Stretches for Sore Muscles - Yoga-Inspired Stretches from Annie Mulgrew of New York. When you work out, your muscles contract and shorten, which can leave you feeling stiff. Stretching increases blood flow to the muscles to relieve this stiffness, lengthen muscles, and improve your flexibility for a greater range of motion (and lower risk of injury) the next time you work out, Mulgrew says.