system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How is low rep strength training different from higher rep size training? How do the muscle fiber grow differently to accomplish this?
gpt
Muscles work by snapping some muscle fibers under whatever stress you are putting them to, then replacing them with more fibers. This allows them to match whatever your needs are using only simple rules, a common theme across all of biology. While strength training, you're focused on the healing part more: you want more muscle fibers in exactly the places you need to strengthen the muscles you are training. Therefore, you do some reps to break a few fibers and then them heal up, making sure your body's systems can keep up with the healing.

In mass training, you're trying to make your body massively overcompensate by breaking tons of fibers with long strings of reps, leaving little time for the healing process to begin. By breaking everything at once, the body rushes in to repair the damage, going overboard in the process so less damage is done the next time you do that. In addition, that many reps quickly runs your muscles out of oxygen, forcing those cells to begin building up lactic acid and burn through their sugar supplies very quickly. The lactic acid also damages cells and sends signals to the surrounding tissue which encourage vascularization, the growing of new blood vessels and widening of existing ones. The increased blood flow also helps bulk up a bit just due to there being a lot more fluid in the area now, and also creates the huge, prominent veins you see in bodybuilders.