"Do, or do not. There is no try" - Yoda, 0BBY

Physiological adpaptations in response to training

At the heart of physical activity is the desire to reap the many benefits that are associated with it. These benefits can have a positive impact on all facets of a person’s individual health. For people wanting to improve their physical conditioning and performance in competitive sporting events, training enables them to positively impact their body in a range of unique ways.

These broad performance goals rely on unique physiological changes or adaptations that occur within the related systems of the human body, such as the cardiovascular, nervous and muscular systems. These adaptations are the result of a training stimulus that causes the human body to adapt, change and become more efficient. In doing so, the body is responding to the stimulus in a way that ensures it is better able to endure or cope with the physical stress being asked of it.

Obviously, there is a close relationship between the principle of training, progressive overload and the physiological adaptations that occur in response to training. For such an adaptation to occur, the stimulus must be greater than what the body is already accustomed to, otherwise the human body will remain largely unchanged.

To achieve these changes and adaptations, a specific, focused and individually tailored training program is required. As greater amounts of scientific data and more training innovations are developed in the areas of strength and conditioning, it can be a complex task to develop a training program that will safely and effectively help a person reach their desired performance goals. The most relevant physiological adaptations are:

Resting heart rate
Stroke volume and cardiac output
oxygen uptake and lung capacity
haemoglobin level
muscle hypertrophy
effect on fast/slow twitch muscle fibres

As each physiological adaptation is studied, links should be made with appropriate training activities and programs that will contribute to its development, as well as the principles of training.

Resting Heart Rate

A person’s resting heart rate (RHR) is a commonly used indicator of their general physical fitness. A lower RHR indicates a strong heart, capable of pumping a greater amount of blood in fewer beats. This is the preferred condition for the human heart, and regular aerobic and anaerobic training designed to appropriately stress the cardiovascular system will lead to a decrease in resting heart rate.

Typically, an untrained adult will have a RHR of between 72 and 80 beats per minute (bpm). However, a trained adult is more likely to have a RHR of closer to 60 bpm. As cardiovascular fitness increases, RHR can become as low as 30 bpm in extremely fit athletes such as ultra-triathletes. These figures are based on averages, as some healthy people have heart rate measurements that naturally fall outside of these norms.

One of the most important measurements for heart health is not the actual RHR of a person, but the time it takes for someone’s heart to return to resting values following exercise. This is known as their rate of recovery, and doctors often use this stress test on heart patients to check the health of their cardiovascular system. As their aerobic endurance improves and the heart strengthens, recovery is much faster. This is evident in elite athletes playing team sports, who are able to quickly recover following high-intensity maximal efforts.

In addition to this, athletes with greater aerobic conditioning will be able to maintain a lower working heart rate at all levels of intensity compared with an untrained athlete, except when the heart is pushed to its maximal capacity. Trained adults are able to achieve a higher maximum heart rate than untrained adults.

Stroke Volume & Cardiac Output

Endurance training involves exercising for a sustained period with an elevated heart rate. Over time, this type of training has a direct training effect upon the heart muscle itself, including both an increase in size (thicker wall of the left ventricle) and contractility (ability to efficiently contract repeatedly and forcefully). In addition to this, trained adults have more blood volume, to accommodate an increase in the number of oxygen-carrying red blood cells. These changes allow the heart a greater capacity to pump more blood per beat around the body by more fully ejecting the blood that has filled into the left ventricle. This is known as an increase in stroke volume. These changes all increase the body’s ability to pump oxygen-rich blood to the working muscles of the body, allowing them to continue to produce energy aerobically.

Cardiovascular development through aerobic and anaerobic training results in an increase in stroke volume at all levels of work intensity. Even though stroke volume plateaus at moderate intensities, a trained adult will always have a greater stroke volume than an untrained adult. This allows the heart to beat less often to deliver oxygen to the required muscles, leading to reduced levels of fatigue. This can be seen in the graph above.

Regular aerobic training leads to a significant increase in the efficiency and capabilities of the entire cardiovascular system. Greater stroke volumes combine with a greater capacity to work at higher heart rates to produce an increase in the product of the two: cardiac output.

