A deep dive into the biological markers of fitness, common misconceptions, and what you can do to get fitter today.

Whether you’re an athlete, a HIIT obsessive, a 5k runner, or just someone looking to wheeze a bit less when you go upstairs, most of us are looking to get fitter in one way or another. 

That said, it’s a weird word, ‘fitness’. The global gym industry was worth $96.7 billion in 2020, so fitness is big business. But what do we actually mean by getting ‘fitter’? 

Kiran Chopra, exercise physiologist at Lanserhof at The Arts Club, defines fitness as having five distinct but interlinked components (pulmonary ventilation; haemoglobin concentration; blood volume and cardiac output; peripheral blood flow; aerobic metabolism). 

Here, we explore what each of these components actually is, how that impacts your fitness and, armed with a scientific understanding of fitness, what you can do to improve yours. 

In physiological terms, what is fitness? 

As far as most of us understand, an increase in fitness comes about through aerobic exercise. Go for a run, and you’ll prompt macro and molecular adaptations which impact the cardiovascular and musculoskeletal system, which in turn leads to increased endurance and better performance. 

Chopra explains that fitness is often measured via our VO2max capacity which is “the greatest amount of oxygen that a person's body can transport through the circulatory system and use in their motor muscles.”

In other words, fitness can be drilled down into how efficiently we can transform atmospheric oxygen into fuel for our muscles. The more efficiently we do this, the better we perform.

What are the main processes involved in oxygen transfer? 

(Spoiler alert, while it’s good to have an understanding about what’s going on in your body, you are able to improve your fitness without understanding every word of the below. If some of it goes over your head, remember you don’t have a degree in physiology, and that’s fine.)

In Chopra’s words, here are the five main signifiers of aerobic fitness:

Pulmonary Ventilation

“When engaging in aerobic endurance training, adaptations often occur during submaximal activity whereby tidal volume (the amount of air that moves in or out of the lungs with each respiratory cycle) increases and breathing frequency decreases, and during maximal exercise when both tidal volume and breathing frequency rise.”

Translation: you can get more air – and thus oxygen – into your body with each breath.

Haemoglobin Concentration

“All bodily tissues receive oxygen from the lungs through haemoglobin in the blood, which also transports carbon dioxide from all cells back to the lungs for elimination from the body. In groups of highly trained endurance athletes, there is a significant correlation between Hbmass (haemoglobin mass) and VO2max as well as between Hbmass and endurance performance. Endurance training over six weeks to nine months commonly comprises a 5% to 10% increase in Hbmass.”

Translation: exercise increases the amount of haemoglobin in your blood, which means the same volume of blood can transport more of that breathed-in oxygen around your body.

Cardiac Output 

“The improvement in maximal cardiac output is the functionally most significant change for long term exercise prescription. Endurance athletes have regularly been found to have increased left ventricle end-diastolic diameter, left ventricle wall thickness, and left ventricle mass, allowing for stronger contractions of the heart per beat.”

Translation: your heart gets bigger and stronger, so it can pump blood – packed with oxygen – around your body more efficiently. That’s why generally the fitter you are, the lower your resting heart rate.

Peripheral Blood Flow

“To meet the increased demand for oxygen by the contracting muscle, blood flow to skeletal muscle can rise up to almost 100-fold from rest. A higher cardiac output and an improvement in vascular conductance in the working muscle work together to boost blood flow.”

Translation: your heart, arteries and muscles all work better together to get more blood, more quickly, into the bits of your body that are working hardest.

Aerobic Metabolism

“Mitochondria are often referred to as the powerhouses of the cell. They assist in converting the energy we receive from meals into energy the cell can utilise. But mitochondria do more than just produce energy. ATP, a complex organic chemical found in all forms of life, is often referred to as the molecular unit of currency because it powers metabolic processes. The majority of ATP is created in mitochondria through a series of processes called the Krebs cycle or citric acid cycle. It is well established that different types of exercise can provide a powerful stimulus for mitochondrial biogenesis (the making of new components of the mitochondrial reticulum) which in turn will allow greater ATP production ie more energy to perform exercise.”

Translation: your cells grow more of the ‘engines’ that create the fuel powering your muscles, so they can worker harder for longer.

Is getting fitter the same as getting faster?

It’s easy, when we think in terms of running, to believe that getting fitter means being able to run faster. Not so. “In terms of improving our fitness, we think of this more so as developing the ability to exercise for longer, not necessarily sprinting faster,” Chopra explains. “For example, aerobic metabolism contributes essentially none of the energy required in the 100m sprint. However, in sports with intermittent sprinting bouts, our aerobic system contributes significantly to recovering between sprints and allowing us to carry out more total sprints throughout a match, for example.”

So if you’re looking to be as fast as Usain Bolt in a 100m all-out, one-time sprint, aerobic fitness isn’t a priority. But, aerobic fitness can allow you to perform more high-intensity sprints in a single session, which means you’ll develop the strength and skill to get quicker more, er, quickly.

What misconceptions do people have about getting fitter?

Many of us think fitness is about battle ropes and burpees, Tabatha and HIIT. “This is simply not the case,” says Chopra. “Yes, certain exercises like kettlebell swings and dumbbell snatches serve a purpose in strength and conditioning, but to get the most out of developing aerobic fitness, sustainability around your ventilatory thresholds and VO2max are crucial in an effective aerobic fitness plan.

“In an ideal world, we would like to do some form of aerobic capacity test whether it be a cardiopulmonary exercise test or a field-based exercise test to determine your VO2max,” Chopra continues.

“If that’s not possible [and let’s face it, it very likely isn’t in your local chain gym] working at certain percentages of your maximum heart rate can provide a close enough representation of what your work rates should be.”

What can I do to build all-round fitness?

In an ideal world Chopra would like you to have access to equipment to track your VO2max, helping you work at a percentage of this while on the treadmill. But, very few of us are likely to have access to this equipment, so let’s keep it simple. 

Getting down to brass tacks, Chopra advises two to three sets, each of six reps of 30m sprints with 20 seconds rest in between.

“This is a simple way to work at an intensity that will help drive some of those physiological adaptations of oxygen delivery, oxygen extraction and oxygen utilisation, allowing us to carry out more aerobic activity and feel fitter,” he says. 

We’re feeling exhausted already.