腱 = 月 (flesh radical, body-part marker) + 建 (jiàn, phonetic — same as 'to build'). Tendons are what 'build' the connection between muscle and bone, and 建 gives you the reading for free.
usage
Common compounds: 跟腱 'Achilles tendon', 肌腱 'tendon (general)', 牛腱 'beef shank' (a popular braising cut). Don't confuse 腱 with 健 (jiàn 'healthy') — same phonetic 建, different radical.
