EMA BW Third-class lever C IMA EMA MF EW Data for third-class lever: Muscle force (MF) = unknown External weight (EW) = 66.7 N (15 lbs) Internal moment arm (IMA) = 5 cm External moment arm (EMA) = 35 cm Mechanical advantage = 0.143 MFX IMA= EWX EMA MF=EWX EMA IMA MF = 66.7 N x 35 cm 5 cm MF=467 N (105 lbs) FIG. 1.23 Anatomic examples are shown of first-class (A), second-class (B), and third-class (C) levers. (The vectors are not drawn to scale.) The data contained in the boxes to the right show how to calculate the muscle force required to maintain static rotary equilibrium. Note that the mechanical advantage is indicated in each box. The muscle activation (depicted in red) is isometric in each case, with no movement occurring at the joint.