To extend the arm, we relax the biceps while the triceps on the opposite side contracts. Draw the structure of skeletal muscle as seen in electromicrographs. Look on page 1040. Electron micrographs are readily available and can be interpreted to show myofibrils and the characteristic patterning of dark and light bands. The detailed structure can be deduced so that thin actin filaments and thick myosin filaments interdigitate.
Ratchet mechanism depends on bridges forming between thick and thin filaments.
The two proteins tropomyosin and troponin act to cover and expose binding sites. Calcium ions activate troponin molecules and ATP is needed by the contracting muscle. The myosin head is bound to ATP and is in its low-energy configuration. The myosin head hydrolyzes ATP to ADP and inorganic phosphate and is in its high-energy configuration. The myosin head binds to actin, forming a cross-bridge.