To calculate a person’s cardiac output, their heart rate should be multiplied by their stroke volume:

heart rate (HR) x stroke volume (SV) = cardiac output (CO)

To determine the cardiac output of a healthy adult with a resting heart rate of 60bpm and a stroke volume of 75mL/beat, the calculation would be:

60 bpm x 75 mL/beat = 4500mL/min

However, it is while exercising that these measurements take on greater significance, with the cardiac output increasing by four to five times per minute. During exercise, the majority of this increased blood flow is directed towards the working muscles and away from less essential body systems, such as the digestive system. Because the trained adult has a lower working heart rate compared with an untrained athlete at a set intensity level, the increase in cardiac output is largely attributed to an increased stroke volume. The difference in cardiac output between an untrained adult and an elite athlete can be as much as double, with extremely fit adults being able to pump 30 to 40 litres of blood out of the heart per minute.

Apart from decreasing fitness levels through reversibility, another factor that leads to a decrease in the functioning of the cardiovascular system is age. As people get older, their heart rate, stroke volumes and therefore cardiac output all gradually decrease in capacity. For this reason, older athletes should take extra precautions when exercising at moderate to high intensities.

Oxygen Uptake and Lung Capacity

Aerobic and anaerobic training lead to the development of both the cardiovascular and respiratory systems. The combined system is often referred to as the cardiorespiratory system, which incorporates the lungs, heart and all blood vessels. The primary role of these body organs during exercise is to provide as much oxygen as is needed or is possible to the working muscles, to ensure that they continue to work aerobically for as long as possible. If the intensity of training is high, and more anaerobic in nature – that is, short- to medium-interval training – the oxygen required is used primarily for recovery, as the body is unable to supply enough oxygen to maintain consistent work intensity due to increasing fatigue from lactate build-up. In both cases, increased oxygen uptake and lung capacity will benefit the athlete, and are a natural physiological adaptation in response to training.

Regular aerobic training results in a variety of changes to the respiratory system. The actual lung capacity may increase slightly for some adults; however, this is more likely to occur in elite endurance athletes such as road cyclists. The most significant change is in the actual amount of air that can be moved in and out of the lungs with each breath. As with the heart’s stroke volume and heart rate, the lungs also develop in their capacity to draw in more air and increase the ventilation rate without necessarily getting larger. The manner in which the air is moved in and out of the lungs is more noticeable, as the lungs prefer to breathe more deeply and forcefully than to increase the ventilation rate (in a similar manner to the heart and the increase in stroke volume at submaximal intensities). The muscles that contribute to the process of breathing in and out increase in efficiency and strength, which equally contributes to increased lung capacity. Some athletes use specific breathing exercises and equipment to enhance their lung capacity.

To test lung capacity, athletes use medical-type equipment that can measure both the depth and force of breathing. It is interesting to note that the lungs are more than capable of breathing in adequate oxygen to supply the working muscles. It is actually the efficiency of the cardiovascular system that is often the limiting factor with regard to cardiorespiratory endurance. Development in this system will contribute to greater oxygen uptake, a key physiological adaptation to regular aerobic training. From deep sleep to high-intensity activity, our bodies have an oxygen demand that needs to be met. As the body’s ability to draw oxygen into the muscle cells is increased through regular training, the athlete will be able to sustain a higher average power output over an extended period, whether it is through running, swimming, cycling or rowing.

Testing oxygen uptake can be difficult without the use of technical equipment capable of measuring the amount of oxygen and carbon dioxide being inhaled and exhaled with each breath. However, maximal tests, where the athlete works to absolute exhaustion, can be a reliable measure of the maximum amount of oxygen that an athlete is able to draw into the working muscles per minute.

This is commonly referred to as VO2 max, which is measured in millilitres of oxygen per kilogram of body weight per minute. The popular multi-stage fitness test (also known as the beep test) and yoyo test both have prediction tables of maximal oxygen uptake, which is a respected measurement of aerobic fitness. The health benefits of an effective and efficient cardiorespiratory system are just as important as the performance benefits for athletes. Reduced risk of cardiovascular disease, type 2 diabetes and being overweight are all positive outcomes of regular training that stimulates the aerobic system to adapt to a suitably difficult training load.

