This is another reference by Paul to the Greek athletic games. In this verse he referred to the boxers in training who practiced shadow boxing, throwing punches at an imaginary opponent.
Unlike these athletes who engaged in make-believe boxing, Paul labored in the cause of Christ with a definite aim and purpose in mind. He made every blow count in a real battle as he worked to establish churches and present the claims of the gospel to the Gentile world.
1 CORINTHIANS 9:26 – So fight I [Paul], not as one that beateth the air.