The VO max is the gold standard for measuring improved performance after exercise training. Here we show that an additional relative improvement in leg power output was achieved by changes in energy coupling, representing better exercise efficiency, after a 6month endurance training programme. Parallel elevations in phosphorylation capacities of mitochondria (ATPmax/Vv[mt,f]) and muscle (ATPmax) accompanied these whole-body adaptations. Together, these changes in energy coupling and capacity at muscle and whole-body levels elevated leg power generation by elderly subjects to partly reverse the decline in exercise performance that comes with age.