Perhaps there's some unmeasured influence, but this study was looking only at the difference between growth within subjects vs between subjects. If the subjects were all "newbies", then that doesn't explain the results.

They're essentially saying that individual genetics explain the majority of the variation seen as a response to muscle stimulus in their test subjects, not the mass used, because the variation within the test cohorts was greater than the variation between them. You can argue that, if they didn't test experienced lifters the results might be different in that population, but you can't dismiss the results on those grounds.

> not the mass used.

Completely anecdotal, but when I was 18, in highschool, I trained in the gym in my hometown, supervised with a trainer, 12 reps per muscle group, very modest gains.

I move to university, start reading a fitness forum where people were saying do max 6 reps if you want big gains.

I also started supplementing with whey protein, and within 3 months the gains were spectacular, everybody noticed, I felt on fire, best time of my life, I miss so much how great I felt in my own body.

I've seen other colleagues and how they trained -- I can say there was 100% correlation that those people who were not training hard also did not have big gains. People who had enough breath left in them to chat in the gym simply did not gain as much as people I saw as training hard.

Also for me, the 6 reps to exhaustion felt completely different then 12 reps (again, to complete exhaustion) -- immediately after the training it felt amazing to be alive, the world became a comfortable place, my anxiety completely vanished, and in the night and morning after an intense training (especially the legs and back) the erections and libido boost were out of this world, something I never felt with the 12 reps regimen.

The activation energy or stimulus required for hypertrophy in untrained individuals is so low that it’s hard to differentiate the results. Studies like this absolutely need to be done in trained individuals if you want reliable data.