to make a part of your body have less fat and more muscle by doing exercise(通过锻炼)(使)(身体部位)结实,(使)紧实
Cycling is one of the best ways to firm up your thighs.想让大腿变得结实,骑自行车是最好的方式之一。
My stomach has begun to firm up since I started exercising.自从做运动以来,我的腹部开始变结实了。