The stress response is your body's instinctive reaction to a dangerous or stressful situation. This reaction can be both helpful and harmful. For example, one of the instinctive responses to stress or danger is muscle contraction. This is helpful during times of physical stress, such as when muscles contract tightly around a broken bone to prevent further injury. But some people respond to emotional stress — such as an argument at work — with the same clenched muscles, which can lead to a painful muscle contraction headache. Look in the mirror the next time you're in a tense situation. If you see hunched shoulders, a clenched jaw and an angry look, you may be someone whose headaches are caused by a muscle contraction response to anger and worry.