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I heard that when a human is standing upright and perfectly still, no muscle is contracting. Is this true?

A certain amount of muscle activity is required for any posture, even standing still. It is true that in a relaxed upright stance, most of the muscles are at their quietist. This has been measured by electromyographic (EMG) studies. True stability comes from the contraction of opposing muscles. This is called cocontraction. In the upright position, muscles on both sides of the joints are working together to hold the posture steady. Quite the opposite is true for patients with musculoskeletal problems. For example, when the normal adult is walking, the trunk muscles are silent. However, in the patient with back pain, the muscles on both sides of the trunk work together to support the body against gravity during this activity. EMG (electromyographic) studies show that when a steady posture is needed, cocontraction of muscles on both sides of the joints takes place. When injury happens, changes occur in the way muscles contract. In the case of back pain, cocontraction may increase to increase spinal stability.

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