it's not just muscles, he didn't have the physiology to be a great climber pre cancer. it was thought he might have the build to be a decent classics rider
HGH, EPO, Blood doping etc can really add to your ability to maintain the required effort to climb in the high alps and repeat those efforts over 3 or 4 passes.
you can tell it's had an effect in the last couple of years with blokes having to dose their efforts on climbs and not being able to really blast away continually. it's been more sustained pressure rather than, blast, rest, blast, rest, blast, rest, blast which really takes it out of you.
not that doping hasn't been going on for 50 years or so. Tom Simpson died on Ventoux in 1967 with 3 vials of speed in his jersey, one already used.