プログレッシブ和英中辞典(第３版)の解説
ちからこぶ【力×瘤】
〔腕の〕the biceps ((複 ～, ～es))
- 腕を曲げると上腕に力こぶができる
When he bends his arm, the flexor muscle stands out on his upper arm.／When he flexes his arms, his biceps stand out. (▼この場合は複数形)