ELI5: Is it better to do many reps, or lift heavy, when trying to build mass at the gym, and what effect does it have on your muscles biologically?

There are fast twitch and slow twitch muscle fibers. Slow twitch contain more mass and support heavier load. Fast twitch contains less mass and handle less weight but support quick rapid movements. People have different body types and the ratio of fast to slow twitch varies.The types are Ectomorph vs mesomorph vs endomorph. In order to gain hypertrophy you will need fast twitch muscles to apply intensity and slow twitch to handle heavier weights.

Progress comes from increasing intensity, the amount of time your muscle experience load, less rest between sets, and increased reps.

After making progress in each of those areas you build the strength neccesary to undertake heavier weights. So the goal of adding mass will be achieved by correctly performing the excersize with heavier weight. After you add weight, you will drop the reps, and repeat the cycle. Each week you make progress in one or more areas.
An example to Consider - a 12 reps 3 sets of 100 lbs is the same as 6 reps 3 sets of 200. One month you may be working on improving your strength and 12 reps of 3 sets becomes 15 reps of 3 sets. The next month you can now do 6 reps and 3 sets of 205. Than you may choose on month 3 to do a 12 reps 3 sets of 110. Month 4 you can now try to use both low and hi reps in what is called a pyramid set. 12 reps of 110 than /10 reps of 115/ 8 reps of 115/ and 6 reps of 120. After month of that you might do 3 sets of 6 120. and gain your mass.

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