The sport science binge continues. This book does a great job of getting into the nitty gritty how each of your body's systems works in the context of sport. One chapter outlines the specific adaptations we're aiming to induce through endurance training, which is the clearest and most comprehensive list I've seen so far.
In the twentieth century, engineers discovered that hollow structures made of composite materials are strongest. Bones consist of calcium, which is a hard mineral, and collagen, which is a flexible protein. If bones were just made of calcium, they would shatter like glass; and if they were just collagen, they would bend like rubber. However, combine the two materials and we have strong, hard bones that are able to bend in response to forces being applied on them.
Bones are constantly being broken down and then rebuilt. This is the responsibility of three microscopic organisms:
Muscles have a blood supply that delivers oxygen and nutrients; the nutrients are carried and stored in a fluid called sarcoplasm. The muscle fibres are bathed in this fluid, which contains fats and glucose to produce energy, and proteins and enzymes to enable chemical reactions to occur.
(Muscles with more precise jobs have more nerves per muscle fibre than those responsible for producing power). For example, the quadriceps muscles at the front of the thigh have a nerve to muscle fibre ratio of about 1:400 and the gluteus maximus (buttocks) have a ratio of around 1:1,000. The functions of these muscles are predominantly to produce power for movements and to maintain posture. However, muscles in the face which produce fine, delicate movements have a much lower ratio of nerve to muscle fibre. For example, the muscles that move the eyeballs and the tongue have a ratio of lower than 1:10 to provide the fine control of movement that they require.
The all or none law states that if a motor unit is recruited, all the muscle fibres attached to that nerve will contract fully or not at all.
The cow needs muscle fibres that will provide the endurance to stand relatively still in a field for most of the day, so it consists of predominantly slow twitch muscle fibre. As slow twitch muscle fibres contain myoglobin and have many blood vessels, the cow’s flesh will be red coloured. The breast of the chicken consists of fast twitch muscle fibres that enable it to move quickly and to take flight, hence the white coloured flesh. However, because the white, fast twitch fibres fatigue quickly, chickens can only fly for short periods of time. The legs of the chicken are darker in colour as they have a greater number of slow twitch fibres that enable the bird to stand for long periods of time and run if necessary.
Golgi tendon organs (GTOs) are found close to the junction between the tendon and the muscle. GTOs monitor tension within a muscle and they are activated to override or inhibit the stretch reflex. This is known as the inverse stretch reflex and GTOs cause the muscle that has been in a state of contraction to relax again. They allow the muscle to be stretched slightly further or loaded slightly more. The inverse stretch reflex takes around 10 seconds to be activated once the central nervous system has sensed that the muscle is not at risk of injury. When an athlete undertakes a developmental stretch, they will stretch the muscle until the muscle spindles are activated and then hold that position until the GTOs generate the inverse stretch reflex and relax. Then the muscle can be stretched slightly further until the muscle spindles activate the stretch reflex again.
Once ATP has been broken down to release its energy, it is no longer able to supply any further energy and therefore it needs to be resynthesized (reproduced). The component parts for ATP, which are ADP and phosphate (P), are still present in the cells but they need energy to reattach the free phosphate and resynthesize ATP. The energy needed to reattach the phosphate group is supplied by the breakdown of glucose and fats.
A marathon runner taking 2.5 hours to complete the distance will resynthesize around 80 kg of ATP during the run.
Glycogen that is stored in the liver can return to the bloodstream as its main role is to keep the brain supplied with a steady supply of glucose. When glycogen enters the bloodstream, it is once again referred to as glucose.
Aerobic metabolism: This is referred to as aerobic glycolysis and it produces more ATP per molecule of glucose than anaerobic glycolysis. This is because glucose can be completely broken down, rather than the partial breakdown that occurs in the lactic acid system, so that all bonds are broken down to release energy for ATP resynthesis. Aerobic glycolysis resynthesizes 38 ATP from a molecule of glucose in comparison to 2 ATP resynthesized by anaerobic glycolysis.
The breakdown of glucose or fat with oxygen will produce energy, water and carbon dioxide. However, the body is able to get rid of water and carbon dioxide through respiration and sweating. Consequently, the aerobic system produces no fatiguing waste products, unlike the lactic acid produced through the lactic acid system.
Inspiration is the result of the volume of the chest cavity increasing and the pressure within it falling. This decrease in pressure causes air to be drawn in. During expiration the chest cavity decreases in volume and the internal pressure increases, causing air to be forced out.
