aching of limbs, muscular pains
痠 centers on deep, dull pain in the body, especially aching limbs and muscular soreness.
痠 combines 疒 (sickness) with 夋, which likely contributes the sound サン and may relate to a dull, lingering quality. The exact historical development is uncertain.
The sickness enclosure 疒 wraps around 夋, like a dull ache spreading through the body. Picture limbs feeling heavy and sore inside this enclosure.
For サン, imagine aching limbs after a long day in the sun: sun -> サン, and the heat makes the muscular pain worse.