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FUNCTIONAL ANATOMY education Levers are rigid devices that transmit or modify forces to create movement. Recognizing the function and purpose of the different types of levers in the human body may help inform treatment goals and application for specific regions of the body. COMPONENTS OF A LEVER Every lever system has three specific components, beginning with an axis (or fulcrum). This is the part that the lever itself turns around. For example, in a pair of scissors, the axis is the pivot point between the handles and the blades. A wrench is a lever that uses the center of the bolt you are turning as an axis. In the body, joints serve as the axis. For example, the knee joint serves as the pivot point or axis between the upper and lower leg. The next two components are sources of mechanical energy that work in opposition. The first is resistance, defined as the mechanical energy that is being overcome by activating the lever. When examining human movement, gravity, friction, or some other external force that the body must work against provides resistance. The second source of mechanical energy is called force and is provided by muscle contractions. Using our scissor example, squeezing the handles generates force and the item you are cutting provides Levers in the Human Body By Christy Cael resistance. In our wrench example, the effort you use to turn the wrench is the force and the friction created by the threads of the bolt provide the resistance. TYPES OF LEVERS The three components of a lever system are arranged in different configurations to accomplish different tasks. First-Class Levers A first-class lever is characterized by a central axis, with the force on one side and the resistance on the other. Seesaws are a common example of a first-class lever (see Image 1). A rigid plank is placed on a central stand and one person sits on each end. The two can balance on the central axis, or one can move skyward while the other moves down. This type of lever is designed for balance. First-class levers are utilized where the body requires balanced strength. Lifting your head up after looking down is a first- class lever at work. The weight of the head is forward relative to the vertebral column. The forward pull of gravity forms the resistance of the lever. The joints between the cervical vertebrae form the axis. The trapezius muscle and its synergists that extend the head provide the force to move the lever. Resistance is on one side, the axis is in the middle, and the force is on the other side. This type of lever at this location promotes central positioning of the head on the vertebral column. Second-Class Levers A second-class lever has the force on one end, the axis on the other end, and the resistance between the two. Wheelbarrows are a common second-class lever (see Image 2). The wheel serves as the axis on one end. The bucket in the center is filled and provides resistance in the center. Lifting the handles provides force on the other end. Second-class levers are very powerful, but offer limited range of motion and speed. 44 m a s s a g e & b o d y w o r k j u l y / a u g u s t 2 0 1 8 First-class lever 1