1 Corinthians 9:26 (ESV)

26 So I do not run aimlessly; I do not box as one beating the air.

Paul moves on to another sport from the Isthmian Games, namely, boxing. Boxing was considered to be the hardest sport of all, apparently having very few rules. Two men would beat each other until one of them gave up or lost consciousness. Leather straps, tied around the hands and wrists, served to increase the damage that a boxer could inflict on his opponent’s body and face.

Paul might be referring to so-called shadow fighting when he states that he does not box as one beating the air. In his Christian life, he is running and boxing to win, even if it requires pain.