When muscles are exercised often and properly, they keep the arms firm and sinewy.如果能经常正确地锻炼肌肉的话，双臂就会一直结实而强健。She's been trying to bulk up at the gym.她一直在健身房努力锻炼肌肉。Smith buffed up for his role as Muhammad Ali.史密斯为了扮演穆罕默德·阿里锻炼肌肉。She started lifting weights to build muscle.她开始举重，锻炼肌肉。