Limiting factors for maximum oxygen uptake and determinants of endurance performance

In the exercising human, maximal oxygen uptake ((V) over dot O-2max) is limited by the ability of the cardiorespiratory system to deliver oxygen to the exercising muscles. This is shown by three major lines of evidence: 1) when oxygen delivery is altered (by blood doping, hypoxia, or beta-blockade), (V) over dot O-2max changes accordingly; 2) the increase in (V) over dot O-2max with training results primarily from an increase in maximal cardiac output (not an increase in the a-(v) over bar O-2 difference); and 3) when a small muscle mass is overperfused during exercise, it has an extremely high capacity for consuming oxygen. Thus, O-2 delivery, not skeletal muscle O-2 extraction, is viewed as the primary limiting factor fur (V) over dot O-2max in exercising humans. Metabolic adaptations in skeletal muscle are, however, critical for improving submaximal endurance performance. Endurance training causes an increase in mitochondrial enzyme activities, which improves performance by enhancing far oxidation and decreasing lactic acid accumulation at a given (V) over dot O-2. (V) over dot O-2max is an important variable that sets the upper limit for endurance performance tan athlete cannot operate above 100% (V) over dot O-2max, for extended periods). Running economy and fractional utilization of (V) over dot O-2max also affect endurance performance. The speed at lactate threshold (LT) integrates all three of these variables and is the best physiological predictor of distance running performance.