Which factor is not a valid indicator for VO2 max assessment?

Maximal voluntary contraction is not a valid indicator for VO2 max assessment because VO2 max, or maximal oxygen uptake, specifically measures the maximum capacity of an individual's body to transport and utilize oxygen during intense exercise. The assessment focuses on cardiovascular fitness and aerobic capacity, which are directly correlated with oxygen consumption.

In the context of VO2 max testing, the other factors mentioned provide essential data points for accurately determining an individual's aerobic capacity. A plateau in oxygen uptake indicates that a person's body is reaching its maximum oxygen utilization, which is a hallmark of VO2 max. Reaching maximal heart rate suggests that the individual is exerting maximum effort, necessary for assessing the top level of performance. A significant increase in the respiratory quotient (the ratio of carbon dioxide produced to oxygen consumed) indicates a shift from aerobic metabolism to anaerobic metabolism, which also provides insight into the limits of aerobic capacity.

Thus, the factor of maximal voluntary contraction does not relate to the assessment of VO2 max because it measures neuromuscular output rather than the physiological metrics associated with oxygen uptake and aerobic endurance.