請問臂長是指哪裡？ - 單車

我在測量車架尺寸網站測的時候搞不懂臂長是哪裡到哪裡

原文是

Arm:（臂長）

Continue to hold the cylindrical object and extend your arm in front of you,

holding your arm parallel to the ground. Try not to allow your shoulder

to come forward. Make sure that the object in your hand is perpendicular

to your arm. Locate the Pivot of the arm to shoulder point and
～～～～～～～～～～～～～～
measure from this point to the center of the object,

which should fall in line with your knuckles.

Measure both arms and average the measurements.

問題是Locate the Pivot of the arm 這要怎麼看，中心點或旁邊？


另外肘長有一樣的疑問

Forearm （肘長）

Hold a cylindrical object (a magic marker is ideal) in your fist.

Lower your arm so that it is to your side, and extend your forearm

in ahead of you. Keep the object in your hand perpendicular

to your forearm, and measure from the center of the object,

which should fall in line with your knuckles,

to the boney protrusion at your elbow pivot.
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這the boney protrusion at your elbow pivot又是指哪裡？

我摸不太出來，隨便量 誤差有2、3公分

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