ance ▁of ▁natural ▁fast ▁tw itch ▁mus cle ▁fib ers , ▁which ▁aid ▁in ▁quick er ▁reaction ▁times . ▁Scient ists ▁have ▁concluded ▁that ▁el ite - level ▁s print ing ▁is ▁virt ually ▁impossible ▁in ▁the ▁absence ▁of ▁the ▁A CT N 3 ▁protein , ▁a ▁" speed ▁gene " ▁most ▁common ▁among ▁persons ▁of ▁West ▁African ▁descent ▁that ▁rend ers ▁fast ▁tw itch ▁mus cle ▁fib ers ▁fast . ▁Top ▁spr inter s ▁of ▁differ ing ▁ancest ry , ▁such ▁as ▁Christ ophe ▁Lem ait re , ▁are ▁believed ▁to ▁be ▁exceptions ▁in ▁that ▁they ▁too ▁likely ▁have ▁the ▁gen es ▁fav ou rable ▁for ▁s print ing . ▁ ▁End urance ▁running ▁ ▁Many ▁Nil otic ▁groups ▁also ▁excel ▁in ▁long ▁and ▁middle ▁distance ▁running . ▁Jon ▁Ent ine ▁has ▁argued ▁that ▁this ▁sport ing ▁pro w ess ▁st ems ▁from ▁their ▁exception al ▁running ▁economy . ▁This ▁in ▁turn ▁is ▁a ▁function ▁of ▁sl im ▁body ▁morph ology ▁and ▁sl ender ▁legs , ▁a ▁pre ponder ance ▁of ▁slow ▁tw itch ▁mus cle ▁fib ers , ▁a ▁low ▁heart ▁rate ▁gained ▁from ▁living ▁at ▁high - alt itude , ▁as ▁well ▁as ▁a ▁culture ▁of ▁running ▁to ▁school ▁from ▁a ▁young ▁age . ▁A ▁study ▁by ▁P its il ad is ▁et ▁al . ▁( 2 0 0 6 ) ▁question ing ▁ 4 0 4 ▁el ite ▁distance ▁run ners ▁from ▁Ken ya ▁found ▁that ▁ 7 6 % ▁of ▁the ▁international - class ▁respond ents ▁ha iled