We are only able to expel a maximum of around 4 litres of air or else our lungs would collapse. The 1 litre remaining in the lungs is called the residual volume. The amount of air we inhale with each breath is referred to as the tidal volume.
Inhaled air has 21 per cent oxygen, of which the blood can take in a small amount and the rest is exhaled. Because exhaled air still has high levels of oxygen, we can use exhaled oxygen to resuscitate someone else. Exhaled air has an increased amount of carbon dioxide that has replaced oxygen.
The movement of gases between the alveoli and the blood is due to an imbalance between their relative concentrations. In the alveoli, there is a higher concentration of oxygen (pO2 of 105 mmHg) than in the blood contained within the capillaries (pO2 of 40 mmHg). This difference creates a pressure gradient. The capillaries that surround the alveoli are only one cell thick and there are tiny spaces between the cells in the artery walls that allow gases to pass through them. The difference in the pressure of oxygen forces oxygen from the alveoli into the blood until the pressure is equal in both the lungs and the blood. The partial pressure of carbon dioxide (pCO2) is also different between the lungs and the blood. Air in the lungs has a low pCO2 of 5 mmHg compared to the pCO2 of 45 mmHg in the capillaries. This pressure differential causes carbon dioxide to move from the blood to the alveoli where it can be exhaled. This movement of gases is referred to as diffusion.
Despite seeming to be a fluid, blood consists of both fluid and solid parts. The fluid part is called plasma, which is pale yellow in colour and provides the liquid for the solid components to float in. It makes up 55 per cent of the blood volume. The other 45 per cent of blood volume is made up of blood cells, of which there are three types: red blood cells (erythrocytes), white blood cells (leucocytes) and platelets (thrombocytes). Around 99 per cent of the solids in the blood are red blood cells, and the white blood cells and platelets account for the remaining 1 per cent of solids
A red blood cell has a lifespan of around 120 days before being removed from circulation by the spleen and lymph nodes.
Unlike red blood cells, the white blood cells can leave the blood vessels and move to wherever infection is present. The cardiovascular system is merely a transport system for them.
The heart is divided into sides; the left side is responsible for pumping oxygenated blood to the body and the right side for pumping deoxygenated blood to the lungs.
The atria are collecting chambers, where blood waits before entering the ventricles. The ventricles are pumps that provide force to propel blood around the body:
Coronary arteries are the arteries that actually supply the heart muscle, or myocardium, itself. The arteries are called ‘coronary arteries’ because they sit on top of the heart muscle like a crown. The heart supplies itself with the most oxygen-rich blood available because these coronary arteries branch off the aorta almost immediately after the blood has passed through the aortic valve. The heart knows that its own continued function is central to the survival of the body as a whole, therefore it needs the best blood supply. The blood supplied to the heart muscle has newly arrived from the lungs, where it has been oxygenated. However, it can also contain toxins that have been breathed in from the environment. For example, cigarette smoke contains thousands of toxic substances that are damaging to tissues. So while the heart muscle gets the first choice of fresh oxygen, it can also receive the greatest concentration of toxins that are inhaled in cigarette smoke. These toxins can damage the blood vessels and negatively affect the function of the heart.
The definition of an artery is that it is a blood vessel taking blood away from the heart, while a vein always returns blood to the heart. Arteries predominantly carry oxygenated blood and veins predominantly carry deoxygenated blood.
Roughly speaking, the human body contains around 60,000 miles of blood vessels which, if organized into a straight line, would reach around the Earth three times. This seems almost unbelievable but it illustrates the vastness of the network of capillaries and how tiny some of these blood vessels are in size.
Capillary means ‘hair like’ and describes the microscopic size of these blood vessels. Some are so tiny that they only have enough space for one red blood cell to pass at a time.
The higher reading represents systolic blood pressure, which is the pressure in the arteries during the contraction phase of the heartbeat. The lower reading represents diastolic blood pressure, which is the pressure in the arteries during the relaxation phase of the heartbeat.
The pumping action of skeletal muscles plays a key role in returning blood to the heart and if we were to suddenly stop running, the muscles of the calves and upper leg would suddenly stop acting as the muscle pump. Cardiac output would still be high, sending blood to the legs but we would get blood pooling, or venous pooling, in the legs. The result would be that there would be significantly less blood returning to the heart and cardiac output would be compromised. This is why we can feel dizzy or light-headed after exercise if we stop suddenly, as all our blood is pooling around our ankles, causing a reduction in blood supply to the brain.
This concept led to physiologists commenting that if you want to be an Olympic-level performer it is most important to choose your parents wisely.
