When you wear lifted heels, they shift your body’s centre of gravity forward. This creates a chain reaction of events to pull you back in alignment and prevent you from falling on your face. The Achilles tendon is shortened, which causes the calf muscles to flex increasingly. Hamstrings are locked in an eccentric position, while an excessive curvature in the lower back with the pelvis tilted forward makes the buttocks stick out. This tension creeps up the chain all the way to your neck, leading to the fairly ubiquitous complaint of a tight neck. The higher the heels the greater the effect.