Visualization of the cellular and molecular structure of human skeletal muscle. The contraction of skeletal muscles is accomplished, on a molecular level, by the interaction of two long parallel-running proteins, one ropelike and the other more like a ladder studded regularly with sticky heads. The proteins (myosin and actin) touch, swing past each other, release, then repeat the motion, generating force and turning chemical energy into physical energy. Pooled and concentrated, they produce enough torque to contract the whole arm.