Since the 1950’s, researchers have thought that lifting weights mostly just makes you stronger, then once you are strong enough to lift really heavy weights, your muscles get bigger too.
Buckner and colleagues (2016) discuss a bunch of research that has come out since then. They say it shows that when lifting big weights, some people get big muscles, some people get stronger, some people get both, and some people get the shaft. So, not everyone gets the same response from lifting. And also, bigger is not necessarily equal to stronger.
The authors say that people get bigger muscles from lifting light weights lots of times OR from lifting heavy weights a couple times, but only lifting heavy weights makes you stronger. This means strength and muscle size might be sort-of related, but they aren’t the same thing. How does that work?
Basically, the authors suggest that you can get BETTER at lifting heavy weights. Meaning, your central nervous system (brain and spinal cord) and other neural (nerve) parts get better at making your muscles exert force.
Also, people stay strong for a long time after stopping lifting (like half a year even if you are young), but that your muscles get smaller pretty fast (you go back to normal in just a few months if you haven’t been lifting long).
Of course, then they sum it up all sciencey at the end by saying that we aren’t totally certain about all this, and we have to do more research (said every scientist ever).
Buckner SL, Dankel SJ, Mattocks, KT, Jessee, MB, Mouser, JG, Counts, BR, Loenneke, JP. (2016) The problem of muscle hypertrophy: revisited. Muscle and Nerve (published online) DOI: 10.1002/mus.25420