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The functional priorities of your body become clear when you consider the fact that it is teeming with fast, agile, third-class levers and very few strong second-class levers. Mobility trumps force. For instance, the majority of the joints in your upper appendages are constructed as third- class levers. This makes sense since the sole purpose of your shoulder, arm, and wrist is to ensure that your hand is able to mobilize through a broad range of motions. THIRD-CLASS LEVER Moe is the "Shovel Master." He might extract only a small load of dirt with each pitch, but the swing of his tool allows him to remove soil rapidly and plop it in an assortment of locations (Image 22). A third-class lever is all about range of motion and speed. By placing the effort (Moe's hand) between the axis (his other hand at the end of the handle) and the load (dirt), this lever type trades away force amplifi cation for maximum range of mobility. As with fi rst- and second-class levers, altering the relative lengths of the lever arms (in this case by moving your grip) will change the mechanical advantage and the effort required (Images 23 and 24). Brooms, tweezers, baseball bats, tennis rackets, and golf clubs all act as third- class levers. The sporting instruments, in particular, highlight the pros and cons of this lever type. On one hand, a third-class lever affords a big windup and follow-through, and allows the action to occur rapidly. However, it doesn't provide much force, so the athlete needs to supply the necessary exertion—thus the need for a big stick to hit a tiny ball, followed by a tremendously loud grunt. Because humans are designed for locomotion, it is not surprising that your body (in the limbs, especially) is comprised mostly of third-class levers. Flex your elbow (Image 25), wrist or knee (or even clench your teeth) to feel this lever type in action and feel the swiftness and agility it provides. Andrew Biel, LMP, is the author of Trail Guide to the Body: How to Locate Muscles, Bones and More (Books of Discovery, 2014) and the president of Books of Discovery. This is an excerpt from his forthcoming book Trail Guide to Movement: Building the Body in Motion (Books of Discovery, 2014). He lives outside of Lyons, Colorado, with his wife, Lyn Gregory, and two children, Grace and Elias. Placing his hands close to each other shortens the (biomechanical) effort arm and lengthens the resistance arm. An ineffective way to shovel. particular, highlight the pros and cons of this affords a big windup and follow-through, and (Image 25), wrist or knee (or even clench your teeth) to feel this lever type in action and feel Placing his hands close to each other shortens the Resistance arm { Axis Effort Resistance Moe shoveling. With an extra-long effort arm (provided by an extra-long human arm), Moe could increase his leverage while using the shovel. With an extra-long effort arm (provided by an extra-long human arm), Moe could increase his leverage while using the shovel. 23 Effort arm { Resistance arm 22 24 resistance arm. An ineffective way to shovel. Flexing the elbow demonstrates a third-class lever. Resistance Effort 25 STORY 6 Axis