Treadmill entertainment can contribute to bad posture habits. If you are hunching your shoulders and looking down to read a magazine or watch video on the treadmill console, you are reinforcing very bad walking posture habits.
Good walking posture is with the head up and eyes forward. If you need entertainment when on the treadmill, position your video or reading material so you are looking straight ahead at it, not down or up.
When I go into a gym, I often see a full row of hunchbacks on the treadmill. This bad posture can lead to low back pain, neck and shoulder pain, and doesn't allow you to take full, complete breaths. It also reinforces the bad sitting posture many of us have from hours in front of the computer. Your time on the treadmill should be spent building good posture habits, not contributing to the damage you already do to yourself the rest of the day.
Every few minutes throughout your workout, give your shoulders a backwards roll to check that you aren't hunching them.