Agree with billy on squats ^^^^
Adding to it - Most people arnt aren't flexible enough to squat properly... what helps especially in kiteboarding is 'Depth' in a No weights squat. Your body is exposed to more extreme angles and depth through your knees when your on the water, than you actually realise...
Here is a good start point - do 10 warm up squats first - then hold onto something secure and sit as deep as you comfortably can for 30 - 60 secs x 3 sets every second day..
The pic is what you should be aiming for, within reason... it will help to stop blowing your knees out due to trained nerve / muscle co-ordination when you land hard or go deeper than your normal Range of Motion ...
As you progress and gain greater flexibility - your next step is the Front squats.
Its the one on right with bar in front and for obvious reasons as the pic shows - the normal squat loads up your lower back too much for most people so safety first.
Only do it with a very light weight as your aim is kiteboarding 'not' Powerlifting. You can do 1 set of 10 (very easy) 3 x per week - as the weight is very light... and also you dont want to overwork / fatigue then injure yourself - watching everyone else kite !
Youll feel the difference very quickly.. the article is brief but good information and to the point
alumniwellness.com/2011/04/5-reasons-to-do-front-squats/ Also a great stretch for that exercise and lower back is the Hip flexor stretch.
Little known - this group when tight, is responsible for 80% of all lower back problems..
strengthen your core is what you always hear... thats an industry lie bla bla bla
See that gnarly big muscle that wraps around your lower back, when it gets too tight from lack of Muscle Flexibility, its what pulls your lower back out...
Any thing you do in a sitting positon - office, motor vehicle, Pc trolling ha ha for long periods make it tighten up - thats why after a long time, you stand up and stretch your back out - unconciously putting flexibility back into these muscles.. Ah haa you get the picture now Huh