Reading through this thread I see my fellow kiters suffer. Working in the field of sporting and work related injuries for long (too long?) time, I attempt to give a brief general summary of my view.
My background...? I had severe knee, lower back, and neck problems from my late teens due to injuries mainly from gymnastics, skiing, and just by stretching limits....
Just to gain your trust:
Now, I am 55 (not kg), my last knee problem was when I collided with a kite surfer mate 8 years ago, my last lumbar problem was when I tried waist harness for a few days two years ago, and the last neck problem was when lots of beautiful women walked around me (before I met my girlfriend

)....
I kite up to 5 days a week (depending on wind), 2-4 hours non stop.
What I do for it.
1. I eat very healthily, apart form some tendency to put good amount of bourbon into my fresh orange juice…....uhhhmmm alcohol is a good preservative…....
2. I never kite without a warm up (sometimes long sometimes short), and I stretch only after a warm up. In fact, depending on how I feel, the first 30-60 min of my kiting is more or less a warm up with smaller jumps and easygoing wave riding. I use a modified seat harness….check the pics on my profile if interested...and ..after every session I sit in my outdoor spa for at least 10-15 minutes under heavy massage jets with the above mentioned preservatives in hand…...and I am immensely grateful for living in OZ, where I can do all this....
The following suggestions are general advice only, I don't take into account anyone's personal circumstances....…..(yes, you can use scotch in the orange juice, but you cannot replace orange juice with coke or red bull)......and these advises are inherently incomplete, so I would be open to anyone's experience, something that worked for them....
Reading my post......yeah, it's long...so feel free to bile out if it doesn't resonate with you..
OK. Here we go....

1. Do not take age as an excuse, take it as a challenge. The body always responds positively to positive changes. I mean diet, lifestyle, and regular exercise, low on alcohol, coffee / red bull and emotional stress.
2. If you have an injury do NOT HTFU, because you will likely to break again. See a professional, and don't take anything less then 100% at recovery.
3. Know your limits so that you can go beyond them sensibly and intelligently.
4. Just like with driving: don’t kite tired.
5. Drink water before kiting. If you spend more than 1.5 hours on water, put electrolytes in your water before you take off. They will keep water in your muscles. Dehydrated muscles cramp, fatigue and break easier.
6. Warm up. Even just jogging down to the beach is fine, as long as you puff at the end. (if you are athletic you can carry my gear as well just to puff at the end....)
7. If you sit most of the day you need more warm up. If you are emotionally stressed, you need more stretches.
8. Stretch only after warm up.
9. Take the first 10-15min of kiting as a warm-up when you are in your 20's (even when your girlfriend is around), and add an extra 5-10 minutes with every 10 years thereafter...
Lumbar pain:
1. Seat harness for most cases. It may restrict your movements, and look uncool, but sitting at home or lying in bed with pain is not too cool either. If you ride the waves, or like to ride toe side check out my harness modifications.
2. Do not unhook. Riding unhooked is similar to lifting, which is a no-go with lumbar problems.
3. Avoid landing toe side from a jump. It’s a bit tricky not to jump when going through the shore break toe side, but do your best, and land softly. It is better to just splash in the water and get wiped out, then landing in an awkward position and go home crook.
4. Strengthen your belly muscles. Even just lifting up your board above your head with each jump will do, apart from some sit-ups on windless days. Pilates is great, but it may cause lower back strain if you are not careful.
5. Strengthen your knees so that you can keep them bent and flexible all time. Rigid / weak knees at landing from a jump = pressure on L4 L5 = trouble.
6. If you don’t have the luxury of a massage jet in a spa, then have regular massages or other forms of body work, and use some liniments on your back before and after kiting. What I use is called “Zen” and probably available in pharmacies (I am not associated with the manufacturer in any ways.)
7. Sort out all injuries with professionals. Scar tissue formation, calcifications are sources of further injuries and pain. Just because it has stopped hurting, it may not necessarily be healed.
8. Partially dislocated hip (or any other) joint, that some of you had, usually means ligament stretch or break, and it is a bad news. It needs to be sorted out professionally and has to go through a rehabilitation process. Otherwise it will keep coming back.
Knee pain:
1. If it is painful check it out with your doc for ligament or cartilage tear. And if the tests come positive, repair it (usually surgery, and rehab). If no tear, then it is muscular weakness, mainly the quads (front thigh muscles)...and this is the better news (HTFU as one may say)
2. Jog, and jump around a little while, warm up your knees before you go on the water. Then stretch, make sure your quads (thigh) muscles are warm and flexible before you go on the water. (you sure get my message by now: warm up precede stretches. This is an unorthodox approach...yes, it works)
3. On the water be gradual, gentle and flexible. This is the secret. If your knees giving notices, it usually means limitation in the strength of your thigh muscles. Don’t ignore it, take a rest. If you HTFU you will break. It’s better to take rest every 30 min and go back, than going home after 60min due to pain.
4. After kiting keep your knee, and quads warm. Don’t hang out in 20+kn even if the weather is warm. The wind chill will make your blood vessels contract, reduce circulation = pain. (I always have this in colder weather….nasty…until I rub something in my knee and put on something warm.)
5. Keep your quads super strong, especially if you kite 2-3 hours without break. Squats, gym work, cycling are great. Sitting is a killer.
6. Use some liniment on your knee after session, as I suggested above…
At last (only for male): don't fight with the waves...make love with them...don't they remind you to beautiful breasts....?