English Standard Version (ESV)

²⁶ So I do not run aimlessly; I do not box as one beating the air. ²⁷ But I discipline my body and keep it under control,^[a] lest after preaching to others I myself should be disqualified.

ESV Study Bible notes:

9:27 This verse has a long history of misinterpretation in terms of punishing one’s own body as a means of spiritual discipline. Paul’s language, however, is governed both by the athletic metaphor of the previous two verses and by the physical demands of his apostolic work (4:9, 11-13; 2 Cor. 4:8-12; 6:3-10; 11:23-12:10; 1 Thess. 2:1-2, 9; 3:7-8). Just as an athlete goes through physical training that is sometimes uncomfortable in order to attain the goal of victory, so Paul endures physical and emotional hardship, and gives up his right to material support, for the gospel’s advancement. (See 1 Cor. 9:12 and 2 Cor. 6:1-10).