There may be two distinct types of hypertrophy—sarcoplasmic hypertrophy and myofibrillar hypertrophy. It’s been theorized that moderate intensity (10-20 rep) training would improve a muscle’s ability to perform series of repetitions by adding fuel (sarcoplasm) to your muscles, whereas heavy (1-5 rep) training would cause the muscle to improve its strength by increasing the contractile elements (actin and myosin) of the muscle.
Myofibrillar hypertrophy=muscle fibres
Both adaptions would cause your muscles to grow bigger, but moderate rep bodybuilding would theoretically cause you grow by increasing the amount of fluid in your muscles, whereas strength training would theoretically cause you to grow by developing new muscle fibres.
A 2014 study by Schoenfeld found that bodybuilding does indeed cause sarcoplasmic hypertrophy (study), so this may actually be true.
The practical implications of this are pretty neat. Bodybuilding leads to “chronic increases in intracellular fluid status”, which means perpetually inflated muscles, and perhaps more interestingly, there’s a large body of preliminary research that indicates that this could mean an increased ability to build even more muscle as well as greater resistance to muscle atrophy.
What we still don’t know is whether this increased muscle hydration would happen with other types of weightlifting. Another study published by Schoenfeld found extremely similar muscle growth between both powerlifters and bodybuilders, even though their training routines were extremely different (study).
We also don’t know whether sarcoplasmic hypertrophy would look or feel any different from other types of hypertrophy. It’s indeed possible that myofibrillar hypertrophy would result in harder muscles… but that’d just be guessing.
If being big, strong and functional is your goal though, doing both strength training rep ranges and bodybuilding rep ranges is the way to go. (Higher rep ranges than bodybuilding and your muscles wouldn’t really grow any bigger in either way.)
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