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Short History of Energy Transfer

When we examine our evolutionary history, we are reminded of the fact that many of the biologic processes, which are the basis of our present physical performance, are in fact thousands of millions of years old. A treatise of the function of the cell written 1500 million years ago would still be valid today.

It is generally accepted that our Solar System was created 5 Billion years ago. The atmosphere of the planet then was devoid of Oxygen and the protective Ozone layer. The Ultra Violet rays from the Sun could reach the surface of the planet unhindered. This radiation provided the energy for photosynthesis of the non-living organic compounds from CO2, H2O and Ammonia. By a slow long drawn process and by permutations and combinations, glucose molecules, protein molecules etc. were formed.

A few protein molecules arranged themselves in a manner, which was more stable than the rest. To resist their destruction by other scavenger molecules, they built a wall around themselves and learnt to duplicate themselves. Initially the ancient organisms split water by photosynthesis and gradually released free O2 into the atmosphere. This process was a momentous one and it took millions of years to build an atmosphere where one out of every five molecule is O2.

With presence of O2 & O3 in the atmosphere the UV radiation of the sun was prevented, by and large, to reach the surface of the planet non-biologic synthesis of organic matters ceased. The 1.5 billion-year-old unicellular organisms had developed primitive energy transfer systems for their activities, which have withstood the test of time. Our cells follow the same mechanisms for all functions of the body, i.e. ATP hydrolyzed by water
-> ADP + P + Energy.

Each independent cell with the help of a very selfish gene joined hands with other cells to form bigger organisms. To maintain effective transport of nutrients & gases into the cells and metabolic wastes to be excreted outside the cells by osmosis & diffusion, they carried with themselves the same fluid environment of the primordial soup but in a bag of protective skin. The composition of interstitial fluid today is supposedly that of the oceans thousands of millions of years ago. An effective circulatory and excretory system was developed to maintain this milieu interior.

We jump thru the evolutionary cycle with the appearance of modern man. From hunters to gatherers to farmers to urban dwellers, man has come a long way.

But has he? Humans are still an aggressive, selfish, manipulative species. Though the brain has evolved to double the size of its predecessors, ethical conduct still remains in doubt today with altruism a very rare phenomenon. Man is unique in forcing its will on others by aggression or manipulation.

The main task for us today is to make it possible for individuals to accomplish their work without undue fatigue, both physical and mental.

Energy Transfer System

Follows 1st law of Thermo Dynamics

Energy is neither created nor destroyed but transformed from one form to another. Chemical -> Mechanical. For work to be carried out the muscles need to contract and needs energy to do so. The energy currency here is ATP & CP. One mole of ATP hydrolyzed ADP + P - 7.3 Kcal. The above reaction is anaerobic and instant. The total quantity of ATP in the body is 80 - 100 gm. (Enough to perform at a maximum level for several seconds). But ATP has to be re-synthesized quickly for activity to progress. Another high-energy phosphate reservoir is CP.

CP C.Kenase C + P + Energy

This P ion combines with ADP -> ATP

CP yields more energy during hydrolysis than ATP and is present several times the ATP amount in the body.

Anaerobic glycolysis

But ATP is a heavy molecule and could not be stored in the cells in quantity, so had to be replenished constantly. Hence the appearance of glycolytic pathway.

As stated earlier the role of nutrients is to provide energy for the rapid re-synthesis of ATP from ADP. Glucose is utilized by the cells in a series of metabolic breakdown steps to provide the necessary energy. In the evolutionary sequence this forms a very primitive method of energy transfer and is well developed in amphibious, fish & reptiles.

Glucose -> Pyruvates + Energy

Energy + P + ADP -> ATP

The end product of the above reaction is Lactic acid, which lowers the PH of the cell and all activities grind to a halt. Net gain in the anaerobic breakdown of glucose is only 2 moles of ATP and only 5% of the energy in the sugar molecules.

Aerobic Energy Transfer

An alternate mechanism developed to utilize the highly corrosive O2 molecules in the atmosphere for nutrient metabolism to continue. To extract the remaining potential energy in the glucose molecule, an elaborate & intricate pathway has evolved in the mitochondria of the cell. But this process requires Oxygen to proceed with the pyruvic acid and eventually results in the complete metabolism of the glucose molecule to CO2 & H2O with the release of 36 molecules of ATP - Krebs Cycle.

Stored lipids represent the body's most plentiful source of potential energy. At sub-maximal intensity in the presence of O2 and carbohydrates lipids can theoretically provide for 100,000 Kcals of energy. Total Carbohydrates provide for only 2000 Kcals.

