When people don’t exercise, we label them as lazy, but they are actually doing what they we as a species have evolved to do - which is to avoid unnecessary physical activity. In this article we explore the evolution of exercise and discover the link between sprinting and HIIT and the positive impact sprint interval training has on the body.
Running: Our Evolutionary Story
Humans are not the quickest of creatures; while the fastest of our species can briefly get up to speeds of just over twenty miles per hour (or around 10 metres per second), this is far slower than what lions (~30 metres per second), cheetahs (~35 metres per second) and race horses (~19 metres per second) are capable of. Compared to our primate cousins though (like chimps, bonobos and gorillas), we are uniquely adapted for running. Over the last two decades, evolutionary biologists have come to realise that running is a core tenet of our evolutionary story, how our ability to run, both far and fast, paved the way for persistence hunting. This ultimately led to dietary diversification, our migration across the globe and the eventual emergence of early civilisation.
Our ancestors didn’t evolve to exercise; sprinting, jogging, walking, squatting and climbing were just part of their lifestyle. They didn’t need to undertake formal, voluntarily physical activity for the sake of their health and fitness. The hunter-gatherer lifestyle was hard enough without adding discretionary, planned physical activity for the sake of physical improvement to it.
Exercise is actually a recent phenomenon, and is unique to humans. Farmers during the agricultural revolution (about 12,000years ago) exercised as a way to prepare to fight, and ancient texts like The Odyssey, paintings from pharaonic Egypt, and Mesopotamian carvings demonstrate how sports like wrestling, javelin throwing and sprinting helped prospective warriors to keep fit.
But exercise took a backseat to religion (in the Western world at least) after the fall of Rome and didn't have a renaissance until, well, the Renaissance. Medical practitioners like Mercurialis in ‘De Arte Gymnastica’ advocated exercises like gymnastics, fencing, and running to strengthen the body and enrich the mind. Subsequently, during the Enlightenment and the Industrial Revolution, liberal luminaries like Jean-Jacques Rousseau and Thomas Jefferson enthusiastically exalted the utility of exercise, and its culture burgeoned again.
How much should we exercise?
Over the last 150 years as the world has industrialized and many of our medial tasks have become mechanised, our doctors, politicians, and teachers have begun to raise concerns that the youth of their day are less active, less fit, and thus less healthy than the previous generation. And so we promote exercise. Just as we put wheels in cages for mice in labs, over the decades we have invented a plethora of devices, contraptions and methods to enable our fellow humans to undertake exercise-optional physical activity for the sake of health and fitness.
Today, a global network of medical organizations generally agree that adults should get at least 30 minutes of "moderate to vigorous aerobic exercise” at least five days a week.
Aerobic exercise
There are different ways to define "moderate to vigorous aerobic exercise”. First, let’s avoid the jargon and just stick with the term "aerobic exercise." Aerobic exercise is sustained physical activity which your body fuels by burning oxygen. Researchers have developed pretty rigorous methods for measuring the intensity of aerobic exercise in their labs but an easy way to gauge how much you are pushing your body outside the lab is using the talk test: a good example of moderate intensity aerobic exercise would be a brisk walk involving about a hundred steps a minute. This kind of activity will force you to breath fast and deep enough to make singing impossible but not so hard as to prevent you from conversing in normal sentences. During vigorous aerobic activities like fast running (e.g., during the foundational sessions of the SprintHit program), you’d probably be able to speak a few words, but full sentences would be impossible.
Why Is the SprintHit program good for us?
Thousands of studies since the early 1970’s have established the diverse benefits of aerobic exercise. We won’t go into detail in this post but to summarize briefly, some of the earliest known benefits were to the cardiovascular system. Because the fundamental challenge of aerobic activity is to deliver more oxygen at a faster rate to your muscles and various organ systems, your body adapts: the chambers of your heart will grow stronger and be able to pump more blood; in the blood, the number of your red blood cells and the volume of your plasma will increase; out at the margins, the small arteries and capillaries in your muscles and skin (where oxygen is being delivered to fuel your activity) will expand, and in those vessels the proportion of so-called good cholesterol (HDL) to bad cholesterol(LDL) will improve. Beyond the cardiovascular system, aerobic exercise stimulates muscle cells to grow (or hypertrophy) and increase their numbers of mitochondria, the “powerhouses” of cells, where oxygen is burned for energy production. Those muscles will also improve their ability to store carbohydrates and burn fat. The bones of your skeletal system, which act as the anchor to those muscles, will grow larger and denser when we are young and will repair themselves more effectively when we’re older. In terms of metabolic effects, aerobic exercise improves your body's ability to use sugar, lowers levels of inflammation and reduces visceral fat (the fat around your organs). It has many effects too on your endocrine system, beneficially altering your body’s levels of testosterone, oestrogen, cortisol, and growth hormone. Provided you don’t overdo it, aerobic exercise is also good for your immune system, enhancing its ability to protect you from certain infectious diseases. And finally, aerobic exercise increases blood flow to the brain and stimulates brain cell growth, maintenance, and function which can improve your cognition and mood.
Anaerobic fitness
But not all exercise is aerobic. Sometimes we need to exert a maximum, pant-inducing effort for shorter periods. The problem with using oxygen to fuel activity is that aerobic metabolism takes time; your body can’t use oxygen to fuel these short bursts of intense activity. Luckily (or more accurately, by evolutionary design), our bodies have ways to produce energy quickly, for short periods, without using oxygen. This is called anaerobic activity (anaerobic means “without oxygen”). Whereas our ancestors needed to use anaerobic metabolism during the final stages of a hunt to catch their quarry, today, we might use it to catch a bus or during competitive sport.
What sprint interval training does to the body
Short bursts of intense cardio elevate heart rate and oxygen consumption close to their upper limit, typically above 85 or 90 percent of your maximum rate, a level at which even speaking a syllable is a challenge. Athletes have known since the 1920s that repeated surges of anaerobic activity, termed high-intensity interval training (or ‘HIIT’), are an effective way to improve performance but in recent years, researchers have started to uncover its many potential health benefits.
Hundreds of studies have been conducted to date, revealing the beneficial effects of HIIT in diverse groups regardless of age, sex, fitness level, obesity or health status. We now know that because HIIT stresses the cardiovascular system more acutely than moderate-intensity aerobic exercise, it can yield rapid, dramatic benefits. When it’s done properly, HIIT can substantially improve both aerobic and anaerobic fitness, burn fat, improve muscle function, reduce blood pressure, lower harmful cholesterol levels, and stimulate the production of growth factors that help protect the brain (we’ll go into these effects in greater detail in future posts). In short, HIIT comes with many of the same benefits of aerobic training(and some of its own), but requires a fraction of the time to do.
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