Every serious lifter knows the golden rule of getting bigger and stronger: progressive overload. To force your muscles to adapt, you have to consistently challenge them with more than they're used to. For decades, the debate has centered on the best way to do that. Should you prioritize adding another plate to the bar, or should you focus on squeezing out more reps with the same weight?
A new study from the European Journal of Applied Physiology suggests the answer isn't so simple. It turns out, the most effective method might come down to your unique physiology. You might be a "rep responder," a "load responder," or someone who grows just as well with either.
Let's break down this fascinating research and what it means for your training.
(Note: This review is based on the abstract of the study, as the full text was not available.)
The Study: Pitting Reps vs. Load Head-to-Head
Researchers wanted to see how individual muscle growth responses differed when overload was applied in two distinct ways:
- Load Progression (LOADProg): Increasing the weight on the bar while keeping reps the same.
- Repetition Progression (REPSProg): Increasing the number of repetitions performed while keeping the weight the same.
To test this, they recruited 37 untrained participants for a 10-week resistance training program. In a clever study design, each participant had their legs randomized. One leg was trained using the LOADProg model, and the other leg was trained using the REPSProg model. This allowed for a perfect apples-to-apples comparison within the same person, removing variables like genetics, diet, and sleep.
Before and after the 10 weeks, the scientists measured the cross-sectional area of the participants' leg muscles using ultrasound to see which leg grew more.
The Results: Four Types of Responders
After analyzing the muscle growth in each leg, the researchers found that the participants fell into four distinct groups. The results challenge the one-size-fits-all approach to programming.
1. The "Rep Responders" (~34% of participants)
About a third of the group saw significantly better results from adding reps. For these 12 individuals, the leg trained with the REPSProg protocol grew by an impressive 14.2%, while the leg trained with the LOADProg protocol only grew by 3.4%.
2. The "Load Responders" (~19% of participants)
For nearly one in five participants, adding weight was the clear winner. These 7 individuals saw their LOADProg leg grow by a massive 21.5%, compared to 12% growth in their REPSProg leg.
3. The "Equal Responders" (~35% of participants)
Another third of the group—the largest single cluster—responded almost identically to both protocols. For these 13 people, it didn't matter whether they added reps or weight; the muscle growth was statistically the same.
4. The "Non-Responders" (5 participants)
Finally, five participants were classified as non-responders, meaning they didn't achieve significant muscle growth (defined as less than a 5.7% increase) with either method.
What's Happening at the Cellular Level?
The researchers didn't just stop at measuring muscle size. They also took muscle biopsies to look for clues that could explain these different responses.
They investigated changes in myonuclei (the 'control centers' of muscle cells), markers of protein breakdown, and extracellular matrix remodeling, but none of these factors seemed to explain why one person responded better to reps and another to load.
However, they found one fascinating link related to satellite cells. These are muscle stem cells that are crucial for muscle repair and growth. In the group that responded best to adding reps (the "Rep Responders"), the REPSProg protocol led to a much greater increase in satellite cells (108.6%) compared to the LOADProg protocol (48.9%).
This suggests that for some individuals, a higher-repetition progression model might be a more potent stimulus for activating the cellular machinery needed for long-term muscle growth.
Practical Takeaways for Your Training
This study provides some powerful insights that you can apply to your own lifting journey.
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Individuality is King: The most important takeaway is that there is no single "best" way to progress for everyone. Over half the people in this study had a clear preference for one style of progression over the other.
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Don't Dismiss Rep Progression: In the world of strength training, adding weight to the bar is often seen as the only "real" form of progression. This study proves that for a significant portion of the population, progressing by adding reps is not only valid but potentially superior for hypertrophy.
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Find Out What Works for You: How can you know if you're a "rep responder" or a "load responder"? Run a simple experiment. Pick an isolation exercise like bicep curls or leg extensions. For 4-6 weeks, train one arm/leg by trying to add weight each week (e.g., stay at 10 reps, increase the weight). Train the other arm/leg by trying to add reps each week (e.g., keep the weight the same, go from 10 reps to 11, 12, etc.). Keep track of your progress and see which feels more productive.
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Incorporate Both: Since a large group responded well to both methods, a well-rounded, periodized training plan that includes phases of both load progression and rep progression is likely a fantastic strategy for long-term gains. You can have a strength block focused on adding weight, followed by a hypertrophy block focused on adding reps.
Ultimately, progressive overload is the engine of your gains, but this study shows that you have more than one way to step on the gas. Listen to your body, track your progress, and don't be afraid to switch up your progression style. You might just unlock a new level of growth.
Reference: Scarpelli, M.C., Bergamasco, J.G.A., Godwin, J.S. et al. (2025). Individual muscle hypertrophy response is affected by the overload progression model and is associated with changes in satellite cell content. European Journal of Applied Physiology. https://pubmed.ncbi.nlm.nih.gov/40500534/