What is work?

Work can be thought of simply as activity - either mental or physical. Although it may not seem like it sometimes, your body is working constantly even when you're asleep! You carry out mental work whenever you use your brain to do something - reading, watching television, solving a problem, etc. You carry out physical work whenever you use your body to do something - walking, sitting, lifting, etc. Your body still works when you are asleep to digest food, repair tissues, keep you breathing, etc. Here, we will be concentrating mainly on physical work.

Types of physical work
Physical work is carried out by your muscles and is therefore often called muscular work. 
There are 2 types:
Static (or isometric meaning 'same length'): this is where a muscle remains contracted for a period of time but there is no movement, as in holding a picture against the wall or carrying a bag of shopping. Holding a static or fixed posture can be very tiring as your muscles don't get time to relax. A muscle which is heavily contracted squeezes against the blood vessels next to it, restricting blood flow. This cuts down the delivery of oxygen to the muscle and the removal of a waste product, lactic acid, from the muscle. This results in muscular aches or pain. Any fixed posture will bring on these symptoms, for example, standing to attention or sitting upright.
Dynamic (or isotonic meaning 'same tension'): this is where there is rhythmical contraction and relaxation of a muscle which does result in movement, as in pulling open a drawer or walking up stairs. Dynamic work is less tiring and more efficient than static work. This is because during dynamic work a muscle contracts and relaxes rhythmically which makes it act like a pump for the flow of blood in the blood vessels, allowing the blood to supply more oxygen and take away more lactic acid than during static work.

Hold a small book in each hand. Put one arm straight out in front of you and keep it in that position. This arm is doing static work. With your other arm, keep your elbow at your side and move your forearm up and down repeatedly. This arm is doing dynamic work. Which arm gets tired first? It's probably the one holding the book still in front of you and apparently doing nothing!

Types of muscle
There are three types of muscle tissue in your body: smooth muscle tissue - located within the walls of various body organs; cardiac muscle tissue - which forms the heart; and skeletal muscle (striated) tissue - which forms the muscles that make you move. Skeletal muscle is made up of bundles of muscle fibres which can contract together in one direction. During movement, when a muscle contracts, the muscle fibres reduce in length so the amount of movement that the muscle can produce depends on the original length of these fibres. The strength of the muscle will depend on the number of fibres that it contains and the cross-sectional area of the muscle.

Muscular endurance and strength

Muscular endurance is the ability of a muscle or a muscle group to remain contracted over a period of time. Endurance can be static or dynamic. Static endurance can be determined by the length of time a limb can maintain a certain position, whereas dynamic endurance can be measured by the number of times a limb can perform a movement against a certain resistance.

Muscular strength is the maximum amount of force that a muscle can exert under maximum contraction. The amount of force that can be exerted by your limbs depends on body posture and the direction of force. For example, when standing, you can exert more force when pulling backwards than when pushing forwards. 

There are several factors that influence your muscle strength and endurance:
Age: your strength increases in your teens and early 20s, reaches its maximum by the middle to late 20s, remains at this level for 5 to 10 years, and afterwards begins to decrease gradually. 
Sex: in general, women are about two thirds as strong as men. This is because men have greater muscle mass as a percentage of body mass compared to women. 
Body build: usually the 95th percentile person of a population will be stronger than the 5th percentile person (see the anthropometry topic for more details about percentiles); the athletic or muscular looking person will tend to be stronger than others; among people of equal body size, differences in strength may be due to the amount of muscle tissue, body shape and proportions.
Fatigue: the build up of lactic acid in the muscles due to static muscle work causes a gradual decline in muscle strength; fatigue can be delayed by adopting comfortable working postures, changing your posture now and again, decreasing the intensity or duration of muscular effort, training or practice, having adequate rest periods, and good nutrition.
Exercise: you can increase your muscle strength and endurance through exercise up to the limits of your maximum physical potential, which is mainly determined by the genes you inherit from your parents.
Heat: heat, especially when combined with high humidity, decreases muscular performance, especially endurance.
Cold: cold will not affect muscle strength if you wear adequate protective clothing, but it may affect your manual dexterity.
Clothing & equipment: bulky clothing may make movement more difficult and you may not be able to achieve the best positions for exerting maximum force. Clothing and any equipment you carry will add to your overall weight and therefore you will need extra muscular energy to move.
Motivation and emotional state: fear, anger or excitement can temporarily increase your muscular strength but skill and accuracy may suffer.
Nature of your job: manual workers are significantly stronger than other types of worker (as you might expect!) 
Postural aids: backrests increase pushing strength by directing all your strength forwards. Footrests increase pulling strength by allowing you to brace your legs.

