The term 'firming, as muscles' can be summarized by the crossword solution 'toning, ' which is a six-letter word. Toning is a form of exercise that focuses on strengthening and tightening the muscles in the body. It is often used to improve posture, balance, and flexibility. It can also help to reduce fat and increase muscle mass. Toning exercises can be done with weights, resistance bands, or bodyweight exercises.