What Triggers Muscle
Growth? A Simple Explanation
of Muscle Protein Synthesis

Key Takeaways

• Muscle gain comes from net protein balance over time—when muscle protein synthesis (MPS) repeatedly outpaces muscle protein breakdown (MPB).¹
• Resistance training can increase MPS on its own.²
• Free-form EAAs can stimulate MPS without exercise.²
• Protein sources can produce different anabolic responses because they digest differently and create different blood EAA patterns (how fast and how high EAAs rise).²
• On training days, amino acids + carbohydrate taken before lifting produced a greater net anabolic response than the same dose taken after.³
• Small EAA doses have stimulated MPS in some studies, sometimes matching larger whey doses in that study context.⁵ ⁶
• EAAs helped preserve lean tissue during a 28-day bed rest (disuse) model.⁴
• During a calorie deficit, adding extra EAAs to whey improved post-exercise whole-body protein balance vs whey alone or a mixed meal.⁷

Overview

Muscle growth happens when the body's muscle-building processes outpace its muscle breakdown processes. The central driver is muscle protein synthesis (MPS)—the process by which amino acids are used to build new muscle proteins.¹

Muscle proteins are constantly being turned over: some are being built (MPS), and some are being broken down, a process known as muscle protein breakdown (MPB). What matters for muscle gain is the net balance between the two.¹

Protein turnover at a glance:

 

Two inputs can directly stimulate this system:

• Resistance training (mechanical tension)¹
• Essential amino acids (EAAs) from protein or supplements (nutritional stimulus)²

Resistance training by itself increases MPS by turning on the muscle-building machinery.¹

Free-form essential amino acids (EAAs) by themselves can also stimulate MPS (and whole-body protein synthesis) because a strong rise in circulating EAAs is an anabolic signal—not just raw material.²

When resistance training is combined with EAAs, both mechanisms contribute: exercise “sensitizes” muscle and increases delivery/uptake, and the EAA rise provides the key building blocks and signal—often producing a larger overall anabolic response and improving net protein balance compared to either stimulus alone.² ³

Resistance Training as a Trigger

Resistance exercise is a potent, reliable trigger for MPS. Acute training generates mechanical tension and small-scale damage in fibers; this mechanotransduction activates anabolic signaling and supports muscle repair.²

As a result, MPS rises after training and can remain elevated for many hours (often up to ~24–48 hours depending on the person and training stimulus).²

Two key concepts:

• Mechanical loading: placing sufficient tension on muscle (through load, range of motion, and time under tension) to activate anabolic signaling.
• Progressive overload: gradually increasing training stress (load, sets, reps, or density) to keep the stimulus above your current capacity.

Training variables that influence the MPS response:

• Exercise intensity (relative load)
• Total volume (sets × reps × load)
• Degree of muscle damage and tension (including controlled eccentrics)
• Progressive resistance over weeks and months

Dietary Protein and Amino Acid Supplements as a Trigger

Dietary protein and EAA supplements support muscle building because they supply EAAs—the nine amino acids your body can’t make and must get from food—which are required to build new muscle proteins. If one or more EAAs are missing (or too low), the body can’t build muscle protein as effectively.²

Why do different proteins build muscle differently?

Different protein sources digest and absorb at different rates, which creates different blood EAA patterns (how fast and how high EAAs rise). This is why different protein sources can produce different anabolic responses: they digest differently and create different blood EAA patterns.²

High-quality protein sources and their EAA completeness:

Sources that produce a faster and higher rise in circulating EAAs tend to produce a stronger anabolic response.²

Essential Amino Acid Supplements and Muscle Protein Synthesis

Free-form EAA supplements can stimulate MPS and whole-body protein synthesis on their own because they deliver the full set of indispensable amino acids in a form that can raise circulating EAAs efficiently.²

A consistent finding in scientific literature is that the anabolic response is closely tied to the magnitude and timing of the rise in circulating EAAs.² In plain terms: how high EAAs rise in the blood (and how quickly) helps predict how strongly protein synthesis responds.²

This is what makes free-form EAAs uniquely effective: they deliver all essential amino acids in a form that can create a strong EAA signal without requiring a large meal or slow digestion.² That combination—complete amino acid profile plus an efficient rise in circulating EAAs—helps explain why free-form EAAs can stimulate MPS on their own and why they can complement training, protein meals, or periods when food intake is lower.²

Do you need 10–15 g of EAAs for results?

EAA supplementation does not require one universal “minimum gram dose” to be effective. Different doses and formulations can stimulate MPS, and several studies show meaningful effects from small EAA doses in the right context:

In older women, a leucine-enriched EAA bolus produced a robust MPS response at rest and after exercise, comparable to (and in parts of the response trending higher than) a larger whey dose in that study context.⁵

In young females after exercise, 1.5 g EAAs produced post-exercise myofibrillar protein synthesis rates that did not differ from 15 g or 20 g of whey protein in that study context.⁶

Low-dose takeaway: In some contexts, smaller EAA doses have produced robust MPS responses.⁵ ⁶ A consistent theme is that the EAA signal (a complete EAA profile and a strong rise in blood EAAs) helps explain why results can differ by source and dose.²

Real-world proof point

In a 28-day bed rest study (a NASA-relevant disuse model), EAA + carbohydrate supplementation helped preserve lean tissue and reduce muscle loss compared to control. This supports the idea that EAAs can act as a powerful anabolic lever even when training isn’t happening.⁴

