According to GRADE and EBM methodology, here is what ideal scientific evidence would look like to definitively prove or disprove this specific claim, ordered from strongest to weakest evidence.
Randomized Controlled Trial
A 12-month RCT comparing two groups of resistance-trained humans: one group undergoing a training protocol optimized to induce sarcoplasmic hypertrophy (e.g., high-volume, moderate-load, short rest) and another undergoing a protocol optimized for myofibrillar hypertrophy (e.g., low-volume, high-load, long rest). Muscle biopsies would quantify sarcoplasmic components (glycogen, mitochondria, fluid) and myofibrillar protein content via proteomics at baseline, 6 months, and 12 months, with functional outcomes (strength, endurance) tracked. The study would isolate whether sarcoplasmic changes persist and contribute meaningfully to net muscle growth independent of myofibrillar accretion. Longitudinal Cohort Study
A 2-year prospective cohort tracking 200 resistance-trained individuals with detailed, repeated muscle biopsies (every 6 months) and imaging (MRI/DEXA) to correlate changes in sarcoplasmic components (glycogen, mitochondria, fluid) with changes in myofibrillar protein content and muscle size. Statistical models would test whether sarcoplasmic expansion predicts long-term muscle growth after controlling for myofibrillar accretion, training volume, and nutrition. A case-control study comparing elite bodybuilders with high muscle mass but low strength-to-mass ratios (presumed high sarcoplasmic hypertrophy) versus strength athletes with similar muscle mass but high strength (presumed high myofibrillar hypertrophy). Muscle biopsies would quantify component ratios, and the study would test whether those with higher sarcoplasmic content show less persistent growth over time or reduced functional adaptation. Controlled Animal Model Study
A study in genetically identical mice subjected to two distinct resistance training protocols—one inducing sarcoplasmic expansion (high-rep, low-load) and one inducing myofibrillar growth (low-rep, high-load)—with serial muscle biopsies over 16 weeks. The study would determine whether sarcoplasmic changes regress after training cessation and whether they contribute to net muscle mass independently of myofibrillar accretion. In Vitro Cell Culture Study
A cell culture study using human myotubes exposed to two distinct stimuli: one mimicking high-volume training (e.g., repeated calcium pulses) to induce sarcoplasmic expansion, and another mimicking high-load training (e.g., mechanical stretch + IGF-1) to induce myofibrillar accretion. The study would track persistence of each adaptation after stimulus withdrawal and quantify whether sarcoplasmic changes are transient and non-contributory to net protein accumulation.