Immobilize the neck for only a suitable time as prolonged immobilization may decrease muscle strength.
Cervical collars can be used for whiplash injury to hold the neck and head still.
Range of motion exercises:
Early mobilization and range of motion exercises are most effective. Exercises like chin tuck help to retrain the deep muscles in the neck. After the car accident, the longus capitis and longus colli muscles of the neck have often decreased activity. Other exercises may include rotating the neck in both directions, tilting the head side to side bending the neck toward the chest, rolling the shoulders etc.
Strengthening the muscles, improves posture, and restores normal movement. Strengthening exercises like the serratus wall slide is a light strengthening exercise, it is a good re-training exercise for the serratus anterior muscle.
Stretching exercises are done to increase range of motion, exercises like Levator release with a ball can be used to reduce muscle tightness where the shoulder and the neck meet.