Hello,
Did you receive any physiotherapy following your operation at all? Without assessing exactly what muscles are weak it is somewhat difficult to design a rehabilitation protocol for you.
If you were given exercises to do after your operation, these should work on building up these muscles and should be OK to continue with.
If not, you should probably see if you can visit a physiotherapist to design you a rehabilitation plan which will help improve the muscles in the weaker areas.
This page http://www.sportsinjuryclinic.net/cybertherapist/front/frontthigh/quads/quadstrength.php gives you some information on quads strengthening which may be of assistance.
I hope this is of some help!
Ruth Cheesley BSc(Hons) MSc MCSP
Chartered Physiotherapist