Resistance training with many repetitions leads to an increase in the fluid and energy-storage components of muscle cells, while training with fewer repetitions leads to an increase in the...

We analyzed the available evidence on whether high reps build sarcoplasmic hypertrophy and low reps build myofibrillar hypertrophy. What we’ve found so far is that 39 studies or assertions support the idea that training with many repetitions leads to increases in the fluid and energy-storage parts of muscle cells, while training with fewer repetitions is linked to increases in the contractile protein structures inside those cells [1]. Sarcoplasmic hypertrophy refers to growth in the non-contractile parts of the muscle — things like water, glycogen, and other stored energy — while myofibrillar hypertrophy refers to growth in the actual protein filaments that make the muscle contract. The evidence we’ve reviewed suggests these two types of growth may respond differently to training volume and intensity. We did not find any studies or assertions that contradict this pattern. However, it’s important to note that this is based on 39 assertions, and we have not reviewed the full methods or measurements used in each. The term “hypertrophy” here describes changes in muscle composition, but we cannot say how much each type contributes to overall muscle size or strength in real-world settings. The evidence we’ve reviewed leans toward the idea that rep ranges might influence which part of the muscle grows more — but we don’t yet know how meaningful this difference is for most people training for health or performance. In everyday terms: lifting lighter weights for more reps may swell your muscles with more fluid and energy stores, while lifting heavier weights for fewer reps may thicken the actual muscle fibers that generate force. But whether this changes how strong or toned you look over time is still unclear from what we’ve seen so far.