The immediate physiological responses to training are numerous, but for Year 11 HMS these are limited to heart rate, ventilation rate, stroke volume, cardiac output and lactate levels. The immediate physiological responses to training are proportional to the intensity of the training. Physical activity demands oxygen delivery along with the removal of carbon dioxide and lactic acid. The immediate changes help to achieve a higher delivery of oxygen, faster removal of carbon dioxide and conversion of pyruvic acid to lactate. 

The physiological responses to training can take around 3 minutes to fully adapt to the intensity of training and the speed at which the responses return to resting levels relates to fitness levels and the body’s ability to recover from training sessions. The reason for many of the immediate physiological responses to training is the increased amount of carbon dioxide produced by the working muscles, stimulating an increase in heart rate, ventilation, stroke volume and cardiac output. Lactate levels increase in response to an increased use of the lactic acid energy system and the muscle’s need to remove lactate to delay fatigue.

During exercise the body undergoes several immediate physiological changes that allow for an increased movement of blood around the body. This increased blood flow has two functions: 

• increase oxygen delivery 
• remove waste products such as carbon dioxide 

Use the acronym H.S.C.V.L (HSC = Very Long!)

Heart rate is the number of times the heart beats in a minute. Heart rate responds to training by increasing from the resting value and is often used to set or determine the intensity of the training session. For example, an athlete might train for 20 minutes at 80% maximal heart rate (MHR).

Before the body even begins to move a trained athlete’s body will begin to respond to the familiar visual stimulus, by increasing the heart rate. Heart rate then increases in response to exercise because the body detects an increase in carbon dioxide in the blood. This increase in carbon dioxide indicates that the body requires more oxygen, which results in the body increasing its heart rate.

The heart rate (HR) can be seen to increase just prior to training in the trained athlete and then increases quickly to around 120 bpm. After this, the body responds to the carbon dioxide levels in the blood to increase the HR as required, with the trained athlete having a lower HR than the untrained athlete. Once training is finished the trained athlete’s HR will return to normal levels faster than the trained athlete.

The body can take 1 to 3 minutes to complete this change in heart rate so that the new heart rate is moving blood around fast enough to deliver the oxygen and remove the carbon dioxide at the rate required. This is because the body is constantly checking the gas levels in the blood and making further adjustments to the heart rate. If a new intensity occurs, the body will adjust the heart rate again according to the intensity. For example, if an athlete is doing hill sprints, their heart rate might be at 174 beats per minute (bpm). If an athlete changes the activity to jogging around a football field, their heart rate will drop possibly to 130 bpm, but this could take 1 to 3 minutes to occur.

After the training session, the heart rate returns to normal as the body is no longer producing carbon dioxide at the higher rate, causing the body to reduce the heart rate.

Stroke volume is the amount of blood measured in mL pumped out of the left ventricle of the heart per contraction. The stroke volume of each ventricle is often the same so that the right ventricle pumps the same amount of blood to the lungs as the left ventricle pumps to the body. Stroke volume’s immediate response to training is to increase. The average human has a stroke volume of around 70 mL, this volume can double during exercise at high intensities.

The continuing rise in stroke volume for some athletes and not others has been attributed to a variety of variables including, sex, blood volume and fitness levels of the athletes. It is thought that the elite athlete is able to continue to increase their stroke volume through to their VO2max, while others cannot, but more study is needed.

The reason for stroke volume increase during training or exercise is threefold. Firstly there is an increase in blood returning to the heart due to muscular contractions, which naturally results in greater diastolic filling of the heart increasing the stroke volume. Secondly, the body has a higher demand for oxygen and therefore the heart contracts more forcefully during exercise. And thirdly, there is less resistance to the blood moving out of the ventricle due to vasodilation (widening) of the blood vessels.

Cardiac output is the amount of blood pumped out of the left ventricle in a minute. It can be calculated by multiplying the stroke volume by the heart rate and is usually given in litres per minute. Stroke volume and heart rate both increase in response to training, therefore cardiac output also increases in response to training.

Cardiac output immediately increases in response to training and is directly related to the intensity of the workout. For example, an athlete who is cycling and producing 200 watts of energy will have a lower cardiac output than when they are cycling at 400 watts.

