out of the parasympathetic dominance that characterizes modern life.

The Best Way to Exercise

If you aren’t willing to put in the work, you won’t get the most out of your exercise. You’ll end up like the unfortunate majority of the human population, struggling to maintain a healthy body weight, and losing sleep over unrealistic diet and exercise programs.

Put your phone away, focus, and challenge yourself to push the limits of your body. Keep rest periods short, aim for personal records, and let the gym be a cathartic escape from your otherwise sedentary lifestyle.

Both your body and your busy schedule will thank you.

  1. Boutcher, Stephen H. “High-Intensity Intermittent Exercise and Fat Loss.” Journal of Obesity 2011 (2011): 868305. PubMed. Web.
  2. Larsen, I. et al. “High- and Moderate-Intensity Aerobic Exercise and Excess Post-Exercise Oxygen Consumption in Men with Metabolic Syndrome.” Scandinavian Journal of Medicine & Science in Sports 24.3 (2014): e174-179. PubMed. Web.
  3. Tremblay, Angelo, Jean-Aimé Simoneau, and Claude Bouchard. “Impact of Exercise Intensity on Body Fatness and Skeletal Muscle Metabolism.” Metabolism 43.7 (1994): 814–818. Print.
  4. Hazell, Tom J. et al. “Effects of Exercise Intensity on Plasma Concentrations of Appetite-Regulating Hormones: Potential Mechanisms.” Appetite 98 (2016): 80–88. PubMed. Web.
  5. Prado, Wagner Luiz et al. “Effect of a 12-Week Low vs. High Intensity Aerobic Exercise Training on Appetite-Regulating Hormones in Obese Adolescents: A Randomized Exercise Intervention Study.” Pediatric Exercise Science 27.4 (2015): 510–517. PubMed. Web. “PubMed Entry.” Web. 25 June 2017.
  6. Sim, Aaron Y. et al. “Effects of High-Intensity Intermittent Exercise Training on Appetite Regulation.” Medicine and Science in Sports and Exercise 47.11 (2015): 2441–2449. PubMed. Web.

Andrew Greene is a personal trainer at Boston Sports Club in Boston. Hailing from Cincinnati, he received his Bachelor’s degree in Biochemistry and Molecular Biology, and is fascinated by the interplay of society, human behavior, and health. In his rare hours outside of the gym, Andrew enjoys writing, reading, and singing like nobody is listening.

To keep up with Andrew, follow him on Instagram @andrew.h.greene or connect with him over email at

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