As Yogi Berra said of baseball, it is 90 percent mental, and "the other half is physical." This 'Yogi-ism' is equally applicable to tennis, a sport in which elite players need to be "intuitive physicists" in order to win at the highest level. 

Each sport is governed by different sets of rules, and those that use balls each have different specifications for their equipment.

When researchers asked runners to repeat a specific phrase in their heads, like "push," the runners performed substantially better than they had prior to the intervention. 

Athletes may be paid millions, but implicit in the bargain is that ownership of their bodies is no longer entirely theirs.