Read the text slowly and carefully. After the first reading, do exercise a: this will you to see whether you have got a good general idea of what the passage says. Then read the text again, paying careful attention to detail.
The human body is made up mainly of bone, muscle, and fat. Some 639 different muscles account for about 45 per cent of the body weight. Each of these muscles has four distinct and measurable qualities which are of interest to us:
(1) it can produce force which can be measured as strength of muscle;
(2) it can store energy which permits it to work for extended periods of time Independent of circulation — this is generally referred to as muscular endurance;
(3) it can shorten at varying rates. This is called speed of contraction;
(4) it can be stretched and will recoil. This is called the elasticity of muscle.
The combination of these four qualities of muscle is referred to as muscular power.
If muscles are to function efficiently, they must be continually supplied with energy fuel. This is accomplished by the blood which carries the energy fuel from lungs and digestive system to the muscles. The blood is forced through the blood vessels by the heart. The combined capacity to supply energy fuels to the working muscles is called organic power.
The capacity and efficiency with which your body can function depends on the degree of development of both your muscular and organic power through, regular exercise. However, the level to which you can develop these powers is influenced by such factors as the type of body you have, the food you eat, presence or absence of disease, rest and sleep. You are physically fit only when you have adequately developed your muscular and organic power to perform with the highest possible efficiency.
Heredity and health determine the top limits to which your physical capacity can developed. This is known as your potential physical capacity. This potential capacity varies from individual to individual. Most of us, for example, could train tor a lifetime and never come close to running a four-minute mile simply because we weren't built for it. The top level at which you can perform physically right now is called your acquired capacity because it has been acquired or developed through physical activity in your daily routines.
Your body, like a car, functions most efficiently well below its acquired capacity. A car, for example, driven at its top speed of, say, 110 miles per hour uses mote petrol per milt than when it is driven around 50-60 miles per hour, which is well below its capacity. Your body functions in the same way, in that the ratio of work performed to energy expended is better when it functions well below acquired capacity.
You can avoid wastage of energy by acquiring a level of physical capacity well above the level required to perform your normal daily tasks. This can be accomplished by supplementing your daily physical activity with a balanced exercise programme performed regularly. Your capacity increases as you progressively increase the load on your muscular and organic systems. Exercise will increase physical endurance and stamina thus providing a greater reserve of energy for leisure-time activities.
(From the Royal Canadian Air Force Physical Fitness)