The quantity of mitochondria can increase up to 50 per cent after only a few weeks of regular aerobic training (McArdle, Katch and Katch, 2022) and in addition to doubling the quantity of aerobic system enzymes within 5–10 days, muscles become increasing effective at using oxygen as a source of resynthesizing ATP.
McArdle, Katch and Katch (2022) identify the number and size of mitochondria as being a limiting factor of an athlete’s aerobic fitness rather than the ability of the cardio-respiratory system to supply the muscles with oxygen. It is likely that oxygen is present but the muscles don’t have the necessary resources to be able to use it.
In addition, aerobic exercise increases the concentration of myoglobin in muscles. Myoglobin is a protein that transports oxygen within the cell. As a result, the oxygen-carrying capacity within muscles increases.
With regular aerobic training the muscles can rely on fat metabolism for more energy at higher submaximal intensities. They become better at burning fat at higher intensities because of several processes, including an increase in fat-mobilizing and fat-burning enzymes.
Prolonged participation in aerobic-based activities leads to several changes in cardiovascular function. Specifically, during exercise cardiac output increases, while at rest heart rate is reduced and stroke volume is increased. These changes are because of cardiac hypertrophy, or an enlargement of the heart muscle. This is often referred to as ‘athlete’s heart’; the heart can become up to 25 per cent larger in size.
The increase in potential blood supply from the enlarged heart is supported by an increase in the number of capillaries in the muscles and around the alveoli. This increased capillarization ensures that the oxygen and nutrients can be effectively delivered and that waste products, such as carbon dioxide and lactic acid, can be quickly removed from muscle tissue.
Plasma, which is the fluid component of the blood, increases by 20 per cent after four aerobic training sessions (McArdle, Katch and Katch, 2015) with relatively little increase in the red blood cell count. However, the oxygen-carrying capacity of the blood does increase because the blood has become less viscous and flows through blood vessels with less resistance.
Red blood cell count will increase as well, but at a slower rate compared to the rapid increase in plasma volume. The two effects combined will increase the oxygen-carrying capacity of the blood.
Resting heart rate decreases with aerobic training and maximal heart rate generally decreases slightly as well, as the larger pump may take more time to contract and relax fully.
The percentage of air that accounts for oxygen, nitrogen and carbon dioxide is identical at sea level and at altitude but the partial pressure of each gas is lower. This reduction in the partial pressure of oxygen in the air means that it is closer to the partial pressure of the oxygen already in the bloodstream and that less oxygen diffuses into the blood.
When a force is sent through a bone, the bone bends slightly and this loading causes osteoblasts to migrate to the area where stress is being experienced. Osteoblasts lay additional collagen fibres at the site of the stress and then collagen is coated with calcium to harden it up. As a result, bone diameter and density start to increase as long as stress is applied to a bone.
Monosaccharides are one unit of a sugar. Glucose, fructose and galactose are the monosaccharides. Disaccharides are two units of sugar joined by a bond. Sucrose, lactose and maltose are the disaccharides.
A food with a high GI results in glucose quickly entering the bloodstream and causing a high spike in blood glucose levels. When blood glucose levels rise, the pancreas is stimulated to release insulin to lower the blood glucose level. Insulin is a hormone that allows glucose to flow into the cells of the muscle and liver with a resulting fall in blood glucose levels.
The potential solution is personalized nutrition by assessing which foods cause spikes in glucose levels to an individual, and which cause a lower response. The foods causing the sugar high can then be avoided. This information can be gathered through an app or measuring your own blood glucose levels after eating.
The recommended amount of protein that should be consumed at each sitting is between 15–25g (Phillips and van Loon, 2011). Protein is best taken in smaller, regular meals, as the liver can only deal with small amounts at any time and any excess protein will either be excreted or stored as fat.
Fatty acids are the smallest unit of a fat and are packaged in the diet as triglycerides, which are three fatty acids attached to a glycerol backbone.
The priority post-exercise is to replace the depleted glycogen stores and replace fluids lost through sweating. It is recommended that athletes start refuelling immediately after exercise has stopped. Glycogen storage is 150 per cent faster in the first two hours after exercise (Ivy et al., 1988), in what is described by athletes as a ‘golden window’. Therefore it is important to eat carbohydrates as quickly as possible after finishing exercise. McArdle, Katch and Katch (2019) recommend 50–75 grams of high or moderate GI carbohydrates in the first 15 minutes after exercise, and then another 50–75 grams in the next two hours.
I'm James—an engineer based in New Zealand—and I have a crippling addiction to new ideas. If you're an enabler, send me a book recommendation through one of the channels below.
