Ergonomics practice questions.

A cordless drill is used overhead on building sites for periods of up to two hours at a time. Which ergonomic factor is most directly responsible for operator discomfort in this situation?

• Clearance between the drill body and the 95th percentile hand breadth
• Colour contrast between the drill casing and the site background
• The 5th percentile grip reach of an operator standing at floor level
• Static muscle loading caused by holding the arm above shoulder height

Answer: Static muscle loading caused by holding the arm above shoulder height

Sustained overhead work is a physiological loading problem: the shoulder muscles contract continuously without the blood-flow relief that movement provides, so fatigue appears long before any fit or visibility problem does.

A researcher records two hand measurements from the same participant: measurement P with the hand held flat and still on a table, and measurement Q while the participant grips and turns a control knob. Which single feature of measurement Q identifies it as dynamic anthropometric data?

• It is recorded in millimetres rather than in degrees.
• It is taken while the body part is working in a task posture.
• It gives a larger value than the corresponding still measurement.
• It comes from one participant rather than from a whole sample.

Answer: It is taken while the body part is working in a task posture.

Static (structural) data are taken in fixed standard postures; dynamic (functional) data are taken while the body works, so they capture the space, angle and force a real task needs. The size of the value does not classify a measurement, and both kinds come from samples.

A design report fixes the distance to a control by adding the 95th percentile upper arm length to the 95th percentile forearm length and calling the sum the 95th percentile reach of the user group. Which error does this contain?

• A reach dimension must instead be taken from the 95th percentile of stature.
• The two segment lengths should have been multiplied, not added.
• Percentiles of separate dimensions do not add to the same percentile.
• Upper arm length and forearm length are physiological measurements.

Answer: Percentiles of separate dimensions do not add to the same percentile.

A percentile marks a position in the distribution of one dimension, and very few people sit at the 95th percentile of two dimensions at once, so the sum overstates the value. Composite dimensions must be measured directly on the sample; a reach dimension is in any case governed by the 5th percentile.

For a seated user carrying out light manual work, which relationship between the height of the work surface and the user's seated elbow height keeps static loading of the shoulder muscles lowest?

• At or just below the user's seated elbow height.
• About 150 mm above the user's seated elbow height.
• Level with the user's seated shoulder height.
• At the height of the seated user's knee.

Answer: At or just below the user's seated elbow height.

Work carried out above elbow height forces the upper arm to be raised and held there, a static load that produces fatigue and aching long before any force is applied. At or just below elbow height the upper arm hangs; only fine visual work is set higher, and then for eye distance, not for the muscles.

A student team measures the hand length of 14 classmates and publishes 5th and 95th percentile values from that data set for use on a product intended for adults of all ages. Which limitation of these values is most serious?

• Hand length cannot be expressed as a percentile value.
• Percentile values require measurements taken in millimetres.
• Fourteen measurements cannot be ranked from smallest to largest.
• The sample is too small and too narrow to represent that population.

Answer: The sample is too small and too narrow to represent that population.

A percentile is only as good as the sample it is read from: fourteen people of one age group cannot describe the spread of a whole adult population, so the tails are guesswork. The arithmetic is valid, the sample is not.

Two workers report discomfort in the forearm during the same task. Which single observation shows that the cause is a static muscular load rather than a repetitive one?

• Both workers describe the discomfort in the same part of the forearm.
• It builds while one posture is held and eases when it changes.
• It appears only after the work surface has been raised.
• It is felt most strongly at the end of a long shift.

Answer: It builds while one posture is held and eases when it changes.

A static load comes from holding a posture or a grip without movement, so it accumulates while the position is maintained and is relieved by changing it. A repetitive load is tied to the number of movement cycles, so it tracks the count of movements rather than the time a position is held.