Reviewing Lever Systems

One of the core concepts of kinesiology, a foundation of physical therapy, is understanding lever systems. A lever system is described when using muscles to move joints as the relation between 3 components: load, fulcrum, and effort. There are 3 ways that these components are organized in order to create force and move a load. This should serve as a good review for anyone studying kinesiology or to better understand how our muscles are affecting our joints.

First Class Lever: This lever system has the fulcrum in the middle and the effort/force and load on opposite sides. A good real-life example is a seesaw and a joint-based example would be the triceps muscle at the elbow. The fulcrum is the elbow, the triceps is on one side of the elbow and the load is placed in the hand on the other side of the elbow. This system is good for strength creation but has slower speed.

Second Class Lever: This lever system  has the fulcrum and effort/force on opposite sides with the load in the middle. The classic joint we think of here is the ankle. The fulcrum is the MTP, the effort is the gastroc/achilles complex, and the load is the body weight in the middle. A real-life example would be a wheelbarrow. The fulcrum is the wheel, the load is in the wheelbarrow, and the effort or force comes from us lifting the handles. This lever system is great for strength, but again has less speed.

Third Class Lever: This lever system has the fulcrum and load on opposite sides and the effort/force in the middle. The muscular example would be the biceps at the elbow as the tendon (effort/force) attaches distal to the elbow (fulcrum) and the load is in the hand. A real-life example would be holding something with tweezers. This lever system is better for quick movements.

Hopefully this will serve as a good review for some of you in school or in the clinic. Often these foundational concepts are overlooked, but can play a role in standard rehabilitation programs. Not only does it help us in knowing how to properly load each joint/muscle, but it also can tell us what those muscles are to do.

-Dr. Chris Fox, PT, DPT, OCS

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