Whether both energy transfer systems developed concurrently or one after the other is hard to tell. But logic tells us that in all probability the anaerobic system was followed by the aerobic one. The first living organism then did not enjoy a long life, or a very sluggish one as aerobic pathway is more efficient for fulfillment of the basic functions, even at rest.

Muscular activity is required in all types of work and in all expressions of life. Industrialization & automation has made us more sedentary and four or five day weeks have greatly increased the intensity of the work periods leading to high mental stress.

There are three types of muscular contraction.

  1. Static (Isometric) - No change in length
  2. Dynamic: Eccentric - Lengthens while contracting
  3. Dynamic: Concentric - Shortens while contracting

Here we are referring to contraction as physiological contraction.

Isometric contraction or static work involves:

- Sustained muscular contraction -> Reduced blood flow -> no increase in muscle O2 consumption -> Anaerobic energy metabolism -> High lactate -> Fatigue, Discomfort, Soreness, Pain.

Dynamic work necessitates:

- Repetitive muscle contraction and relaxation -> High blood flow due to the milking action of muscles on the veins & arteries -> High muscle O2 consumption -> O2 dependent aerobic energy transfer -> Uses glycogen & fatty acids as fuel & converts it to CO2 + H2O.

The implication of the above is that intermittent work at low to medium intensity can be carried out for a longer period of time, if the movements are dynamic in nature.

All out work or work carried by static contractions would be carried out for a minute or so and requires periods of rest in between in highly demanding tasks and high workload emergency situations.

The worker is both a user and a source of energy. The force output of a muscle in finite and depends on the cross sectional area of the muscle. (Strength training increases the cross sectional area of a muscle)

(1 Watt = 0.737 ft. lbs./ Sec.)

PWC - refers to a workers capacity of energy output. In the working environ, one of the challenges is to determine the ratio between workload and work capacity. Thus the former is a relative component of the latter.

A work burden may be too high for one worker but moderate for another and light for the 3rd. The 1st person being more prone to fatigue. This is true regardless of quantity of muscle group involved. Hence the basic task for us would be that of measuring the rate at which work is being done i.e. workload and matching this rate with the workers ability to perform work.

The relationship between workload and work capacity is affected by a complicated interplay of many factors internal as well as external.

Finally the nature of the work to be performed, apart from intensity and duration is of decisive importance when prolonged work stress is contemplated.

The ideal way to perform physical activity is to perform it dynamically, with brief work periods interrupted by brief work pauses.

Similarly the working position is of great importance in that working in a standing position may represent a greater circulatory strain then does working in sitting position. Conversely while standing the worker has enough space to move about thus varying the load on individual muscle groups and facilitate circulation and at times be preferable.

Working technique is of importance in high efficiency & conserving energy. Depending on his psychological trait monotony at work may be stressful to some but relief to others.

Finally of course the tempo or intensity of the work performance is of importance.

According to Eldhom 1 kg of muscle can produce 0.3 hp. Thus a 70 kg man can in theory produce 10 hp. In a single burst but champion cyclists produce 2 hp for 10 seconds.

Further 1 lt. of O2 yields approx. 0.1 hp and work requiring more than 2 lt. of O2 would be termed extremely heavy.

The capacity of an individual to utilize O2 is termed VO2 Max. This is determined my making and individual run on a treadmill or perform task on the cycle ergometer while measuring this O2 shows a linear relationship of workload. But a time comes when further increase in load does not produce increased O2 uptake and clearly tells us that he has reached his limits of O2 uptake. He may continue to work at that intensity for some more time but it will be basically glycolytic in nature.

But there is great inter individual variability in VO2 Max (largely due to genetic factors). Training improves VO2 Max but by only 10% - 15%.

In addition to VO2 Max sustained performance at high rate depends on the ability of an individual to remove waste products (lactates) from the tissues. Unlike VO2 Max blood lactate response can be improved considerably by training.

Assessment of physical work load

  • Either directly measuring the O2 uptake during actual work operation.
  • Indirectly by recording the H/R during performance of the work (H/R recorded continuously during the whole day reveals a general picture of the overall activity level). Because of individual variance to H/R response, the best way to express this circulatory strain as % of H/R reserves. (H/R reserve being HR Max - HR resting). 50% being excessive.

Also H/R can be converted to the corresponding estimated O2 uptake and workload can be presented as % this VO2 Max.

