Does Muscle Size Actually Explain Your Strength Gains?

Muscle size is a major but incomplete driver of strength: neural and architectural factors account for a substantial share, and exercise-induced changes in muscle size do not appear to explain exercise-induced changes in strength. Muscle size and strength are related, but they are not the same thing — and how we interpret that relationship has significant consequences for reading strength training research. A review by Loenneke, Buckner, Dankel, and Abe argued that the increases in muscle size seen after resistance training do not contribute meaningfully to the increases in strength observed over the same period. For lifters trying to make sense of hypertrophy and strength research, this matters.

Key Finding

Strength and muscle size often change at different rates following resistance training, depending on the loading paradigm used. The central argument is that when muscle growth and strength are tracked together, the changes in size do not statistically account for the changes in strength — implying that other factors are doing most of the work when strength improves with training.

In plain terms: growing a muscle and getting stronger are overlapping but distinct processes, and assuming one directly causes the other can lead you to the wrong conclusion.

Study Details

What was studied: The authors examined whether exercise-induced changes in muscle size contribute to exercise-induced changes in muscle strength, synthesizing evidence on how size and strength adaptations relate to one another after resistance training.

Data sources: The argument draws on existing resistance training literature — including the authors' own body of work on muscle size and strength — rather than a single new intervention.

Authors: Jeremy P. Loenneke, Samuel L. Buckner, Scott J. Dankel, and Takashi Abe — a group with an established record of examining the relationship between muscle size and strength and the statistics used to model it.

Methodology: Rather than conducting a new training intervention, this is a review and conceptual analysis of the existing evidence, evaluating whether the correlation between changes in size and changes in strength supports a causal contribution.

Results

The findings are conceptual rather than numerical, but they are practically significant:

  • Changes in muscle size and changes in strength are only weakly linked at the individual level. The magnitude of hypertrophy a person experiences does not reliably predict the magnitude of their strength gain.
  • Neural and intramuscular adaptations — factors like motor unit recruitment, firing rate, and muscle architecture — contribute to strength changes that occur independently of muscle size.
  • Analytical choices matter. Expressing strength relative to muscle size (for example, as a simple ratio) assumes a proportional relationship that often isn't supported by the data, which can distort conclusions about how much growth "explains" strength.

The core message: muscle growth and strength gains should be treated as related but distinct outcomes, and assuming that bigger automatically means proportionally stronger is not well supported.

Limitations

1. No new experimental data. This is a review and conceptual analysis, not an intervention study. It synthesizes existing evidence rather than testing a specific training protocol, so the practical implications require some inference.

2. Complexity of application. The distinctions the authors draw are statistically nuanced and don't always translate cleanly into everyday training decisions.

3. Population and protocol variability. The underlying studies reflect specific populations, training protocols, and measurement methods. General principles hold, but individual responses vary.

What This Means for Your Training

This study won't change your program. But it should change how you read research — and possibly how you evaluate your own progress.

Strength and size are related, but they're not the same metric

It's well established that muscle size and strength don't always change in lockstep. Low-load training and high-load training can produce similar hypertrophy but different strength outcomes. Beginners often see rapid strength increases before meaningful muscle growth occurs. These patterns exist because strength is influenced by neural adaptations, muscle architecture, tendon stiffness, and coordination — not just cross-sectional muscle area.

If you're tracking your training and notice your strength is progressing faster than your visible muscle development (or vice versa), that's not a problem with your program. It reflects the multifactorial nature of strength.

Be cautious about studies that use "strength per unit of muscle size"

You'll encounter research that expresses strength as a ratio — watts per kilogram of lean mass, or force per unit of muscle cross-sectional area. This paper is a direct caution about taking those ratios at face value. If the underlying proportionality assumption isn't tested and confirmed, the ratio may be distorting the picture.

This is particularly relevant when comparing strength between individuals of different body sizes, or when evaluating whether a training intervention "really" improved strength beyond what muscle growth alone would predict.

