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With input from Philip Tan & Fabian Lim
One does not become a marathoner overnight. Endurance running requires an extended period of training. The training provides a stimulus to induce adaptations in our bodies to make us more efficient runners.
Let us first understand how our body works when we run. They are all related, but I’d like to simplify things by looking at the four key systems that enable us to run: the musculoskeletal, energy, cardiorespiratory, and cooling systems.
Body Systems
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| Figure. 2.1. Impulses generated in the brain and spinal cord and transmitted via motor nerves to the muscles, triggering a series of coordinated muscle contractions to bring about movement. |
Musculoskeletal system
Running comprises a series of coordinated movements involving our musculoskeletal system, which comprises our muscles, tendon, bones, and joints. Tendons, which are slightly elastic structures, serve to attach our muscles to bones, while ligaments join one bone to another. Muscles contain contractile elements that require energy to contract. Hence, contracting muscles move our limbs much like a lever. For example, the quadriceps muscles in the front of the thigh help to flex the hip and extend the knees as we run, while the hamstring muscles at the back of the thigh do the opposite.
Each muscle fibre is a cell, containing the active actin and myosin that contract using energy supplied by adenosine triphosphate (ATP). Mitochondria, the cell’s powerhouses, generate ATP from fat and carbohydrate, using oxygen that is brought there from the surface of the cell by myoglobin. The oxygen is brought to the muscle fibre by haemoglobin in the blood vessels, the smallest of which are capillaries that surround each muscle fibre. Motor nerves interface with the muscle fibres via motor end plates. Our brain and spinal cord control movements by sending electrical impulses down these nerves to trigger muscle contractions (see figure 2.1).
Your endurance and speed during the run depend largely on your muscles’ ability to produce energy and force. Generally, there are two main types of muscle fibres existing in various proportions within the muscles. These fibres are differentiated on the basis of their colour, the quantity of mitochondria they contain, and the speed at which they contract. Type 1 or Slowtwitch (ST) fibres are red and have a high concentration of mitochondria and myoglobin. On the other hand, Type II or Fast-twitch (FT) fibres are white and have low mitochondrial content. ST muscle fibres are very efficient at producing ATP from the oxidation of carbohydrates and fat and are recruited most often during endurance events such as the marathon. FT muscle fibres, while able to generate
more force than ST fibres, fatigues easily due to their limited endurance and are hence
recruited mainly for explosive sprinting events.
The bones and joints are an important part of the skeletal system. These structures are subject to substantial impact forces as we run. Even while jogging, the impact forces that reverberate through our skeletal system are about three times our bodyweight.
Energy Systems
When we run, muscles contract and relax in a coordinated pattern to propel the body forward. This work requires energy, which is provided to the muscles in the form of ATP.
ATP is generated from carbohydrate or fat via the aerobic (i.e. requiring oxygen) or anaerobic (i.e. no oxygen needed) system. The latter comprises the ATP phosphocreatine system and the anaerobic lactic system. There are therefore three energy systems that deliver ATP to the working muscles:
- The ATP-PC (Adenosine-Triphosphate Phosphocreatine) system is the instant energy source as it relies on existing ATP that is stored in the muscle cells. When energy is needed immediately, phosphates are split from ATP molecules to release energy. However, this system can only last up to 10 seconds before the ATP runs out, and is at work when you push for a short sprint.
- The anaerobic lactic system breaks down carbohydrate or fat to produce ATP in when the exercise intensity is high. It can generate ATP in the absence of oxygen, but lactic acid (lactate) is generated as a by-product. As our body can only tolerate a certain amount of lactate accumulation, we cannot rely on the anaerobic lactic system beyond two to three minutes.
- The aerobic system is the most important for endurance athletes, as it is a sustainable and efficient source of ATP. Unlike the anaerobic lactic system, the aerobic system requires oxygen to produce ATP from carbohydrate or fat.
All three energy systems are active concurrently, but the contribution varies according to the intensity and duration of your run (figure 2.2).
There are three main energy sources or substrates, namely carbohydrates, fat, and proteins. Carbohydrates are our body’s first-choice fuel, as it is fast-release. Carbohydrates (e.g. glycogen and glucose) are stored mainly in our liver and muscles, and there is about 2,000 kilocalories (kcal) worth of carbohydrate stores. When we run continuously, we will soon run out of carbohydrates, usually 30 km later, and that is when we ‘hit the wall.’
Our other source of fuel is fat. Unlike carbohydrates, our fat stores are practically limitless. Let’s say you weigh 70 kg and your body is 20 per cent fat, then you would have 14 kg of fat. This amount of fat contains 107,800 kcal of energy (1 kg of body fat contains 7,700 kcal), whereas we only need about 3,000 kcal to complete a marathon! Unfortunately, the body fat can only be released slowly — at moderate to fast running speeds, we have to depend mostly on carbohydrates.
Proteins, mostly found in muscles, can also be burnt for energy. However, our bodies are reluctant to do this, as we need our muscles, especially if we are physically active. Hence, it is not a major energy source.
Fig. 2.2. The ATP-PC system is the main source of energy for short bursts of activity lasting up to 10 s. The anaerobic lactic acid system is activated during high-intensity activity lasting two to three minutes. The aerobic system and be relied on for longer durations, but the rate of energy production is low.
