Fatigue and rest breaks

In one line

Fatigue is not eliminated by willpower: it is sized. Every task demands an energy expenditure, a posture and a load, and the standard time has to include the corresponding recovery.

What it is

Niebel and Freivalds explain that the normal time of a time study does not include unavoidable delays or some legitimate lost time, so allowances are added (see standard time). They group them into constant allowances (personal needs and basic fatigue) and variable allowances (for the particular conditions of the workstation). For example, a typical total allowance calculation: 5% for personal needs, 4% for basic fatigue and 1% for unavoidable delay, for a total of 10%.

The UNGS book agrees on the division between constant allowances, for fatigue and personal needs, and special allowances that depend on particular conditions.

The ILO recommendations for variable allowances consider factors such as standing or sitting, abnormal positions, use of force, lighting, atmospheric conditions, required attention, noise level, mental strain and monotony. Niebel and Freivalds point out that these recommendations were developed by consensus between managers and workers from many industries and are not directly backed by measurements; that is why, for demanding cases, they propose calculating them with metabolic models and health and safety guidelines.

How to calculate rest for physical effort

The book presents a criterion based on energy expenditure. Bink (1962) proposed a limit of 5.33 kcal/min of acceptable energy consumption over an 8-hour workday, which corresponds to one third of the maximum consumption of the average American man; for women, following the same criterion (one third of 12 kcal/min), the limit is 4 kcal/min. When the work exceeds that limit, rest is needed, and Murrell (1965) proposed the formula:

R = (W − 5.33) ÷ (W − 1.33)

where R is the fraction of the total time that must be devoted to rest, W is the average energy expenditure during work (kcal/min) and 1.33 kcal/min is the expenditure at rest. If W does not exceed the limit, no additional rest is needed.

Calculate the rest

Real example

Niebel and Freivalds use the case of shoveling coal into a hopper, with an expenditure of 9.33 kcal/min. Applying the formula, R = (9.33 − 5.33) ÷ (9.33 − 1.33) = 0.5: the worker needs to rest for about half of the 8-hour shift, that is, 4 hours.

Another example from the book, about force rather than energy: a worker lifts a 40-pound load (about 18 kg) less than once every 5 minutes. With a maximum sustained force of about 100 pounds (45.5 kg), the calculation gives a variable rest allowance of 7%, which is added to the typical 9% of constant allowance for 16% in total.

How to distribute the rest

The book insists that calculating how much is not enough: how it is distributed matters. It makes no sense to work 4 hours straight at 9.33 kcal/min and then rest for 4 hours: the duration of the work cycle is the main cause of increased fatigue. It recommends short bursts of heavy work combined with short rest breaks (between half a minute and one minute), micro-pauses of 1 to 3 seconds that unblock occluded blood vessels, and active rests in which use alternates between the hands and other muscles. And it mentions that it is better for workers to decide when to rest when they need it, at their own pace, instead of rest breaks prescribed by the machine.

Differences between people, with care

Niebel and Freivalds describe that muscle strength varies a great deal between individuals and depends on factors such as gender, age, handedness and training or physical condition. What they say about gender is an average with many caveats:

• The average woman has between 35 and 85% of the strength of the average man (with an average of 66%); the difference is greater in the upper body and smaller in the legs.
• The book clarifies that the effect is due mainly to body size (muscle mass) and not strictly to gender, and that, because of the wide distribution of strength, there are many women who are stronger than many men.
• Muscle strength peaks around age twenty and then declines by 20 to 25% by age sixty.

For design, the book cites the NIOSH recommended weight limit, calculated so that more than 75% of women and more than 99% of men have enough strength to lift that load, with a maximum energy consumption of 4.7 kcal/min.

The practical conclusion of this site (it is not a rule from the sources): the limits by sex are averages that serve to size a workstation (does it work for most of the people who can occupy it?), not to assign or rule out a person. The most sensible approach is to redesign the task (mechanical aids, heights, smaller loads, rotation) so that it falls within the range of most of the population, and to assess each person by their actual capacity.

Template to use

Factor	Workstation description	Allowance (%)
Personal needs	Constant
Basic fatigue	Constant
Posture (standing, bent over, awkward)
Force or lifting (weight and frequency)
Lighting, temperature and noise
Attention, mental strain, monotony
Unavoidable process delays
Total		Sum

So you do not have to build it by hand, there is an allowances and rest spreadsheet in Excel with the formulas already done: it adds up the allowances, calculates the standard time and the rest for effort with Murrell's formula. The values it comes with are examples and must be deleted.

Benefits

• It avoids standards that are impossible to meet without damaging health.
• It gives a method to justify a pause, a mechanical aid or a rotation with numbers.
• It reduces injuries and absenteeism, which also cost production time.

Limitations to keep in mind

• The ILO values come from consensus and not from direct measurement: use them as a starting point, not as truth.
• The average limits by sex do not describe a particular person.
• Murrell's formula assumes an average energy expenditure: measuring or estimating W at the real workstation is not trivial (tables are used, or heart rate as an approximation).
• The factors of mental fatigue, noise and thermal environment require additional criteria that the energy-based calculation does not cover.

In summary

Fatigue is calculated and distributed: how much rest the task needs (energy, posture, force, environment) and how to spread it over short breaks. Differences between people are real, but they are wide distributions; the goal of work study is to design workstations that work for most people, not to classify people.