cues. In this case the cue is: you're behind—keep up! We can safely imagine that in most families this signal is sent and received hundreds if not thousands of times over the childhood years, sent by older, bigger kids to smaller, younger ones, who respond with levels of effort and intensity that those older children (who share the same genetic inheritance) never had the opportunity to experience. (And recall that myelin is all about impulse speed: the more you have, the faster your muscles can fire—a particularly handy feature for sprinters.)
