Men often worry about their expanding frontside in the form of a growing belly. But they shouldn’t ignore a deflating backside, commonly known as “flat butt.” A flat butt is a sign of weak gluteal muscles and tight hip flexors. “This often happens from too much sitting,” says physical therapist Matt Murray with Harvard-affiliated Spaulding Outpatient Center Malden. “Odds are most men worked in jobs that didn’t require much, if any, physical exertion. Sitting for long stretches was common, and that sedentary behavior tends to worsen during retirement.”