In reply to fmck:
I agree with the above, definitely sounds like the quad tendon, pain on descent being a good indicator.
Below is just a suggestion but I ignored very similar pain in my knees for years, masked it and trained through it stupidly and ended up rupturing both quad tendon's at the same time while running in the fathers race at school sports day....cost me double knee surgery 2 weeks in hospital, 4 months in bed with leg braces, 6 months of work and 2 years of rehab....still not 100%
A small tear or strain possibly. A trip to the physio or to be on the very cautious side visit A and E and they should ultra sound it. There would no real viable signs ( bruising / swelling ) of a small tear of the tendon .
Rest and find a good regime for strengthening the lower quad and stability around the knee.
Mega cissus from my protein is very good for tendon strains
http://www.myprotein.com/sports-nutrition/mega-cissus/10530258.html?affil=m...
Post edited at 16:27