improving your cardio fitness
A key indicator of fitness is how efficiently your body transports oxygen to muscles, via the cardiovascular system, in order to release the energy needed for physical activity. Embarking on a HIIT programme is a great way to improve cardiovascular health.
blood circulation
Aerobic respiration is the body’s primary mode of generating energy. In this process, oxygen is transported through blood to cells where it is required for the chemical reactions that convert energy stores into usable energy for bodily functions – such as the muscle contractions in HIIT. The pumping action of the heart maintains the flow of blood around the body, pushing oxygen-rich blood through arteries and returning de-oxygenated blood with CO2 waste back through veins to be expelled via the lungs.
Training adaptations
HIIT workouts improve cardiovascular efficiency in several ways: by training the heart to work at a faster rate and pump more with each beat; by increasing the overall volume of blood and the quantity of oxygen-carrying haemoglobin; and by increasing the density and improving the function of capillaries around muscles.
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Capillaries
The delivery to muscle tissue of oxygen and nutrients in blood, and removal of waste products such as carbon dioxide, takes place via small blood vessels known as capillaries.
Blood
Blood delivers oxygen from the lungs and nutrients from the digestive system to facilitate the release of energy in cells, while de-oxygenated blood removes the carbon dioxide that we then breathe out.
Scoring your cardiovascular capacity
A popular way of measuring aerobic fitness is to work out your “VO₂ max” score. This denotes the maximum (max) volume (V) of oxygen (O₂) the body can consume – and therefore the quantity of oxygen available in the muscles for aerobic cell respiration – during all-out physical effort. Assessing your VO₂ max can help you decide the level of HIIT routines to start with. As you progress, re-testing your score provides a benchmark for tracking progress.
Take the Cooper test
Developed by Dr. Ken Cooper in 1968, the Cooper test is a simple way to measure your VO₂ max. To complete it, run as far as you can in 12 minutes and use the total distance you have run to calculate your VO₂ score, following the mathematical formula below (using either the km or miles, as appropriate).
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performing the test
For an accurate result, run on as flat and even a surface as possible – an athletics track is ideal. Set your timer to count down from 12 minutes, run as far as possible, and record the total distance.
MITOCHONDRIAL FUNCTION
Mitochondria are organelles within a cell that regulate metabolic activity and generate chemical energy; they are present in muscle fibres and crucial to the performance of physical activity. Multiple studies show that mitochondrial function improves in response to endurance exercise, and some studies indicate high-intensity exercise may provide a greater stimulus than moderate. In short, undertaking a HIIT programme improves your capacity to generate energy at a cellular level.
Anti-aging effect
Mitochondrial function is known to decrease with age and is associated with diabetes, cardiovascular disease, and Alzheimer’s. Stimulating the synthesis of mitochondria through exercise may therefore help support health in old age.
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Muscle fibre
Subsarcolemmal mitochondria sit below the sarcolemma, the plasma membrane that surrounds a fibre. Intermyofibrillar mitochondria sit between the rod-like myofibrils that house the contracting filaments of muscle.
Generating energy
The first stage of energy release takes place in the sarcoplasm, where glucose is converted to pyruvic acid. This acid then moves to the mitochondria where, via chemical reactions in the presence of oxygen, it is turned into ATP.