Track the right things, and track them separately

The practical implication for lifters is straightforward: track strength and muscle development as distinct outcomes, not as a single composite metric. Logging your session performance over time — weights, reps, sets — gives you a direct record of strength progression. Body composition changes are a separate signal.

This is exactly the kind of longitudinal data that makes tracking your training valuable. When you have months of session logs, you can see whether strength is moving with or independently of body composition changes. That separation is meaningful, and it's something Kenso is built to support — your training history is there precisely so you can identify patterns that a single session or a single metric can't reveal.

When reading research, ask about the method

Next time you encounter a study claiming that a particular program "increased strength beyond what muscle growth could explain," it's worth asking: how did they account for muscle size? Did they use a ratio, and did they test whether that ratio was appropriate for their data? The answer changes the interpretation.

This doesn't mean dismissing research — it means reading it with the same rigor the authors of this paper are advocating for.

The Bigger Picture

Strength training research is more methodologically complex than most training content acknowledges. Studies on hypertrophy and strength are only as useful as the analytical tools used to interpret them — and this paper is a reminder that even well-established assumptions carry pitfalls worth examining.

For lifters committed to evidence-based training, the takeaway isn't to distrust research. It's to understand that the relationship between muscle size and strength is genuinely complex, that simple ratios can mislead, and that tracking both variables independently over time gives you better information than any single snapshot metric.

Kenso's rule-based progression engine and AI Coach are built around the same principle: consistent, longitudinal data about your actual training performance is more informative than any one-time measurement. To be clear, Kenso doesn't compute hypertrophy-to-strength normalization or run statistical size-vs-strength analyses — the AI Coach reads your logged training history (weights, reps, sets, RPE, and any Apple Health data you connect) to help you adjust programs. The more complete your training history, the clearer the signal.

Frequently Asked Questions

Does building more muscle always lead to greater strength?

Not always in direct proportion. Muscle size is one contributor to strength, but neural adaptations, motor unit recruitment, muscle architecture, and coordination also play significant roles. Research consistently shows that strength and hypertrophy can change at different rates depending on the training stimulus.

What is ratio normalization in strength research, and why does it matter?

Ratio normalization means dividing a strength measure by a measure of muscle size to get "strength per unit of muscle." This approach assumes a strictly proportional (zero-intercept) relationship between the two variables. When that assumption isn't met, the ratio can produce biased or misleading results, which is why allometric scaling or regression is often preferred.

Should I track muscle size and strength separately in my training log?

Yes. They reflect different adaptations and don't always move together. Logging session performance (weights, reps, sets) gives you a direct record of strength progression, while body composition changes are a separate signal. Tracking both independently over time gives you a clearer picture of how your training is working.

Why might my strength increase faster than my muscle size, or vice versa?

Early strength gains in new lifters are thought to be substantially driven by neural adaptations — your nervous system becoming more efficient at recruiting muscle fibers — before significant muscle growth occurs, though the exact magnitude of the neural contribution is debated (Sale DG. Neural adaptation to resistance training. Med Sci Sports Exerc. 1988;20(5 Suppl):S135-45). Conversely, some training protocols (particularly higher-volume, lower-load work) may produce more hypertrophy relative to maximal strength improvements. Both patterns are common.

How should I interpret hypertrophy and strength studies I read online?

Look for how the study measured and accounted for muscle size when reporting strength outcomes. Be cautious about studies that express strength as a simple ratio without confirming the proportionality assumption. Allometric scaling and regression-based approaches that statistically control for muscle size are generally more robust, though all methods have appropriate use cases.

Citation

Loenneke JP, Buckner SL, Dankel SJ, Abe T. Exercise-Induced Changes in Muscle Size do not Contribute to Exercise-Induced Changes in Muscle Strength. Sports Medicine. 2019;49(7):987-991. doi:10.1007/s40279-019-01106-9