MLB says players can use certain wearables during a game2
With the increase in the number of hard throwing pitchers requiring elbow-related Tommy John surgery (named after the pitcher who first had the procedure done), it is hoped that keeping track of stress on the elbow will allow teams to rest pitchers who are in risk of damaging their elbow. Active pitchers who recently have had the procedure include Marlins' ace Jose Fernandez and Mets' Cy Young candidate Matt Harvey.
The Zephyr Bioharness heart and breathing monitor can help teams monitor stress among starting pitchers and catchers. It can also help them develop a post game recovery routine for these players, building up their stamina.
The data generated from the wearables will not be released to fans and media, and will be kept by the team and shared with the player. In fact, data generated by the devices cannot be downloaded until after the current game is over.