Ace the IB SEHS Test 2026 – Flex Your Knowledge and Leap into Success!

Muscle contraction is driven by the interaction of contractile proteins inside the muscle fiber, specifically within the myofibrils arranged as repeating units called sarcomeres. In each sarcomere, the thick filaments (myosin) interact with the thin filaments (actin). Through the cross-bridge cycle, powered by ATP and regulated by calcium, the myosin heads pull on actin, causing the sarcomeres to shorten and, collectively, the muscle to contract. The other connective-tissue wrappers—endomysium surrounding each fiber, epimysium around the whole muscle, and fascia outside the muscle—provide structure and transmit force but do not generate or drive contraction themselves.