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Aerobic Exercise

    Most of us recognize the benefit of exercise and the potential improvements to our general health and well being. However few of us, including the sporting heroes whose physical exploits we may admire, fully understand the physiology of exercise and the best means of achieving specific exercise objectives.

    To get the most out of any exercise program it is essential to have a clear understanding of what your exercise objectives are. These may include developing bulging muscles, improving general strength, reducing or maintaining weight, improving sporting competitiveness or simply avoiding the health consequences of a modern sedentary lifestyle.

    Many of the commonly held beliefs relating to exercise are poorly substantiated and some are completely fictional. A classic example is the old adage "no pain no gain" which unnecessarily associates exercise with discomfort.

    Here we explore some of the realities of exercise physiology, and try to highlight the best ways to achieve certain exercise objectives and hopefully dispel some of the myths.

Let us start by separating exercise program into two distinct groups, resistance based and aerobic based.

Resistance based exercise

    Resistance training is designed to improve either muscle strength or size. The muscle cells are purposely damaged through a process of overloading, the body reacts instinctively to repair the damaged cells so they can cope with any future overload, increasing their size and strength in the process.

    The muscles themselves do not actually increase in number, as some might think: the human body has a genetically defined number of muscle cells.

    Muscular fitness is a combination of strength, endurance and flexibility. Resistance training occurs over a short time frame and does not necessarily improve endurance capacity or for that matter flexibility. There may even be a decrease in endurance capacity because as the muscle cells grow the fluid between the cells, essential to oxygen transportation, is reduced.

    There is also evidence to show that resistance training can cause an increase in blood pressure.

Aerobic based exercise

    Aerobic training relates to the processes by which the body generates the energy to perform work. The principal fuels used to produce energy are the body's stores of fat, carbohydrate or protein. These fuels can be converted into energy by one of two processes, the aerobic metabolic process or the anaerobic metabolic process.

    The aerobic process consumes fuel in the presence of oxygen (supplied by the flow of blood) producing by-products, carbon dioxide and water, which are expelled by respiration and perspiration. The aerobic process provides the majority of the energy used by the slow acting muscle fibers (crucial to endurance activities).

    The Anaerobic process occurs when there is not enough oxygen in the blood to produce energy aerobically. This process consumes carbohydrate as its primary source of fuel and does so in the absence of oxygen, producing a by-product called lactate. It is lactate which produces the muscle soreness and fatigue associated with excessive exercise. The anaerobic process provides the majority of the energy used by the fast-acting muscle fibers (crucial to strength and power activities).

    When we start exercising, energy is initially produced anaerobically until the respiratory and cardiovascular systems respond and supply the oxygen necessary for aerobic energy production, hence the increase in breathing and heart rates. Once oxygen supply is sufficient, most of the energy will be produced aerobically, with the balance supplemented anaerobically. The lactate formed by this residual anaerobic production is easily dissipated by the body's organs, avoiding any onset of fatigue.

    As exercise intensity increases the muscles' ability to produce energy aerobically will reach a limit (defined by the capacity of the respiratory and cardiovascular systems to supply additional oxygen). At this point the body cannot supply additional oxygen, and energy production becomes anaerobic. This transition point is the maximal aerobic output and is called the aerobic threshold. Exercise above this level causes a rapid build-up of lactate, leading to muscle fatigue which will cause cessation of exercise.

    Knowledge of the aerobic/anaerobic process is essential to the successful attainment of specific exercise aims. Fat burn (weight maintenance), cardio-vascular training (cardio-vascular or endurance fitness) and anaerobic training (tolerance to fatigue) all rely on an understanding of the way our body produces energy.

Weight maintenance

    At lower intensities our body uses a mixture of fat and carbohydrate as its source of fuel. As the maximum aerobic output is approached, the percentage of fat consumed as fuel reduces to zero. Also, as the duration of exercise is increased the percentage of fat consumed tends to increase.

    If your objective is weight maintenance, then it is necessary to burn as much fat (as opposed to carbohydrate) as the source of fuel as possible. This is best achieved at lower intensities and over longer durations. As soon as the intensity is increased the aerobic process starts to burn more carbohydrate and the weight maintenance effect will be reduced. A low intensity (60 - 70% of the maximal aerobic output) is typically that at which you can hold a conversation: it is by no means strenuous and is about that achieved by a brisk walk.

Aerobic training

    A sustained exercise program will improve the efficiency with which the respiratory and cardiovascular system can supply oxygen. This improves lung function, heart function, vascular efficiency and capillary growth, leading to improved well being and endurance.

    At about 70 - 80% of the maximal aerobic output, lactate begins to accumulate in the blood supply at a greater rate than it can be extracted by the liver, kidneys and other organs. Exercising above this intensity will cause progressive accumulation of lactate in the blood, increased heart and breathing rates, cause muscle fatigue and will eventually lead to the cessation of exercise. Prolonged exercise at or below this intensity will maintain lactate at non-fatiguing levels and exercise duration will be limited solely to the depletion of available fuel stores.

    If your exercise objective is aerobic (cardio-vascular/endurance) training then it is necessary to exercise at an intensity which will avoid fatigue due to lactate build up. This is best achieved at moderate levels of intensity over medium/long durations. A moderate exercise intensity (70 - 80% of the maximal aerobic output) is about that achieved by a steady jog, avoiding the onset of muscle soreness.

Anaerobic training

    Anaerobic training causes the build-up of lactate (as rapidly as one minute after exercise is commenced). Lactate saturation will necessitate either a rapid reduction in exercise intensity or complete cessation.

    The accumulation of lactate limits the contribution of the anaerobic process to total energy production. Though energy may be generated rapidly, total work capacity and total power output is limited.