Q: “I understand that heavy training is the key to building muscle, but I have some lagging muscles that just aren’t responding. Sometimes I do a lot of endurance work, but I’ve always avoided that during cycles. Could I get a size benefit from anabolic steroids while doing endurance training?”
A: There are so many things that are true most of the time, but really should not be taken as iron-clad, no-exceptions rules.
One of them has to do with light-weight, high-rep work, or with positives-only (no resistance on the return) work.
Yes, generally the way to build muscle is absolutely not as so many women like to work out, with light weights allowing very large number of repetitions. And distance running certainly will not build the muscle that sprinting can, for example.
However, when all else has failed, when everything sensible has been tried with heavy training to build a lagging muscle, sometimes brutal endurance training can be thing that wakes a muscle up (figuratively speaking.)
A personal example would be my front delts. I must have either strange genetics or a strange pattern of nervous system firing, as my front delts have always been much smaller than either the medial or rear delts. Nothing in the gym ever fixed this. No amount of military presses, Smith machine presses, front raises, plate raises, cleans, snatches, cable work, or upright rows. All kinds of programs were tried. Just not much resulted, although I could strict military press bodyweight – which to be sure is not excellent strength but isn’t terrible, particularly for having no front delts.
To my absolute astonishment, 90 minutes of outrigger canoe (Hawaiian ocean canoe) paddling three times a week at the very least doubled my front delt size, and took only months to do so. From past experience, I would never have accomplished this from weight training, and certainly never did in the past. Now, this was absolutely brutal on the shoulders: a forced death march so to speak. But they responded. Figuratively speaking, I suppose the body had no choice.
There are also of course many other cases of brutally exhausting calf work finally overcoming a plateau, or forearms finally responding after taking up brutal amounts of work.
Possibly, if you do endurance work of that sort for a lagging muscle, you might get similar benefit, and anabolic steroids most likely would help.
More generally, anabolic steroids can help avoid losses while doing endurance work.
As for doing things like 10K runs, muscle gains are unlikely from such work, except perhaps calves or tibialis if such work is entirely new to them and they find it brutal. More likely however stair work or running in sand would be better.
Most times the answer is no to your question, but sometimes yes.