Maximal oxygen consumption, or VO₂ max, is the highest rate at which the body can take in, transport, and utilise oxygen during intense exercise. It is widely recognised as one of the most valid indicators of aerobic capacity and cardiovascular fitness. In line with the new IB SEHS specification, students are expected to understand what VO₂ max represents, how it is measured, and the factors that influence it, including age, sex differences, body composition, lifestyle habits and training status.

VO₂ max is typically expressed in millilitres of oxygen used per kilogram of body mass per minute (mL/kg/min). It reflects the integrated performance of the respiratory, cardiovascular and muscular systems during exercise. A higher VO₂ max indicates that the body is more efficient at delivering oxygen to working muscles and utilising it to generate energy through aerobic pathways.

Age has a predictable impact on VO₂ max. In general, VO₂ max peaks in the late teens to early twenties and declines gradually with age. This decline is due to reductions in maximal heart rate, cardiac output, and muscle mass, as well as changes in mitochondrial density and capillarisation. While training can slow this decline, it cannot fully reverse it. Understanding the role of age allows us to interpret VO₂ max values appropriately across the lifespan.

Sex differences also play a significant role. On average, males tend to have higher absolute and relative VO₂ max values than females. This is primarily due to differences in haemoglobin levels, heart size, lung volume and muscle mass. Males typically have a larger stroke volume and a higher oxygen-carrying capacity. However, when adjusted for lean body mass, the gap narrows, highlighting the importance of considering body composition when evaluating fitness potential.

Body composition is another key factor. Since VO₂ max is often expressed relative to body weight, individuals with a higher proportion of fat mass may have lower relative scores even if their absolute oxygen consumption is similar. Lean body mass, particularly muscle, is metabolically active and directly contributes to aerobic performance. Reducing excess fat mass while maintaining or increasing muscle mass can improve VO₂ max relative to body weight.

Lifestyle factors such as smoking, poor diet, high stress, and inactivity can reduce VO₂ max by impairing cardiovascular and respiratory function. In contrast, a physically active lifestyle supports higher oxygen transport capacity, better lung function, and improved muscular efficiency. Even within genetically similar populations, lifestyle habits can lead to significant differences in aerobic fitness.

The level of fitness is perhaps the most immediately modifiable factor. Regular aerobic training increases cardiac output, stroke volume, capillary density, mitochondrial function, and oxygen extraction at the muscular level. Endurance athletes typically have exceptionally high VO₂ max values as a result of years of structured training. While genetics sets the upper limit, consistent training can lead to significant and lasting improvements.

In summary, VO₂ max is shaped by a complex interaction of biology, behaviour and training. It provides a window into an individual’s aerobic fitness, but must be interpreted in context. For athletes, coaches, and scientists, understanding the variables that influence VO₂ max enables more targeted training strategies, fairer comparisons, and improved long-term performance planning.