What do slouching, back pain, and a middling forehand or weak shot off the tee have in common? Often it’s a weak core—the girdle of muscles, bones, and joints that links your upper and lower body. Your core gives you stability and helps power the moves you make every day. Whether it’s bending to pick up a laundry basket, swinging a golf club, paddling a kayak, or reaching to pull a vase from the top shelf of a cabinet, a strong and flexible core makes the move more fluid, efficient, and robust. Strong, well-balanced core muscles can also improve your posture and help prevent back injuries. And if back pain does strike, core exercises are usually part of the rehab regimen.