The term "de-training" refers to the process of losing physical fitness due to a decrease in exercise or activity. Antonyms for "de-training" can include words such as "training," "conditioning," "strengthening," "exercising," and "toning." These words describe the act of improving physical fitness through regular exercise and activity. Other antonyms can include words such as "maintaining" and "sustaining," which suggest the importance of consistent physical activity to prevent the loss of fitness. By contrasting with "de-training," these antonyms emphasize the importance of staying active and making physical fitness a priority in our lives.