There are not many sports where the ability to hold tension is an assest. Runners tend to accumulate tension in the jaw, neck, and shoulders, as the fast-twitch muscles in these areas respond quickly to stress by rapidly contracting. Tight or hunched shoulders not only hinder relaxed running form by jarring the arm swing, but sap vital energy and inhibit deep breathing. Yoga tackle tension through stretching, breathing and relaxation exercises.
Double shoulder rolls exercise will help you prevent your arms swinging freely especially for those with poor posture or hours spent hunched over a desk.
To experience how clenching your fists affects arm swing stand up, make 90 degree shape with your arms and clench your fists tightly. Now swing your arms as if running. Notice how this immediately restricts your arm movement and tightens the muscles of your shoulders and upper back. Repeat. This time soften your hands, letting your first finger rest lightly on your thumb. Relax your hands. Imaging you are holding potato chips between your finger and thumb and their still there at the end of your run.