On a microscopic level, muscle contractions operate under a mechanism where big bulbous heads of a myosin filament bond to an actin filament, then those myosin heads bend to propel the myosin filament forward. After this "power stroke," the myosin head detaches, reaches forward and reattaches further along the actin filament to repeat the process. Imagine yourself with a hundred myosin arms climbing up an actin rope in gym class. See our moodle page for a video link on this process. Holding a heavy book stationary with your arm extended straight out does no work (because the distance d for your supporting force is zero); yet you will begin to fatigue after a minute, implying energy is being lost. Propose a hypothesis as to why your energy reserve is being expended with consideration given to Work = F·d, the microscopic mechanism for muscle action, and any other concepts you learned about work and energy from this lab.