Effect of Isotonic Exercise

With regular exercise a person’s cardiovascular fitness can be immensely increased as cardiac output is increased, therefore enabling a greater rate of O� being delivered to the body’s tissues. In cases of strenuous exercise, such as with athletes, the cardiac output is found to be double that of a person doing very little exercise, resulting in the enlargement of the heart and increasing the stroke volume. During isotonic exercise muscle contractions remain almost constant while the muscle may change in length.

As a result of exercise, heart rate increases as the demand for oxygen has been made greater. This is achieved by the means of two different mechanisms. The first of these mechanisms is the psychological effect as the body prepares itself for the task ahead. The increase of ventilation as a result of anticipation of exercise is known as the neural changes which send excitatory impulses to the inspiratory area in the medulla oblongata in the brain.

Stimulation of the limbic system also occurs as the body begins to prepare itself. Heart rate increases to ensure that enough blood is being pumped around the body. This initial response to the impending exercise is equivalent to that of the “fight-or-flight” response. Nerve impulses, initiated from the hypothalamus in the brain are sent to the sympathetic division of the ANS, which then starts to rapidly organise the body’s essential resources, such as oxygen and glucose, for the onset of physical activity. Vast amounts of these resources are transported to organs such as the brain, skeletal muscles and the heart.

The second mechanism is the actual increase in heart rate during exercise. This is a more complex sequence of events where the increased cellular metabolic processes taking place in active muscles cause relaxation of smooth muscle in the local arterioles. Arterioles have an essential role in regulating blood flow into capillaries by regulating the resistance; the mechanism used for opposing blood flow. Resistance is achieved by friction between the blood and the walls of the blood vessel. When relaxation of the smooth muscle occurs, the diameter of the blood vessel is greater and resistance is reduced as friction has also been reduced.

This is known as vasodilation. In contrast when contraction of the smooth muscle occurs, the diameter of the blood vessel has been reduced and there is an increase in both friction and resistance. This is known as vasoconstriction. During exercise there is a high demand for oxygen therefore as a result blood flow needs to be increased and resistance decreased. The relaxation of the smooth muscle in the local arterioles allows this happens. The process of vasodilation also causes a reduction in the Total Peripheral Resistance (TPR) which further caused a reduction in blood pressure.

Blood pressure (BP) is the force that drives the blood around the body. The greater the pressure exerted means that the flow of blood will also be greater. When the TPR causes a reduction in blood pressure, pressure sensitive receptors known as baroreceptors detect the change and set in motion a series of events to increase it. These receptors can be found in the walls of the aortic arch and internal carotid arteries. They send impulses to the cardiovascular system in the medulla of the brain to assist in the regulation of blood pressure. The two reflexes that are important in the regulation of blood flow are the carotid sinus reflex and the aortic reflex.

The carotid sinus reflex is initiated by baroreceptors in the walls of carotid sinuses which are located in the arteries of the neck. This reflex assists in regulating blood pressure around the brain. Nerve impulses are then projected from the carotid sinus baroreceptors to the cardiovascular centre in the medulla oblongata in the brain. The aortic reflex is initiated by baroreceptors in the aortic arch which regulates systemic blood pressure. Nerve impulses are projected to the cardiovascular centre by sensory neurons. The cardiovascular centre then works to increase blood flow and pressure by increasing heart rate and the force of the hearts contractions by the means on the sympathetic innervation of the heart.

An additional effect of physical activity is an increased amount of venous return to the heart which further increases the force of the hearts contractions. During isotonic exercise an increased affect on systolic blood pressure occurs whilst diastolic blood pressure remains relatively normal due to the fact TPR has been reduced. Systolic blood pressure is the force exerted on the artery walls during ventricular contraction and is normally 120mmHg in an adult. It is the highest pressure found in artery walls whilst diastolic blood pressure is the lowest. Diastolic blood pressure occurs during ventricular relaxation and is normally about 80mmHg in an adult.

The aim of this practical is to investigate the effect upon which isotonic exercise has on heart rate and blood pressure. Method Within this practical ECG recordings will be used to determine the heart rate of the subject and an electronic sphygmomanometer to establish the blood pressure readings. The three levels of exercise that will be performed are 40, 80 and 120 Watts. Before getting started on the bike the subject needs to secure the ECG electrodes in the correct positioning on the body. The electrodes should be placed in the lead II arrangement.

When the subject is ready they should get positioned on the bicycle and acclimatise until their heart rate and blood pressure return to baseline levels. The subject then should start exercising on the bicycle for 5 minutes at each workload. Heart rate and blood pressure should be recorded at rest and at each workload, moving to a higher workload as soon as the time is up and each set of data has been recorded. At the end of the highest workload the subject should be allowed to rest and recordings of the heart rate and blood pressure should be taken until they reach baseline levels.

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