CoachA muscle is a fleshy motor attached to the bones. A muscle can contract — shorten and thicken — and in contracting it pulls the bone it is attached to. Muscles only pull; no muscle can push.
Examples
Example 17.8 (Feel the pair at work)
Grip your upper arm with the other hand and slowly bend the elbow: the muscle in front swells and hardens — it is contracting. Now straighten the arm: the front softens, and behind the arm its partner tightens instead. You have just felt Proposition 17.7 happen.