brachialis muscle
Anatomical term for the muscle in the upper arm that flexes the elbow.
The brachialis muscle is the muscle that works when you bend your elbow.
上腕三頭筋 (triceps brachii) is on the back of the upper arm and extends the elbow, opposite to the flexing action of 上腕筋.
Compound of 上腕 (じょうわん, 'upper arm') + 筋 (きん, 'muscle'). The reading follows standard on'yomi.