I had the same problem, back pain caused through age and degeneration from sitting at a computer screen for decades. I had niggling pain in the lower back and every year or so I would be crippled and could barely stand. The cause was the channel the nerves passed through at the base of the spine was too narrow. All the pain dated from long before I took up kiteboarding.

I switched to a waist harness and did lots of toeside riding. In recent years I have almost exclusively ridden directional boards (strapped surf and freeride raceboards). I have had no pain since. I attribute the improvement to the flexing and strengthening that I got through excessive amounts of kiteboarding, the use of the waist harness, and very good technique.

The key thing is technique. Straight back, hips forward, front leg mostly straight, trim the board with a bent back leg. Straight arms, steer from the shoulders. Let the kite do all the work.

I think seat harnesses are bad for your back for two reasons. When riding toeside they lock down on you hips and it is like having a big spanner attached. The torque forces are huge. Even worse, the seat harness lets you get away with really crap technique. Poo man stance, bent forward.

Of course, I am just some dick on the internet. Just because it works for me, it might not work for you.

PS I am 58. Kiteboarding for 15 yearsl

PS We have recently been learning to ride a hydrofoil so we are thrown back to beginner stage. It would be a lot easier in a seat harness to cope with kooking around in the water with the kite at zenith and the board trying to kill you.

PPS. My most favourite thing of all time is upwind runs. We blast hard and fast upwind for an hour before cruising back downwind through the surf. Riding fast upwind is so much kinder on the body than riding fast downwind and SlammingIntoTheBackOfEachF!u!c!k!i!n!gWave.! Cruising in and out and playing with every wave at each beach all the way down is pure joy.

PPPS My back problem was the gap at the base of the spine was too narrow. This rubbed on the nerve sheath of the nerve bundle that passed through it and caused inflammation which caused pain. The surgeon said he could do nothing to treat it, other than inject Cortisone into the nerve sheath. This doesn't fix the pain, it just deadens it so I cannot feel it. I have no idea how bending and flexing and twisting and having fun can fix that sort of problem, but the pain has gone and that it good. My surgeon and I were of the "use it or lose it" school. Go hard and have fun.