You've seen the posts: influencers swearing by "lengthened partials" and telling you that the deepest part of the stretch is where all the magic happens for muscle growth. The idea is that training a muscle at a longer length—like the bottom of a dumbbell fly or a sissy squat—is the secret key to unlocking new hypertrophy.
But does this popular training theory hold up when we put it under the scientific microscope? A brand new systematic review and meta-analysis from 2025 set out to answer that exact question. Let's break down what the evidence says and what it means for how you train.
A quick note: This article is based on the abstract of the study, as the full text is not yet publicly available.
What Did the Scientists Investigate?
Researchers, including well-known names in the field like Brad Schoenfeld and James Steele, wanted to get a clear picture of how muscle length during resistance training impacts regional muscle growth. In other words, if you train a muscle when it's more stretched out versus more contracted, does one method build more muscle, and does it affect different parts of the muscle differently?
To do this, they conducted a systematic review and Bayesian meta-analysis. Think of a meta-analysis as a "study of studies." They rounded up all the relevant research—12 studies in total—that specifically compared training at longer versus shorter muscle lengths. These studies manipulated muscle length in two main ways:
- Range of Motion (ROM): Comparing partial reps in the stretched portion of an exercise to partial reps in the contracted portion.
- Exercise Selection: Comparing exercises that challenge a muscle at a longer length (e.g., a seated leg curl) versus a shorter length (e.g., a lying leg curl).
The analysis focused on hypertrophy in young adults and looked at growth at three different sites along the muscle: proximal (closer to the body's center), mid-belly, and distal (further from the body's center).
The Verdict: Longer vs. Shorter Lengths for Growth
After pooling all the data, the conclusion was surprisingly clear: the difference in muscle growth between training at longer versus shorter muscle lengths was, in the researchers' words, "trivial."
Across the board, the hypertrophic effects were very similar regardless of which protocol was used. Here’s a quick look at the findings for different muscle regions:
- Proximal (Top Part): Trivial difference in growth.
- Mid-Belly (Middle Part): Trivial difference in growth.
- Distal (Bottom Part): A very slight trend toward more growth with longer-length training, but the effect was tiny and the results were too uncertain to be considered practically meaningful.
Essentially, the data showed that both training styles produce similar gains. The supposed massive advantage of emphasizing the stretched position didn't show up when all the evidence was combined.
The Most Important Caveat
Before you declare lengthened partials a myth and change your whole routine, the researchers highlighted a critical piece of context we need to consider.
When they looked at the studies included in the analysis, the average difference in muscle length between the "long" and "short" training conditions was only about 22%. This is a crucial point. The studies weren't necessarily comparing the absolute most extreme stretch to the most extreme contraction. Instead, they were comparing conditions that were, on average, only moderately different.
This means we should be cautious when interpreting the findings. The results tell us that relatively small differences in mean muscle length during training probably don't matter much for hypertrophy. They don't necessarily tell us what would happen if you compared, for example, the bottom 25% of a squat to the top 25%.
Practical Takeaways for Your Training
So, what does this mean for you in the gym? Here are the key, evidence-based takeaways.
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Don't Obsess Over the Stretch: Based on this comprehensive analysis, there's no need to overhaul your training to exclusively focus on the stretched portion of every lift. The belief that this is a superior, non-negotiable hypertrophy trigger appears to be overblown.
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Full Range of Motion is Still a Great Default: This research doesn't argue for cutting your ROM. A large, controlled range of motion that takes a muscle from a lengthened to a shortened position remains a fundamentally sound principle for muscle growth. This study simply shows that emphasizing one part of that ROM over another may not provide a significant extra benefit.
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Focus on the Big Rocks: Instead of worrying about subtle manipulations of muscle length, your energy is better spent on the things that we know for sure drive growth: consistency, training with high effort, managing your total volume, and applying progressive overload over time.
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Choose Exercises That Work for You: Select exercises that allow you to achieve a deep stretch, feel the target muscle working, and progressively add weight or reps over time without pain. If an exercise that supposedly biases the lengthened position feels awkward or causes joint discomfort, you're likely better off choosing an alternative.
Ultimately, this meta-analysis serves as a great reminder to stick to the fundamentals. While exploring new techniques can be fun, the foundation of getting bigger and stronger remains unchanged. Train hard, be consistent, and focus on proven principles.
Reference:
Varovic D, Wolf M, Schoenfeld BJ, Steele J, Grgic J, Mikulic P. (2025). Does Muscle Length Influence Regional Hypertrophy? A Systematic Review and Meta-Analysis. International journal of sports medicine. https://pubmed.ncbi.nlm.nih.gov/40570881/