What is endurance exercise?
Endurance is defined by the capacity to perform repeated muscle contractions or sustain a single contraction over time. Endurance training (ET) is also known as aerobic exercise, which outlines the importance of the need for oxygen during muscle contractions. It focuses on the activities that increase breathing and heart rate in order to maximise the take-up of oxygen in the respiratory system so it can be circulated in the blood and delivered to the working muscles, like the heart. These exercises include those like swimming, jogging, running, biking and walking.
Cardiac output and Heart Rate
Endurance can be increased through gains in muscle strength and changes in local metabolic and cardiovascular function. Regular aerobic training improves your cardiac output, via the increase in stroke volume (SV). As a result of hypertrophy, having a higher stroke volume makes the transport of oxygen more efficient. SV is a determinant of cardiac output, along with heart rate (HR). The goal of ET is to reduce resting and exercising HR in order to put less strain on the cardiac muscle, and in turn reduce the risk of myocardial infarction (MI), a symptom of coronary heart disease (CHD). SV can be measured using an echocardiogram (ECG). It measures the volume of blood pumped from the left ventricle per beat. Taking into account the factors that affect it: volume of venous return, ventricular distensibility, ventricular contractibility and aortic/pulmonary pressure. In order for the transport of oxygen to be most efficient, HR needs to initially be increased.
In a study conducted by Cornelissen et al (2009), male and females participated in a maximal graded cycle-ergometer test. They cycled at different intensities to measure their endurance. HR was measured at rest, during exercise and during recovery from maximal exercise, before and after ET. Figure 1 shows that training at higher intensity (HI) has more significantly recognisable positive results. Decreased HR following the training programme can be directly correlated to a higher maximal volume of oxygen (VO2max). VO2max is a measure for the amount of oxygen the body can use during exercise.
The study design used was a randomised crossover over a sixth month period. This duration of study is necessary as researchers believe physiological changes in fitness are only detectable after at least six months. The design used allows for precision of results, however it means that any effects from one treatment may affect the outcome the next treatment, invalidating the results. Overall concluding that ET increases SV and decreases HR when a person has a resting HR of 60bpm (bradycardia).
Figure 1: Relationship between heart rate before and after high and low intensity endurance exercise
Blood pressure and Blood Flow
Lots of research shows that low aerobic fitness is correlated with an increased risk of cardiovascular disease. Aerobic exercise is known to increase the size of the coronary artery in the heart. This is good as it reduces the risk of artery blockages from blood clots and plaque deposits (atherosclerosis). Therefore, allowing better blood flow with less resistance. Training the body allows more fat from food to be metabolised using ATP formed from aerobic respiration. Linked with this, another risk factor of CHD is high blood pressure (BP). ET can lower the volume of low-density lipoproteins (LDL) in the blood. This form of cholesterol is linked to a decrease in BP. It can also increase the concentration of high-density lipoproteins (HDL), that are more commonly known as ‘good cholesterol’, which are involved in the regulation of BP. High levels of HDL can transport lipids back to the liver for recycling and disposal, therefore, are an indicator of a healthy cardiovascular system The triglycerides that do not get metabolised can build up and eventually cause symptoms of CHD.
Carbon dioxide (CO2) and waste products (lactic acid) increase in the blood after exercise, this is detected by chemoreceptors in the medulla oblongata, which stimulate the sympathetic nervous system to initiate an increase in HR to pump more blood around the body. This raises the pH to a less acidic pH level. These neural factors aid the regulation of blood flow, the vasomotor centre responds to the changes in BP and acidity. If these factors increase, baroreceptors and chemoreceptors send impulses to stimulate the sinoatrial (SA) node in the heart to increase or decrease HR. This allows the cardiac muscle to work more efficiently to ‘shunt’ blood to prioritised areas via vasodilation and vasoconstriction of arterioles.
Figure 2: Relationship between systolic blood pressure before and after low and high intensity endurance exercise.
In the same study by Cornelissen et al (2009), BP at rest and post exercise recovery was measured before and after training. Figure 2 outlines how BP after training at both low intensity (LI) and HI in comparison to the untrained state is significantly lower, with the systolic BP at LI (Ftr = 18.35; P<0.001) and HI (Ftr = 34.16; P< 0.001) showing that BP decreases after training. With the P values of both these experimental states being P< 0.001, it can be assumed that the data is significant and therefore shows an inversely positive correlation between ET and decreases in BP. Further emphasising the idea that lowering BP can lower the risks and symptoms of CHD.
Smoking, high cholesterol, hypertension and diabetes are all lifestyle factors that can cause atherosclerosis. If diagnosed with CHD, not only can factors like these be changed, but undergoing a regular exercise programme 3-4 days a week could significantly reduce symptoms and improve overall physical and mental health. Stress is a big factor in the causes of CHD. During periods of stress, adrenaline and noradrenaline are released as a fight or fight response from the amygdala, these hormones increase HR, SV and BP. This allows more oxygenated blood flow to muscles with a higher metabolic demand. ET increases VO2max to allow the body to function in its optimal state. With this optimal function, the body can withstand stresses and decrease the risk of MI, eventually leading to a better quality of life.
Written by Georgia Murray
Word Count: 977 words
Reference list:
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