However, all sportspeople benefit in some way from being aerobically fit. Archers with a lower resting heart rate can calmly shoot between heartbeats; netball centres are able to recover faster after successive and intense bursts of activity; marathon runners can sustain a greater pace over the course of the entire race. The training principles of progressive overload and specificity are essential to ensure that these adaptations continue. Training at the appropriate threshold of intensity, as well as using the type of training needed, will specifically develop the required energy systems for a particular sport. Also, reversibility is very relevant with regard to oxygen uptake. An athlete who stops or slows their volume and intensity of training will quickly notice a decrease in aerobic and anaerobic performance. Consider an athlete following an injury and the noticeable loss of reported fitness and endurance.

Haemoglobin

Haemoglobin is the molecule in blood that binds with oxygen and transports it around the body in the blood. It is contained within the red blood cell and is responsible for giving the cell its red colour when bound with oxygen. Aerobic training stimulates the body to produce more haemoglobin in order to increase the oxygen carrying capacity of blood.

Haemoglobin levels increase in response to training and improve the body’s ability to transport oxygen to the muscles where it is needed for energy production. This energy production is done through the aerobic system, producing ATP for movement. This energy system produces carbon dioxide and water as by-products, which are easily removed by the body.

This increased usage of the aerobic energy system delays the need to rely on the anaerobic energy systems and therefore helps avoid fatigue caused by the build up of acid in the muscle. Therefore, an increased haemoglobin level increases the workload at which the athlete reaches their anaerobic threshold. This allows the athlete to have a maintain higher intensities for longer periods of time, because they remain within their aerobic training zone.

This also improves performance by allowing faster recovery from acid build up, which then allows higher anaerobic intensities to be used with shorter rest periods between each anaerobic workload.

Muscular Hypertrophy

Muscular hypertrophy is an increase in the size of the muscle cross-sectional area because of an increase in myofibrils (the tissue component of the cell responsible for contraction) within the muscle cell (myocyte). A myofibril is a rod like unit within a myocyte that contracts to produce movement. The larger the myofibril, the larger the myocyte and the more force the muscle can produce.

Muscular hypertrophy results in an increase in muscular strength and muscular endurance. This helps improve performance by allowing the athlete to exert a greater force and to repeat movements more often. Often hypertrophy will also increase muscular contraction speed allowing greater power to be produced during contraction. This is very beneficial in sports that require, strength, power, or muscular endurance. Such sports include: shot put, sprinting, rugby, NFL, AFL, Ice-hockey, and martial arts.

Effects on fast and slow twitch fibres

Training has an array of effects on fast and slow twitch muscle fibres which are specific to the type of training. Some of these adaptations are similar, such as: increased capillary density, which increases the delivery of blood to the muscle cells. An increase in mitochondria in the muscle cell, which increases ATP production from the aerobic energy system. As well as increased myoglobin, which transports oxygen from the blood through the muscle to the mitochondria. However, other adaptations are specific to the muscle type.

Slow twitch (or type I) muscle fibres are used for movements that have a long duration. They are red in colour because of the extra blood supply they have in order to assist the aerobic energy system. The adaptations within these muscles assist in the use of the aerobic energy system and include increased: mitochondria, capillary density, aerobic enzymes needed for ATP production in the aerobic energy system, glycogen and fat stores, and myoglobin. All of these adaptations help in the delivery of ATP through the aerobic energy system.

Fast twitch (or type II) muscle fibres are the fibres used for strength, power, and movements of high intensity and short duration. They can be linked with the two anaerobic energy systems, which means the adaptations in these fibres help in the use of these systems. Adaptations include increased anaerobic enzymes for glycolysis, increased PC stores, hypertrophy and increased removal of lactate, which helps reduce the acidic levels in the muscle.

It is also widely recognised that fast twitch muscle fibres can be further split into two other categories:

Type IIa - a fibre that displays characteristics of both FT and ST fibres
Type IIb - a classic fast twitch fibre suited to maximal power production

Slow twitch muscle fibres

These are also described as red muscle fibres because of their high blood supply, being provided by the increased number of blood capillaries present . This increased surface area allows for greater gaseous exchange – primarily oxygen into the working muscle and carbon dioxide removal. As a result, type I ST muscle fibres are adapted to use oxygen more efficiently and work aerobically. They contract slowly and release energy gradually as required during endurance activities over a longer duration, such as jogging, cycling or swimming. They are more efficient in the use of fats as a fuel source during exercise at a comfortable steady pace. This promotes glycogen sparing and conservation, in preparation for periods of higher intensity efforts. They are fatigue resistant at submaximal intensities; however, overall power output is low.

