Muscle contraction occurs when the myosin heads rotate and pull on the actin filaments.
The details of how the myosin heads move are not well understood, however. Our measurements provide direct evidence in support of this lever-arm model. Instead, we label various parts of the myosin with different dyes and watch how the light emitted from the dyes changes as the protein moves. Instead of using conventional dyes, however, Selvin uses a rare-earth metal atom in combination with a regular dye. The metal atom is attached to the myosin molecule and lights up with a distinct color when excited by a laser beam.