“Flexibility is the ability to move through a joint’s full range of motion,” says Alex Rothstein, CSCS, an exercise physiologist and exercise science professor at New York Tech. Brad Walker, an exercise scientist at The Stretching Institute, says that, “Improved flexibility in the right muscle groups plays a key role in the body’s ability to maintain good posture. It’s like going from zero to 100 miles per hour, whereas having good flexibility would be like going from, depending on how flexible you are, 50 or 80 to 100.