Suuna being lewd but so pure at same time must be hard for Ruka.
She surely want to does what sefure usually do, but she also doesn't want to defile Suuna.
Not totally unreasonable - lower back and hip pain is often associated with muscle pain below the hips. That whole area is a big mass of muscle and ligament supporting a complex skeletal structure, muscles in the upper thigh can have a direct impact on the state of the lower back, and vice-versa.
Though I think it's more the front of the thighs than the back . . . and I'm pretty sure whoever suggested this to Suuna would have absolutely told her that if they knew, since it would have been even funnier. 'cause we all know the real reason this was suggested . . .
And while practising straddling the rolled up blanket, the masseur felt this strange, enjoyable warmth. She assumed it was the glowly feelings brought on by performing a good deed for another. Ah, that sweet, innocent super-top.