Quads lag and how to reduce it

I had multiple injections in infancy and contracted right quads as a result. Stiff hip until I learnt to walk, then stiff knee. Operated on (quadricepsplasty) when I was four and I got about 90 degrees of bend and no fixed or variable lag, as far as I remember. I never was athletic and always did favour my left leg, so gradually I lost bend again in my right. When I was 18, I had another plasty.. The surgeon had hoped to get me to 90 degrees again but I woke up with just 40, I would say and he said that my knee and quads had a large amount of adhesion. He extended that to 90 with another flexion under anaesthetic a couple of weeks later .... but I now have both fixed and variable lag (about 10 degrees of fixed and a further 10-15 degrees of variable). I have lived with this for over 20 more years now but is there anything I can do to reduce either lag? I have had periods when I have been MUCH more muscular (cycling 20 miles a day) and the left leg has had the usual four bunches of muscles on the thigh but on my right thigh I could touch bone on the outside of the thigh a little above the knee and the (quadriceps?) muscle "lump" was diagonal from my hip down across to the inside of my knee.
So ... are there any ideas as to how to eliminate my lag - so that I can actually lock my knee, for example?