A cyclist's position, especially on a road bike, requires you to hold the head at an exaggerated curve to counter the rounded position of your upper back. That can be a lot of weight to hold up considering that the head, weighing in at 8 to 11 pounds on average, is supported only by the muscles of the neck and shoulder. And the more aggressive the position you ride in (such as when you're in the drops), the more your upper back will round and the less muscular support you'll get from the shoulders.
Staying in this position for prolonged durations, as you do during a long ride, can lead to muscle imbalances and pain. The front of the neck becomes lengthened while the back of the neck shortens. Consequently, muscle tightness and knots can develop on both sides of the neck, limiting range of motion and altering your posture.
Fortunately, only a handful of your 24 hours is on the bike, leaving you enough time in the day to reverse the strain. By building an aligned posture, you can learn to optimize your strength and balance your body to decrease stress on the overworked muscles of the upper spine and prevent injury.
Try these three exercises right after a ride to rid yourself of a pain in the neck.
Posture Exercise #1: Shoulder Shrugs
Stand with your back against a wall. Keep your heels touching and your feet hip-width apart, facing straight ahead. Take note of how your shoulder blades are in contact with the wall. Most likely you'll feel only the inner edges in contact. Pull your shoulder blades down and together to flatten them against the wall. Make sure not to bring the shoulders up. You should not feel increased tension near your neck.
Also note that your lower back shouldn't exaggerate its arc. Make sure the rib cage stays neutral, not pointing up. Keep the shoulder blades against the wall. Slide them up towards your ears and then push them down towards your feet. Don't let your shoulder blades drop and continue to press through the entire range of motion.