The amount of force that can be exerted by your limbs depends on your body posture and the direction of force that you apply.

Physical changes during muscular work

• You need and use more oxygen
• Your breathing rate and depth increases
• Your heart rate and blood flow increases
• You sweat more
• The chemical make up of your blood and urine changes

Oxygen supply and consumption

Muscular work involves the contraction of muscles, a process which requires energy. This energy is obtained through the breakdown of energy-rich glucose or fat. When you are not being very active, oxygen is readily available and enough is supplied to the muscles to allow them to completely break down glucose and fat into energy, water and carbon dioxide. This process is called aerobic respiration. When you are being very active, your muscles need more oxygen but it can't be supplied fast enough by the blood and glucose can't be broken down completely. Lactic acid is a by-product of this anaerobic respiration process and builds up in your muscles which you feel as an ache or pain in your muscles. When you rest again and you supply enough oxygen to your muscles, this lactic acid is converted into glycogen and the muscle ache disappears. People who are physically fit can sustain the aerobic process for a longer time than those who are unfit, that is, they are able to supply oxygen to the working muscles at a higher rate and therefore lactic acid doesn't accumulate as fast as in unfit people.

For light work, you need from 0.5 to 1.0 litre of oxygen per minute
For moderate work, you need from 1.0 to 2.0 litres of oxygen per minute
For heavy work, you need more than 2.0 litres of oxygen per minute

During light or moderate activity your oxygen supply keeps pace with the requirements of your muscles. At this stage work is being done aerobically. A steady state of oxygen consumption is reached and maintained throughout the period of muscular work. However, during industrial or recreational activities, a steady rate of work is seldom maintained for a prolonged period so the steady state is rarely attained.

During heavy work, the oxygen supply increases but soon becomes inadequate and the aerobic processes are replaced by anaerobic. At this point your muscles become short of oxygen which results in an 'oxygen debt'. This 'oxygen debt' refers to the extent to which anaerobic processes contribute to the performance of work. This must then be paid off at the end of the work period. The harder you work and therefore the greater the anaerobic contribution, the longer it takes you to recover.

Heart rate

During increased activity the volume of blood flowing into the heart increases as the working muscles help to pump the blood back to the heart. This stretches the heart muscle which then contracts more forcefully, so more blood is expelled with each beat (called the stroke volume). The nervous system also increases the number of heart beats per minute (the heart rate). Both of these together result in much more blood being pumped by the heart.

During hard work your heart rate increases up to about three times its resting value (for example from 60 beats to 180 beats per minute). When the activity stops, your heart rate returns to resting levels. The speed with which the rate falls back to resting levels depends on your fitness; the fitter you are, the faster your recovery. Your level of fitness affects your physical state during work and your recovery in several ways:
 

Children have higher heart rates than adults, for example, your maximum heart rate is about 220 at birth and falls by approximately 1 beat per minute each year of your life. 

Fatigue

Increasing your ability for work

Physical activity
Mechanical devices and electronic instruments are taking over the more physical parts of many jobs, with the result that workers do not need to be so fit to do the same job. This decrease in activity at work is thought to be one of the factors contributing to the increase in coronary heart disease in developed countries. Coronary heart disease is twice as frequent in sedentary people as it is in active people. Medical evidence also suggests that an increase in physical activity acts both as a preventive measure against coronary heart disease and results in improved physical fitness, that is, a greater capacity to cope with work, leisure activities and neuromuscular stress.

Nutrition 
A diet that contains a variety of foods in sufficient quantity to maintain normal body weight and support growth, is usually adequate for all individuals whether they are sedentary or athletic. A normal, balanced diet means eating appropriate amounts of carbohydrates, proteins, fats and oils, vitamins and minerals. Carbohydrates include sugar and starchy foods and form the body's main source of energy. They are an essential, but not exclusive, fuel for muscles. Proteins are used for growth and for repairing body tissues. Protein may also be used as fuel if carbohydrates and fats are not available, for example, during starvation. If you are training for an athletic event, it is recommended that your diet should include approximately 60% carbohydrate and 15% protein. Fats are an important source of energy at rest and for low intensity exercise, and are readily stored in the body. All foods contain stored energy. The amount of energy that your body can obtain from food is measured in kilojoules (kJ). Your body can obtain 17kJ from 1 gram of carbohydrate or protein and 38kJ from 1 gram of fat but as it requires more oxygen to do this, it is not used when the oxygen supply can't meet the extra requirement, for example when you are doing heavy work. Different activities require different energy levels. The table below gives examples of energy requirements for some activities, in kilojoules per hour (for a 25 year old man weighing 65kg).