Levers in the Human Body

By Christy Cael
[Functional Anatomy]

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 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.
A second-class lever in the body is found in the ankle, where power and propulsion are critical. Here, the axis is the ball of the foot, and strong calf muscles (plantar flexors) attaching to the heel provide the force. The resistance comes from the weight of the body compressing down through the tibia between the two. This powerful lever propels the body when walking, running, and jumping. It also helps explain why the calf muscles are so large compared to the smaller shin muscles (dorsiflexors).

Third-Class Levers

Third-class levers are those with the resistance on one end, the axis on the other, and the force between the two. A shovel is a third-class lever (see Image 3). The ground provides resistance when you dig the end in. Force is provided when you lift the middle of the handle. Your other hand provides the axis at the far end of the handle. These levers emphasize speed and range of motion.
Third-class levers are the most common type of lever in the human body and are demonstrated by flexing the elbow to raise the hand toward the shoulder. The elbow joint is the axis, while the biceps brachii and brachialis muscles just distal provide the force. Resistance is the weight of the forearm and whatever is held in the hand.

Application to Bodywork

Identifying which type of lever is used in a given region clarifies function and assists in making appropriate treatment goals for optimal performance, injury prevention, and treatment. Supporting balanced muscle length and strength in the vertebral column makes sense, as does recognizing the tendency of the ankle plantar flexors to shorten with use in explosive activities, since second-class levers emphasize force over range of motion. Most of the mobile joints in the body require full range of motion to function optimally, as this is characteristic of third-class levers.

Christy Cael is a licensed massage therapist and certified strength and conditioning specialist. Her private practice focuses on injury treatment, biomechanical analysis, craniosacral therapy, and massage for clients with neurological issues. She is the author of Functional Anatomy: Musculoskeletal Anatomy, Kinesiology, and Palpation for Manual Therapists (Lippincott Williams & Wilkins, 2009). Contact her at christy_cael@hotmail.com.