I understand that you train low rep, high weight for strength, low weight high rep for endurance, and hypertrophy is somewhere in the middle. But what actually happens in the body to cause these differences in muscle growths due to different rep ranges?
In: Biology
strength requires (in addition to neurological adaptation) and increase in contractile fibers, aka muscle fibers. this increase in muscle fibers is one aspect of hypertophy. these changes are not that big in appearance, so you can get a lot stronger at single repetitions without necessarily getting big.
endurance is aided by sarcoplasm, which is a fluid containing water and sugars that your muscles can use for energy. this is stored near the muscle fibers for them to use, and though it does not contribute to contractile force, it helps the contractile fibers work over time (eg for high rep sets). this fluid occupies a lot of volume, and will contribute significantly to a “swollen” appearance, or the visual aspect of hypertrophy.
Longer endurance activities activate aerobic energy systems utilizing oxygen to work in order to complete the activity. For short, strength exercises you are utilizing anaerobic pathways. If you focus on one of these in particular your muscles and body adapt to favor whichever you use the most frequently, therefore preparing you for what it now assumes will be something you need in the future.
There’s also a lot of more complicated pieces to this regarding hormones, muscle damage etc but long story short, the body adapts based on what you do often so that you will be prepared in the future. Your body is simply trying to prepare better for the next time it is under stress, and prioritizes strength or endurance based on what stimulus it is receiving frequently
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