Sports that are better suited to athletes with higher percentages of ST fibres include:

• marathon running

• long-distance swimming

• ultra-ironman triathlons.

Key characteristics of type I ST muscle fibres (in comparison to type II FT muscle fibres) are:

a greater number of capillaries and therefore blood supply, promoting greater oxygen supply

• the metabolisation of ATP at a slower rate

• increased amounts of mitochondria, the organelle in muscle cells responsible for aerobic energy production

• increased amounts of myoglobin, which plays a similar role to haemoglobin in storing and transporting oxygen within the cell to the mitochondria for aerobic respiration and energy production

• increased amounts of oxidative enzymes to assist with aerobic energy production.

Obviously, aerobic-based endurance training will have the greatest impact upon type I ST fibres. Specific physiological adaptations in response to aerobic training include:

• an increase of over 15 per cent in the number of muscle capillaries

• increased myoglobin stores to support the increased oxygen supply

• increased size, efficiency and amount of mitochondria, accounting for greater aerobic capacity

• some increase in hypertrophy, often seen as an increase in lean muscle mass and tone.

Fast Twitch Muscle Fibres

These are often described as white muscle fibres. Because (type II) FT fibres largely rely on anaerobic pathways – that is, without the need for oxygen – the body does not need to supply these fibres with a rich source of blood and oxygen – hence the difference in colour between the two types:

FT fibres have significantly greater power and force production, with greater resistance to short-term fatigue during anaerobic activities. However, if the activity lasts for more than a minute or so, these fibres fatigue quickly due to the decreased oxygen supply.

Sports that are better suited to athletes with higher percentages of FT fibres include:

• sprints (100 to 400 metres)

• shot put

• weightlifting.

Key characteristics of type II FT muscle fibres (in comparison to type I ST muscle fibres) include:

• fewer capillaries (therefore blood and oxygen supply) and fewer mitochondria

• more fuel for anaerobic energy pathways, namely creatine phosphate and ATP

• the metabolisation of ATP at a faster rate

• increased amounts of glycogen

• larger motor units and neurons that stimulate FT fibres, capable of faster and more powerful stimulation, described as increased excitability and contractility

• increased glycolytic enzymes to assist with the fast metabolism or breakdown of glycogen in the absence of oxygen.

Training methods that target FT fibres depend upon the specific nature of the sport. Medium to heavy resistance training, aimed at increasing power, strength or muscle bulk, will lead to specific adaptations. Likewise, anaerobic training, using short intervals where anaerobic pathways are utilised, will also lead to development of type II FT fibres.

Specific physiological adaptations in response to anaerobic training (either resistance or short intervals) include:

• increased stores of ATP, creatine phosphate, glycogen and glycolytic enzymes, all leading to increased production of ATP from anaerobic energy pathways

• increased synchronisation and coordination of motor unit recruitment, leading to increased neural activation, and therefore greater force and power production

• considerable potential for muscle hypertrophy with specific training programs, focusing on heavy resistance training

• greater tolerance to increased muscle acidity and more efficient lactate clearance

• some increase in hypertrophy, often seen as an increase in lean muscle mass and tone.

As mentioned earlier, type II FT fibres can be categorised into either type IIa or type IIb.

Type IIb is a purer or more classic version of fast- twitch muscle fibres, and the previous description is very accurate for these. However, type IIa requires some further explanation. These are often described as intermediate fibres, because they contain characteristics of both fast- and slow-twitch muscle fibres. They have increased capillary supply and mitochondrial function compared with type IIb fibres, but also an increased amount of anaerobic capacity compared with type I fibres. They can produce higher power outputs over a longer period, and can recover in shorter amounts of time due to their increased oxygen supply. From a training perspective, type IIa fibres will respond specifically to the type of training being performed. Therefore, they take on characteristics and capacities of either type I ST or type IIb FT, depending upon the training stimulus. For this reason, they can be described as being interchangeable, capable of enhancing either the aerobic or anaerobic power of the athlete. People with higher percentages of type IIa fibres are very adaptable athletes, who succeed in sports that require a mix of both energy systems, such as AFL, netball and general classification cycling, where riders must have endurance to climb mountains as well as ride fast in a time trial.

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