Yann points to an article on a mutation in dogs that leads to increased muscle mass and speed:

Here we describe a new mutation in MSTN found in the whippet dog breed that results in a double-muscled phenotype known as the "bully" whippet. Individuals with this phenotype carry two copies of a two-base-pair deletion in the third exon of MSTN leading to a premature stop codon at amino acid 313. Individuals carrying only one copy of the mutation are, on average, more muscular than wild-type individuals (p = 7.43 × 10-6; Kruskal-Wallis Test) and are significantly faster than individuals carrying the wild-type genotype in competitive racing events (Kendall's nonparametric measure, τ = 0.3619; p ≈ 0.00028). These results highlight the utility of performance-enhancing polymorphisms, marking the first time a mutation in MSTN has been quantitatively linked to increased athletic performance.

The myostatin gene in humans plays a similar role in muscle growth-- a deletion in the gene leads to extreme muscularity, as evidenced by the "Baby Superman". The gene has also been shown to be under recent selection in humans--variants presumed to lead to more muscularity are far more common in Sub-Saharan Africa. This paper in dogs shows that increased muscularity also leads to better performance in competititve racing events; it's not such a strech to believe the same could be true in humans as well.