二头 = 'two-headed' — the biceps literally has two heads (long head + short head) where it attaches to the shoulder. 三头肌 (sāntóujī, triceps) follows the same logic with three heads. Anatomically transparent.
usage
Common gym vocab pairs: 二头肌 (biceps), 三头肌 (triceps), 胸肌 (xiōngjī, pecs), 腹肌 (fùjī, abs), 背肌 (bèijī, back). 肌 (jī) is the suffix for muscle group.
