When your trunk is unstable, your arms and legs start moving in directions other than straight ahead. With every step, you’re using energy to bring your arms and legs back toward the midline of your body instead of moving them forward. This problem is usually caused by weak abs and butt muscles. Strengthen those key postural stabilizers, and more of your motion will be directed in front of you rather than to the side. Put another way, for the same energy cost, you’ll run faster.