keep fit - Wiktionary, the free dictionary

From Wiktionary, the free dictionary

keep fit (uncountable)

1. Exercises designed to promote physical fitness if performed regularly. (The addition of quotations indicative of this usage is being sought:)

keep fit (third-person singular simple present keeps fit, present participle keeping fit, simple past and past participle kept fit)

1. To exercise for physical fitness.

   I keep fit by kickboxing every week.