Like a Frilled Lizard Baby
Frilled lizards have reptilian musculature. I know because they are reptiles. And they can run upright. Check it out.
This is not exactly what you want. I don't believe the lizard can walk on two legs. Only run. They stay upright when at top speed.
But it proves the concept works. Just make some qualitative adjustments to your lizardman's anatomy so they can stay upright while walking or standing still.
Edit: In the comments @AustinHemmelGarn says the frill is unnecessary to stay upright. Chinese water dragons run like this too.