The human endurance athlete: Heterogeneity and adaptability of selected exercise and skeletal muscle characteristics
In human subjects, large variations between individuals (up to 3-fold) exist in the capacity for endurance exercise performance. In a heterogeneous population, endurance performance is strongly related to whole body maximal oxygen uptake (VO2max). This is in part genotype dependent (∼25%) but is adaptable with training. However, this relationship does not hold within a homogeneous group of well-trained runners. Other physiological characteristics must contribute to endurance performance and these may include specific advantageous skeletal muscle phenotypes. Muscle fibre type distribution is also heterogeneous, although less adaptable. In contrast, muscle oxidative enzyme capacity is highly adaptable with training. The genetic influences on these muscle characteristics have been indirectly investigated by comparing African endurance athletes, who dominate world-class events, to Caucasian endurance athletes. We have established that African runners have greater resistance to fatigue than Caucasians (p < 0.01) and 50% greater oxidative enzyme activity in vastus lateralis samples (p < 0.05), despite somewhat lower Type I fibre proportion. These differences were not inherently present in a group of sedentary Africans, suggesting that the genotypic influence on athletic performance may be a superior adaptation to training, rather than a baseline genetic effect. Combined physiological and genetic studies are likely to elucidate a polygenetic basis for superior endurance performance.