This season, high-school players in California must use aluminum bats that are BBCOR (bat-ball coefficient of restitution) certified. The rule, also enforced in the NCAA, limits how much bounce a ball can have off a bat. The more a bat gives on impact, the less energy a ball loses and the faster it flies; metal bats, for instance, bow inward and spring the ball forward like a trampoline. Pitchers are three times as likely to be injured by balls hit by non-BBCOR metal bats as by balls hit with wooden ones. To meet the standard, which will go nationwide in high schools next year, companies need to make their aluminum bats more rigid.