誰能說一下為什麼試驗組和對照組的英文可以翻譯成treatment/control arm,為什麼是arm?

我只是搬運工，原文鏈接：藥物臨床研究設計中的"arm"_Dr_Devil_新浪博客

近日在幫助王薇寫《鎮痛藥物臨床研究設計的思考》中發現了個單詞"arm"，查閱了多個搜索引擎發現這個單詞在國內翻譯解釋很稀少。因此，在國外翻譯網站上搜索了英文解釋，記錄下來以便將來之用。

Arms

Clinical trials can include multiple "arms." Each arm is a study group of patients receiving a specific treatment or combination of treatments that is being compared to other treatment arms as well as to the control arm. The "control arm" is the best available standard treatment.

或者

Arm

[In a controlled trial.] Refers to a group of participants allocated a particular treatment. In a randomised controlled trial, allocation to different arms is determined by the randomisation procedure. Many controlled trials have two arms, a group of participants assigned to an experimental intervention (sometimes called the treatment arm) and a group of participants assigned to a control (the control arm). Trials may have more than two arms, with more than one experimental arm and/or more than one control arm.