Question 3 of 7: Energy Consumption and Human Strength

Question 3: Energy Consumption and Human Strength (20 marks: i–7, ii–6, iii–7)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

(i) Factors Affecting the Level of Energy Consumption on a Task

The metabolic energy cost of a task depends on several interacting factors. The physical workload itself — the weight handled, the force exerted, and the speed/pace at which the task is performed — is the primary driver, since energy cost rises roughly with the rate of mechanical work done. The muscle groups and posture involved matter strongly: large-muscle, whole-body dynamic work (walking, lifting) is metabolically far more expensive than small-muscle, localized work (fine assembly), and static (isometric) muscle contraction is disproportionately costly and fatiguing relative to the small amount of external work it produces, because blood flow to the contracted muscle is restricted. Environmental conditions — ambient temperature and humidity, in particular — raise energy cost by adding a thermoregulatory (sweating, increased heart rate) burden on top of the task's own mechanical demand. The method and skill with which the task is performed changes cost too — a well-designed, practiced method with efficient body mechanics costs measurably less energy than an awkward or unpracticed one for the identical output. Finally, individual factors — a worker's fitness level, body size, age, and degree of acclimatization — set the metabolic "price" a given task exacts on that particular person, so the same task can be well within limits for one worker and excessive for another.

(ii) Keeping Energy Expenditure Within Reasonable Limits

Because sustained work above an individual's aerobic capacity leads rapidly to fatigue, reduced productivity, and elevated injury risk, energy expenditure is managed by several complementary means. Rest allowances are built into the standard time whenever the task's energy cost exceeds a comfortable working level — the harder the task, the larger the proportion of paid time spent recovering, following the logic that total energy expended (work plus recovery) must not exceed what the body can sustainably supply over a shift. Job/task redesign reduces the demand directly — mechanizing or partially automating the highest-cost elements, reducing the weight or distance involved, or changing posture/method to substitute dynamic for static muscular effort. Work-rest cycling and job rotation spread high-energy-cost work across the shift or across several workers rather than concentrating it continuously on one person, allowing physiological recovery between bouts. Pacing the work rate to the individual (rather than machine-paced at a fixed, possibly excessive, rate) lets a worker self-regulate effort against fatigue. Together these keep the sustained average energy expenditure within the range the body can support indefinitely without an accumulating "oxygen debt."

(iii) Human Strength: Definition and Measurement

Human strength is the maximum force (or torque, about a joint) that a muscle group can voluntarily exert against an external resistance. It is measured in two principal ways, differing in whether the body is allowed to move during the test. Static (isometric) strength is measured with the joint held at a fixed angle against an immovable resistance — a strain-gauge dynamometer or load cell records the peak force the person can produce while no external movement occurs; because the joint angle is fixed, isometric strength must be measured (and reported) at a specific, stated angle, since strength varies substantially across the range of motion. Dynamic (isokinetic) strength is measured with a specialized dynamometer (e.g., a Cybex-type machine) that mechanically holds the angular velocity of the joint constant while the person pushes or pulls maximally against it, recording the torque produced continuously through the full range of motion — this captures how strength varies with joint angle and with movement speed, which a single-angle isometric test cannot. (A third, related measure — isotonic strength — is the maximum constant external load a person can move through a full range of motion at a self-selected, non-fixed speed; it is closer to how strength is actually applied in most manual work but is harder to standardize than either isometric or isokinetic testing.)