The fourth dimension: physiological resilience as an independent determinant of endurance exercise performance

Endurance exercise performance is known to be closely associated with the three physiological pillars of maximal O2 uptake (V̇O2max$\dot{V}_{{\rm O}_{2}{\rm max}}$), economy or efficiency during submaximal exercise, and the fractional utilisation of V̇O2max$\dot{V}_{{\rm O}_{2}{\rm max}}$ (linked to metabolic/lactate threshold phenomena). However, while ‘start line’ values of these variables are collectively useful in predicting performance in endurance events such as the marathon, it is not widely appreciated that these variables are not static but are prone to significant deterioration as fatiguing endurance exercise proceeds. For example, the ‘critical power’ (CP), which is a composite of the highest achievable steady‐state oxidative metabolic rate and efficiency (O2 cost per watt), may fall by an average of 10% following 2 h of heavy intensity cycle exercise. Even more striking is that the extent of this deterioration displays appreciable inter‐individual variability, with changes in CP ranging from