The total stress of the workload can also be seen reflected in the hormonal response by measuring the urinary epinephrine & nor epinephrine.

But in a majority of professional activities, office work, light industry, Labs and hospital work, and retail and distribution work, the energy output is less than 5 Kcal/min. or 1 lt. of O2. In Iron & Steel industries and building industries, there are many jobs that demand an energy expenditure of 7.5 Kcal/min. or higher. In mining it could reach as high as 10 Kcal/min. Generally speaking it has been observed that a person can sustain work continuously for 8 hours if he is allowed to work at 30% of his VO2 Max. and no more than 40% of his muscular strength capacity to be used continuously. (Max permissible limit for energy output for men working at the same task year round is around 5 Kcal/min.)

Because of VO2 Max variability a similar task may burden a person of 2 lt. VO2 Max while seen light for a person with 4 lt. VO2 Max. The lower limit should be kept in mind then job task is designed i.e. fit the least fit of workers.

If a task requires 2 lt. O2/min. 2 persons with VO2 Max of 2 & 6 liters do the same job. One does it early because he spends only 33% of his capacity while the other breaks down.

Determining the energy requirements of different jobs makes it possible to identify jobs which can be performed only by fitter members of the work force and where possible redesign jobs to lower the required energy expenditure thus making them available to a wider range of people.

For occasional lifting tasks (1 hour or less / day) max energy expenditure should be 9 Kcal/min. The upper limit of energy expenditure is 16 Kcal/min. for men and 11 Kcal/min for women for 4 min.

Factors affecting work capacity

Personal factors
  • Body weight
  • Body composition
    Thus VO2 Max = ml./min./Kg.
    Leaner people will move faster than heavier ones all other things being equal. Shedding fat may improve VO2 Max.
  • Age - VO2 Max declines after 20 years of age. A 60 yr. Old has 70% of VO2 Max as that of 25yr. Old.
  • Sex - Women have lower VO2 max than men. Have lower Hb; higher fat and lower strength values than their counterparts.
  • Alcohol - Increases cardiac output is sub maximal work -> low cardiac efficiency may also cause hypo-glycaemia.
  • Smoking - 4% Co. Hb has 200 times more affinity for Co. Smoking thus reduces work capacity by reducing the O2 carrying capacity of blood. Chronic damage to the respiratory system impair the ventilatory capacity of the lungs.


VO2 Max increases 10%

Strength increases.

Nutritional Status & General Health

Balanced diet.


Directly related to work output.

Environmental Factors
  • Air pollution - High resistance of air flow of the respiratory airway more than 6.5 points/million of Co requires to accumulate in sub maximal exercise. In urban areas, due to less vehicular flow 37 - 54 RPM is seen.
  • Climate - Extreme environments
  • Noise - Which increases H/R - high noise is a stressor.
  • Altitude - Strength is not affected. Stamina is VO2 decreases 10% every 1000 m above 2000 m.
  • Protective clothing & Equipment - Bulky clothing-Clothing which trap heat can increase H/R, increase sweating, decrease electrolytes.
  • Subjective measures of physical effort - Borg RFE Scale.


Though a person can work at 30 - 35% of his VO2 Max for 8 hours theoretically, the task should not be performed continuously without rest.

An average work rest cycle (including work + rest period) of 30-35% should be established for the shift.

How to calculate rest period?

Murrells Formula

Rest allowances = w ( b - s ) / b - 0.3

w = length of working period

b = O2 uptake

s = Standard uptake for continuous work

Work hardening programs

- Fitness training

- Mental training techniques

O2 uptake of various tissues at Rest for a 65 kg man
O2 uptake% of resting metabolism
Skeletal Muscle4518
Estimated Genetic contribution %
VO2 Max20-30%
Sub Max exercise response20-30%
Muscular fitness20-30%
Blood lipid profile30-50%
Resting B. P.30%
Total Body fat25%
Regional fat distribution30%
Habitual activity level30%

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About the Author
Rajeev Kumar
CEO, Computer Solutions
Jamshedpur, India

Rajeev Kumar is the primary author of How2Lab. He is a B.Tech. from IIT Kanpur with several years of experience in IT education and Software development. He has taught a wide spectrum of people including fresh young talents, students of premier engineering colleges & management institutes, and IT professionals.

Rajeev has founded Computer Solutions & Web Services Worldwide. He has hands-on experience of building variety of websites and business applications, that include - SaaS based erp & e-commerce systems, and cloud deployed operations management software for health-care, manufacturing and other industries.

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