The reasons why cardiac output increases in response to training include:

1. Greater blood flow back to the heart, creating a larger stroke volume.

2. Less resistance to blood flow due to vasodilation.

3. An increased demand for oxygen and nutrients by the working muscles.

4. Larger production of carbon dioxide during exercise.

The body responds particularly to the increase in carbon dioxide, by increasing the body’s cardiac output. This is achieved by increasing the stroke volume of the heart and increasing the heart rate. It is the increases in stroke volume and heart rate that produce the increase in cardiac output. Without these increases, the cardiac output would remain the same.

The graph below depicts the immediate physiological responses of the heart to training. It is clear that there is a rise in SV and HR, which in turn causes an increase in CO. It is SV that increases first and then HR, making SV responsible for the initial rise in CO and then increases in HR responsible for further increases. Both HR and SV slowly return to resting levels after training, which naturally brings CO to resting levels also.

Whether a person is considered ‘trained’ or ‘untrained’ their heart pumps approximately 5 Litres of blood per minute at rest. 5L = 5000ml, therefore CO = 5000ml at rest.

Ventilation rate is a measure of how many breaths a person takes per minute, and is also known as the respiratory rate. As with the heart rate, an athlete’s ventilation rate will have an immediate increase in response to training. This is for the same reason that there is an increase in HR, the body is responding to the increasing concentration of carbon dioxide in the blood. In order to remove the carbon dioxide, the body has to breathe it out. By increasing respiratory rate the body increases the amount of carbon dioxide removed, while at the same time increasing the amount of oxygen inspired.

The size of the increase in ventilation rate is directly connected to the intensity of the training, so that the more intense the training the higher the ventilation rate. For example, the athlete who was training at 85% MHR will have a higher ventilation rate than when they are training at 65% MHR. This is because there is less demand for oxygen and a smaller amount of carbon dioxide being produced.

Similar to the changes in HR, the ventilation rate (VR) increases slightly just before training in anticipation of movement. The athlete’s VR then increases according to the intensity, with the untrained athlete requiring higher VRs than the trained athlete. After training, the trained athlete returns to normal faster than the untrained.

If there is a change in the intensity the athlete will have a change in ventilation rate. This means the athlete who was jogging at 60% will have an increase in ventilation rate when they complete a 60m sprint at 100%. This change in rate comes in response to the increased demand for oxygen delivery and carbon dioxide removal.

After training an athlete’s ventilation rate will return to normal levels as the demands for oxygen delivery and carbon dioxide removal decrease.

Lactate levels refers to the amount of lactate and/or lactic acid in the blood. Lactic acid is produced by the Glycolytic (lactic acid) energy system and is quickly converted to lactate before being transported to the liver where it is converted to glucose. During exercise lactate levels rise in proportion to the intensity of the training (as shown in the graph).

This graph depicts the changes in lactate levels as intensity increases. At lower intensities the aerobic energy system can manage the supply of energy, keeping lactate levels around 2 Millimoles/L. Once the Lactate Inflection Point is reached; however, lactate levels continue to rise until fatigue.

Lactate levels in the blood rise in response to the body using the lactic acid energy system, which is required for higher intensities of training (~85%+). This means that during lower intensities, there is a slight rise in the lactate levels, but this level (normally around 2 Millimoles/L) is maintained throughout the training, before returning to resting levels once training stops.

In contrast, anaerobic training, such as short interval training, causes much larger increases in lactate levels due to the use of higher intensities that specifically rely on the lactic acid energy system. A sample comparison can be seen in the table below, where the aerobic training is constant, while the anaerobic interval training continually rises in response to the repeated bouts of high intensity workloads. 

As training begins both aerobic and anaerobic training cause an increase in lactate levels, but there is a substantial difference in the levels. Aerobic training then maintains the lactate levels, while anaerobic training causes the levels to continually rise until training is ceased. Once stopped both lactate levels slowly return to resting levels.

The rise in lactate levels is caused by using the lactic acid energy system. The more the body relies on this system, the more lactate is produced, causing the rise in lactate levels in the blood. This means that at lower intensities lactate levels will not increase as much. So for example, if the interval is 10s of work and 20s of rest the lactate level may stay under 3 Millimoles per Litre.