just strap it . if you want protection you will need a proper brace . better start doing squats .

Chemist warehouse has a range braces and joint support straps. Low prices too.

i have a Drytex Hinged air Donjoy that make me feel all cyborg & supported. The air chambers on the front aid support to the side of your knee most needed. It is for water use, not all braces are.. Works brilliantly with boardies or short leg wetties, have not used it with full length wetties. Goodluck iggypop.

Hi
I have same problem after few hours of choppy conditions.
I found for me a softer board that eats up the chop helped me.
I probably a but older that most of the guys around so I put my
stuffed knees down to old age mostly.
I did at one stage do a google search for knee braces and bought one(found heaps)
oneline which a guy had used for rehab.(ebay I think)
Has alloy side supports with adjustable swing side pivots and has heavy
velcro above and below the knee.
tried it once and found it another hassle to worry about on the beach and in the water.
Do google seach, you will find heaps.
I do agree with exercise and chop eating board though.