Cardiovascular System
The muscles of endurance runners require oxygen and energy substrates. These are delivered to the muscles by the cardiovascular system. Oxygen in inhaled air is brought into contact with blood carried in fine blood vessels (capillaries) within the lungs. As blood passes through the capillaries in the lungs, carbon dioxide (a waste product of energy production) is released and exhaled out of the body. Simultaneously, oxygen is taken up by haemoglobin within our red blood cells. The haemoglobin transports the oxygen from the lungs to all cells and ultimately into the mitochondria for the production of ATP.
While the lung serves as the interface to transfer oxygen from the air to the haemoglobin in our blood, the heart pumps the blood round the body, so that the oxygen from the lungs can reach the muscles. The cardiac output, or the volume of blood that is pumped out of the heart every minute, reflects the effectiveness of this pump. For most individuals, this is about 5 L/min. The cardiac output is dependent on the stroke volume (the amount of blood pumped out with each cardiac contraction) and the heart rate:
cardiac output (CO) = stroke volume (SV) x heart rate (HR)
The cardiac output required to feed our muscles is mostly dependent on our running speed. Hence, if two runners are running at the same speed, both will have similar cardiac outputs, but the fitter runner with the more powerful pump (i.e. bigger SV) will have a lower heart rate than the unfit runner. With a lower heart rate, the heart is less taxed at that speed compared to the unfit runner.
Endurance is reflected by one’s maximal aerobic capacity, or VO2 max. The VO2 max is like the engine capacity of a car — the bigger it is, the better, and it is dependent on a few variables, where:
VO2 max = CO x arteriovenous oxygen difference
The arteriovenous oxygen difference is the difference in oxygen concentration between the blood entering and exiting the muscle, and reflects the muscle’s ability to extract oxygen, which, in turn, is dependent on the capillarity (number of capillaries surrounding the muscle) and amount of mitochondria in the muscles.
While is it good to have a large ‘engine capacity,’ this must be taken in context to the size of the vehicle — having a large engine in a heavy car does not make it a fast car. What you want is a large engine in a light car. Hence, the relevant parameter is not the absolute but the relative VO2 max, where:
Relative VO2 max = VO2 max ÷ bodyweight
Cooling System
Heat production can increase by 10- to 20-fold during intense running. The body is only able to use approximately 25 per cent to 30 per cent of the energy from food to perform physical work, leaving about 70 per cent ‘wasted’ as heat, which is transported from the muscles to the skin and lungs by circulating blood. The skin and lungs then dissipates the heat to the environment. The regulation of body temperature (thermoregulation) has a significant influence on performance in endurance events. If we are unable to remove the excess heat quickly enough, the high body temperature (approximately 40 °C core temperature) can signal the brain to induce fatigue and compromise running performance. Researchers have observed that faster runners tend to have higher body temperatures at the end of the race than the slower runners, suggesting that the ability to tolerate a high body temperature is beneficial to running performance. Native African runners also perform better than Caucasian runners in the heat but not in cool environments. The higher body mass in Caucasians causes their body temperatures to rise faster when running in the heat, which causes these runners to run slower in order to prevent overheating. In contrast, African runners, with lower body mass, have the advantage of lower heat production even when running in the heat. This advantage allows the African to tolerate a higher running pace without overheating.
Apart from heat production by the muscles, another factor that influences body temperature during a run is running economy. An economical runner will expend less energy when running at the same speed as an inefficient runner. Running economy will be discussed in further detail in Chapter 5. The lower energy requirement in the more efficient runner translates into less internal heat production during the run.
The evaporation of sweat accounts for about 80 per cent of heat removed from the body. Evaporation occurs when sweat changes from liquid to gaseous state, and this is facilitated by a low relative humidity. Sweating alone, without evaporation (e.g., sweat dripping off the body), results in fluid loss without significant heat removal. Runners feel more comfortable running in a cooler environment, but do bear in mind that high relative humidity can also exist in cool environment. For example, the relative humidity in Singapore is the lowest at noon (about 60 per cent) and highest at about 1 am (approximately 90 per cent). The cool environment should not lead to a false sense of security from the effects of heat stress.
Heat stress puts a strain on the cardiovascular system by diverting blood away from the muscles to the skin for heat dissipation. The loss of body water through sweating reduces the circulating blood volume, further taxing the cardiovascular system. Running at moderate to high intensity can induce a sweat rate of approximately 1 – 3 litres (L) per hour. Some electrolytes (salts) are also lost from the body through the sweat. If the sweat loss is not replaced during a long run, the blood volume contracts and compromises the cardiac output. The heart compensates for the lower blood volume (and pressure) by pumping more rapidly in order to maintain blood flow to the muscles so that the run can be sustained. This compensatory increase in heart rate, known as cardiovascular drift, forces the heart to work harder. If the cardiac output cannot be maintained, then running performance is compromised and heat injuries such as heat cramps, heat exhaustion, and heat stroke can occur.
It is important to understand the key systems involved in running because those are the areas we are working on improving during training. You will not progress quickly if you train blindly without understanding what you are doing. Think of the key body systems as analogous to parts of a racecar (figure 2.3):
Figure 2.3. Key body systems involved in running and their equivalents in a racecar |
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