Measurement of Health-Related Fitness

Health-related fitness is the set of fitness qualities that matter for staying healthy and free of disease, rather than for winning competitions. To measure one of its key components - body composition - the most popular clinical and school standard is the Body Mass Index (BMI), because it is simple, cheap and needs almost no equipment.

The health-related components of fitness

• Cardiovascular (aerobic) endurance - the heart and lungs' ability to supply oxygen during sustained activity.
• Muscular strength - the maximum force a muscle can produce.
• Muscular endurance - the ability of a muscle to keep working over time.
• Flexibility - the range of movement at the joints.
• Body composition - the proportion of fat to lean mass.

Of these, body composition is the component BMI screens for.

Why BMI is the popular clinical standard

• Simple - a single number from a single formula.
• Low-cost - needs only a weighing scale and a height measure.
• Non-invasive - just height and weight.
• Quick and repeatable - a whole class or clinic population can be screened rapidly, and the measure repeated to track change.

Its computation and interpretive standard

The value is read against the WHO classification bands (see Section 6.5.1 for the full six-band table): underweight below 18.5, normal 18.5-24.9, overweight 25.0-29.9, and obesity from 30.0 upward.

How a school or clinic uses BMI to screen a population

In practice a school or clinic measures everyone's height and weight, computes each BMI, and sorts people into the WHO bands. Those outside the normal range are flagged for further assessment, counselling or referral. BMI thus acts as an efficient first filter over a whole population.

Honest limitation and use alongside other measures

Because it uses only height and weight, BMI cannot separate fat from muscle - a muscular athlete can be misclassified as overweight, and an inactive "normal-BMI" person may still carry too much fat. BMI is therefore used as a screen, alongside the Waist-Hip Ratio (for fat distribution) and the skinfold measurement (for estimated body-fat %), as introduced in Section 6.5. …