Core muscles around your middle get a fast work out on one of these. Those very muscles are the ones that are often linked to back pain when they become out of shape. I've had back surgery years ago and if I don't get enough excercise of those muscles, I begin to get some pain.

But I think Larry was referring to the stowing up on deck or launching part. They are light to lift but a bit cumbersome, especially in some wind. The paddles stow easily in a cockpit locker(most break down) but a SUP is just one more thing to deal with.