Endurance is the foundation upon which fitness is based. Like a house, unless the foundations are strong, things quickly fall apart and crumble, and the same holds true for runners - without a solid endurance base, faster intensive training and racing becomes hard, and injuries and illness are a persistent risk.
The best way to understand endurance is to start by gaining an insight into how the body reacts and responds to running. This, of course, requires energy, which comes from the breakdown of carbohydrate (glycogen) and fat, in the muscle. In order for this to happen in a manner that can be sustained for a long period of time without producing fatigue-inducing by-products such as lactic acid, the provision of energy needs to take place in the presence of oxygen – or “aerobically”. As running speed – or exercise intensity – increases, more energy and oxygen are required, until the point is reached when oxygen can no longer be supplied at a fast enough rate, and energy is then produced “anaerobically”. Endurance running requires a well-developed “aerobic” system, since once a runner reaches a speed where anaerobic energy is required, the effects of fatigue will quickly result in a decrease in speed, and inevitably soon result in the need to stop running altogether.
So the key to endurance training is the development of the body’s aerobic system, so that oxygen can be supplied to the muscles at a rate which is high enough to delay or even avoid the need to produce energy anaerobically.
Understanding the basic physiology behind the delivery of oxygen to the muscles and the provision of aerobic energy will help to explain how the body adapts to meet this challenge.
At rest, we take around 12 breaths, and 20 to 30 litres of air into the lungs, each minute. During maximal exercise, this can increase to up to 30 breaths and 200 litres of air per minute. Crucially though, it is oxygen that makes up approximately one fifth of the air in the lungs, that the body needs for endurance running. Some of this oxygen transfers from the lungs, or more specifically at the alveoli - minute “sacs” which are the lungs’ equivalent of leaves on a tree – it is estimated that there are around three hundred million of them in each lung. These alveoli are surrounded by capillaries that contain blood which has a low oxygen content, having already previously deposited oxygen to the muscles. The alveoli have thin membranes, which enables oxygen in the lungs to transfer across into the capillaries, where it attaches to a substance called haemoglobin, to start its journey to the muscles. The “engine” that powers this journey is the heart, arguably the body’s most important muscle and the world’s moist efficient pump. At rest, the heart beats around 70 times a minute, pumping approximately 5 litres of blood around the arteries and veins (known as cardiac output). During exercise, heart rate can reach or even exceed 200 beats per minute, with cardiac output reaching 35 litres per minute. Blood rich in the oxygen that has been extracted from the lungs’ alveoli is pumped towards the muscles under high pressure, through arteries and then branching out into narrower arterioles and finally, into capillaries. At the same time, blood that has low oxygen content is pumped back to the lungs under lower pressure, through veins.
Blood and oxygen reach deep into the muscles through thin capillaries, where oxygen “disassociates” from the haemoglobin, and enters the muscle to combine with fat or glycogen (carbohydrate) to start the complex biochemical process that produces energy. As exercise intensity increases, so too does the demand for oxygen, often referred to as “oxygen uptake”. All individuals will reach a point where it is not possible to transport any more oxygen from the lungs to the muscles, known as “maximum oxygen uptake”, or VO2 max. This VO2 max value has been used by scientists for many years as an indicator of aerobic or endurance fitness, since runners who are able to perform the best endurance running performances are invariably those with the highest VO2 max values. Importantly, understanding the factors that inhibit and influence VO2 max can help to better understand the adaptations that occur following an endurance programme. We know, for example, that with endurance training, more alveoli are recruited to assist in the extraction of oxygen from air in the lungs. We also know that the heart becomes a larger and more efficient pump, capable of pumping larger volumes of blood with each beat (known as an increase in stroke volume). The density of capillaries around the alveoli and within the muscles also increases, improving the capacity to transport oxygen into the bloodstream, and subsequently to the muscles. Within the muscles, biochemical changes take place that increase the body’s capacity to produce energy, including an increase in the number of mitochondria - the “powerhouse” of the muscle cell where energy production takes place. These “central” adaptations (to the lungs and heart) combine with the peripheral adaptations at muscle cell level, to increase a runner’s oxygen uptake capacity and endurance fitness. Studies have shown that with training, significant increases in VO2 max can occur after periods of just 6 to 8 weeks – but we also know that there is a genetic “ceiling” above which VO2 max will not increase further, lending weight to the statement that in order to be an Olympic Marathon Champion, you have to choose your parents carefully! But scientists have also shown that even when VO2 max has reached its genetic limit, improvements are still possible, and occur because runners can train to tolerate a higher percentage of their maximum capacity, even if this capacity itself does not increase.
The percentage of maximum that a runner runs at is known as their “relative exercise intensity” (REI). We know, for example, that elite marathon runners can sustain around 80% of their REI over the 26.2 mile distance, whilst recreational runners are more likely to sustain a lower 60-70% of maximum. Most physiological responses within the body respond to the REI – core temperature being one of the key ones. The higher the REI, the greater the physiological stress that the body is placed under, so the harder things feel. Training which increases a person’s maximum capacity, whilst enabling a higher percentage of maximum to be tolerated, all contribute to improvements in endurance performance.
So in summary, endurance training provides the physiological adaptations upon which running performance is based. It is essential for the development of the heart, lungs and transport of all-important oxygen to the working muscles. A well-developed endurance capacity enables runners to train and race without producing fatigue-inducing by products such as lactic acid, and subsequently to recover more quickly. As the body adapts, a runner’s maximum oxygen uptake and cardiac output increase, and running at submaximal speeds feels easier. Neglecting endurance training means that the foundations are not in place to support and sustain more intensive running, and as a result, poor performances, fatigue and even injury will quickly follow.
Written by Prof. John Brewer