The most intuitive example of a motor in the human body would be the skeletal muscles. When researches were using optical and electron microscopy to study the muscle fibres they found that at each level of resolution the larger unit was made up of smaller force-generating subunit. At the molecular level myosin and actin were found. See Fig 10.1 pp405: muscles -> muscle fibres bundle -> individual muscle fibres -> myofibril -> myosin filament & F-actin filament -> myosin molecule & G-actin molecule.
The first experiment to provide direct proof of the force generation of myson & actin was by Finer et al., 1994 here they used the familiar combination of adhesion to glass beads which are trapped by optical tweezers which can measure force. When ATP was added to the setup shown in a. a sideways displacement of the actin filament was inferred by the measurement of a force in b.