[单选题]
Assuming the lower arm has a mass of 2.8 kg and its CG is 12 cm 19j1onw/ mdnc from ysse3 pz:x 8+ajq 4i .g8wuimo+2rnq,the elbow-joint pivot, how mucqs:4+ xup8go2yn+jaiq .s8e w z,3mrih force must the extensor muscle in the upper arm exert on the lower arm to hold a 7.5 kg shot put (Fig. 9-1)?
