Aerobic training causes six main physiological adaptations that improve athletic performance: 1) resting heart rate decreases as the heart becomes more efficient, 2) stroke volume and cardiac output increase allowing the heart to pump more blood per beat to deliver more oxygen to working muscles, 3) oxygen uptake increases through improved cardiovascular and respiratory efficiency while lung capacity remains unchanged, 4) haemoglobin levels rise to transport more oxygen in the bloodstream, 5) muscle hypertrophy occurs through increased muscle fiber size from resistance training, and 6) aerobic training enlarges slow-twitch muscle fibers more than fast-twitch fibers.

1. Physiological Adaptations in Response to Aerobic Training
All training is undertaken with the goal of causing the body to adapt. The following 6 adaptations assist in
improving an athlete's performance.
1. Resting heart rate- decreases
Exercise strengthens the heart which increases its stroke volume (how much it can pump in one beat) which
means at rest the heart does not need to beat as often. Aerobic training therefore will lead to a lower resting
heart rate, due to the improved efficiency of the cardiorespiratory system.
2. Stroke Volume & Cardiac Output - increases
Stroke volume (SV) is the amount of blood pumped out of the heart during each contraction (or beat). Stroke
volume increases due to a stronger heart which can pump more blood each beat.
Cardiac output (Q) is the amount of blood leaving the heart each minute, and can be calculated by multiplying
stroke volume by heart rate (Q = SV x HR). Cardiac output increases during exercise as the heart is able to
pump more blood each beat and can therefore deliver more blood if required.
The ability of the body to make oxygen-rich blood available for working muscles is the biggest factor affecting
aerobic performance. The more blood that the heart can eject per heart beat, the more work an athlete will be
able to do.
3. Oxygen uptake & Lung Capacity - Oxygen uptake increases; Lung Capacity is unchanged
The oxygen uptake (VO2) of an athlete is the amount of oxygen which the body uses in a minute. Oxygen
uptake increases with training as the body becomes more efficient and there is more haemoglobin in the blood
to extract oxygen from the lungs.
Lung Capacity remains similar as the lungs themselves do not increase in size with exercise.
The cardiovascular system determines the supply of oxygen-rich blood to muscles. The respiratory system also
plays an essential role in athletic performance as oxygen levels will influence how hard a muscle can work and
for how long.
4. Haemoglobin levels- increase
Haemoglobin levels increase as a result of training, to try and get more oxygen to the working muscles. Red
blood cells transport oxygen to every cell in the body. Each red blood cell contains haemoglobin molecules. The
main function of haemoglobin is to absorb oxygen at the lungs and carry this oxygen to the working muscles
and organs via the blood stream. Haemoglobin absorbs oxygen at a very fast rate and this leads to an efficient
oxygen transportation system within the body.
5. Muscle hypertrophy – increases
Muscle hypertrophy refers to muscle growth together with an increase in the size of muscle cells. Muscles
increase in size with training as muscle fibres become thicker with increased work. This occurs as a result of
strength or resistance training that stimulates activity in muscle fibres causing them to grow. Training needs to
follow the overload principle to bring about muscle hypertrophy.
Hypertrophy does not occur as a result of aerobic training.
6. Effect on fast/slow twitch fibres - increased size of slow twitch; minimal change to fast twitch
Aerobic training will lead to increased size of slow twitch fibres and minimal change to fast twitch fibres. There
are two types of muscle fibre: slow-twitch muscle fibres & fast-twitch muscle fibres.
Slow-twitch fibres contract slowly and release energy gradually as required by the body during steady-state
activity such as jogging, cycling and endurance swimming. These fibres are efficient in using oxygen to generate
energy (ATP), making them resistant to fatigue but unable to produce the power of fast-twitch fibres.
Fast-twitch fibres contract quickly and release energy rapidly however, they fatigue rapidly due to anaerobic
metabolism providing the energy. The body preferentially recruits fast-twitch fibres to perform explosive type
activities such as weight-lifting, field athletics and sprint track athletics.