5.9: The Anti-Gravity Lean
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The structures discussed in the previous chapters were resting on the support base, which was not attached to the support surface (such as your feet and the ground). Therefore only normal force was available to cancel torques caused by gravity and maintain equilibrium. When the support base is attached then tension can help cancel out gravitational torques and the structure can remain in equilibrium even when the center of gravity moves outside the area of support. Such structures are known as cantilevered structures. The animation above shows someone performing the “anti-gravity lean” during which the body is momentarily a cantilevered structure. The maneuver requires that the heels of the shoes be attached to the ground in order to provide a tension force. Cantilevered structures can generate especially large stress and strain on the materials in the structure, for example the Achilles' tendon is severely stressed during the anti-gravity lean. When stress becomes too great then rupture may occur. In a future unit we will examine response of body tissues to stress, such as deforming and rupturing.