Several parts combine into one character.

筋 is a triple-component compound ideograph — unusual in CJK characters, where most have only two main parts. Three radicals contribute: ⺮ (bamboo) on top, 月 (flesh) on the lower left, 力 (power / force) on the lower right. The composite picture is rich: bamboo-like grain (the striated structure of muscle fibers, which split lengthwise like bamboo) + flesh + power = sinew / muscle / tendon. Each component carries a piece of the meaning. The "bamboo grain" metaphor is anatomically apt — muscle fiber bundles do separate along grain lines, like bamboo.

Korean reading "geun." 筋肉 (geunyuk, muscle), 筋力 (geunnyeok, muscular strength), 腹筋 (bokgeun, abdominal muscles / abs — central to Korean fitness vocabulary), 心筋 (simgeun, cardiac muscle / myocardium), 鐵筋 (cheolgeun, "iron-sinew" = rebar / steel reinforcement bars in concrete construction). Important homophone caveat: the Korean syllable 근 represents many unrelated Hanja — 根 (root), 近 (near), 勤 (diligent), 筋 (muscle) — each with completely different etymologies.

Mandarin jīn, 1st tone. 筋 (jīn, muscle / tendon), 筋肉 (jīnròu, muscle), 脑筋 (nǎojīn, "brain-muscle" = mind / thinking — Mandarin uses this metaphor more freely than Korean or Japanese; 动脑筋 dòng nǎojīn = "to use one's brain muscle" = to think hard), 钢筋 (gāngjīn, "steel-sinew" = rebar — same construction metaphor as Korean 철근).

Japanese on-reading キン (kin) — 筋肉 (kin'niku, muscle), 腹筋 (fukkin, abdominal muscles), 心筋 (shinkin, cardiac muscle), 筋肉痛 (kinnikutsū, muscle soreness — extremely common after exercise). Kun-reading すじ (suji) — and here Japanese took the character's metaphorical capacity to an unusual degree. すじ means "tendon / line / streak / stripe," but also "plot of a story / logical line of reasoning / ancestry / source." 話の筋 (hanashi no suji) = "the plot / thread of a story." 筋道 (sujimichi) = "line of reasoning / logic." 筋がいい (suji ga ii) = "has good aptitude / good lineage / promising bloodline." The single Japanese word suji unifies physical sinew, narrative thread, and logical structure — a remarkable semantic web.

Memory aid: bamboo grain (⺮) + flesh (月) + power (力) — striated, fleshy, force-producing tissue.