EAAs: Best Time to Take Them (Daily, Rest Days, Pre vs Post Workout)

Essential amino acids can be beneficial with or without exercise. Free-form EAAs can stimulate MPS on their own, and they can also support muscle maintenance during inactivity.² ⁴ On training days, timing can influence the size of the response—especially when EAAs are taken close to the workout.³

A common oversimplification is “protein after workouts is the ideal time.” The EAA timing research shows something more specific: In a controlled study comparing EAA + carbohydrate taken immediately before vs immediately after resistance exercise, the pre-exercise condition produced a greater net anabolic response (greater net phenylalanine uptake across the leg), largely because amino acid delivery to the muscle was higher when blood flow was elevated during exercise.³

Practical translation:

• EAAs can be a useful daily supplement because exercise is not required for them to stimulate protein synthesis, and they can be helpful during periods of reduced activity or inactivity.² ⁴
• On training days, taking free-form EAAs before training can be a high-leverage strategy. Because free-form aminos require no digestion, they hit the bloodstream rapidly; taking them pre-workout allows the increased blood flow from exercise to 'pump' those building blocks directly into the working muscle exactly when the muscle is most sensitive to the signal. This is especially effective if it has been 3+ hours since your last whole-food protein meal.³
• Post-workout timing can still be useful when it is the first opportunity to consume amino acids—for example, after fasted training or when a protein-containing meal will be delayed after training.³

Beyond specific timing, the bigger lever is repetition: getting complete EAAs into circulation at multiple points during the day so muscle receives repeated anabolic “pulses” across the week.²

In practice, that usually looks like protein-containing meals spaced at regular intervals (often about every 3 hours while awake), with free-form EAAs used between meals when helpful (for example, when meal protein is lower or food isn’t practical).⁸ ⁹

Special Considerations for Muscle Growth

In some situations, maintaining or building muscle is harder—such as reduced training or low activity, calorie deficit, plant-forward eating patterns, or age-related anabolic resistance.¹ ² In these cases, reliably creating a strong EAA signal becomes more important, and free-form EAA supplements are often the most direct and practical way to do it (especially when appetite, food volume, or meal timing make whole-food protein harder to execute consistently).² ⁴ ⁷

Reduced training (injury downtime / deload / travel / low activity)

When training volume drops (travel, deloads, minor injury downtime) or activity is reduced, the risk shifts toward muscle loss because breakdown can outpace building more easily.¹ In these periods, creating regular EAA “pulses” can be especially valuable because EAAs can stimulate protein synthesis even without exercise.²

In a 28-day bed rest study (a strong model of extreme inactivity), providing EAAs with carbohydrate helped preserve lean tissue and reduce muscle loss compared to control.⁴

While most people won’t experience full bed rest, the practical takeaway is that the less you’re able to train or move, the more important targeted amino acid nutrition becomes for maintaining muscle.¹⁴

Energy deficit (cutting / dieting / calorie deficit)

During a calorie deficit, maintaining muscle can be harder due to overall reduced caloric intake, thus the details of EAA availability can matter more. In one study, adding extra EAAs to a whey protein drink improved post-exercise whole-body protein balance during a calorie deficit more than whey alone or a mixed meal.⁷ This suggests that during a 'cut,' a high-potency EAA signal helps ensure your body has enough essential building blocks to support both muscle repair and the protein needs of your vital organs, preventing the body from 'robbing' muscle tissue to fuel other processes.⁷

Plant-forward / plant-only diets

Plant-based eating can support muscle growth, but it often requires more intention because many single-plant proteins are lower in one or more EAAs.² The practical fix is to build meals around complete EAA profiles, either by choosing plant proteins that are complete (e.g., soy) or by combining complementary sources (e.g., legume + grain) so the total meal provides all EAAs.²

Free-form EAAs can serve as an “insurance policy” on plant-forward days: they provide a complete EAA profile in a small serving and can help create a strong circulating EAA signal when a meal is lower in certain EAAs.²

Age-related anabolic resistance

With aging, muscle often becomes less responsive to the same meal or supplement dose (anabolic resistance).² This does not mean muscle cannot be built; it indicates that the nutritional “signal” often needs to be stronger and more consistent.

Practical implications:

• Prioritize high-quality, complete EAA intake at each protein feeding (from complete proteins and/or free-form EAAs).²
• Focus on the blood EAA response (how high and how quickly EAAs rise), since that rise is closely tied to the anabolic effect.²
• Consider the evidence on leucine-enriched EAA mixtures as one strategy to strengthen the overall EAA signal in older adults.² ⁵

Practical Guidelines to Maximize Muscle Protein Synthesis

• Train consistently with progressive resistance (the mechanical stimulus).¹
• Prioritize complete EAA intake (from high-quality protein and/or free-form EAAs).²
• If using free-form EAAs around training, consider pre-exercise timing to take advantage of increased blood flow and delivery.³
• In older adults (or anyone struggling with responsiveness), consider the evidence around leucine-enriched EAA mixtures as a strategy to strengthen the anabolic signal.²
• During energy deficit, nutritional strategies that increase EAA availability can improve post-exercise whole-body protein balance.⁷

Frequently Asked Questions

Can EAAs build muscle without exercise?

Free-form EAAs can stimulate MPS even without exercise because a strong rise in circulating EAAs is an anabolic signal—not just raw material.²

What role do EAAs play in muscle building?

EAAs are the indispensable building blocks for new muscle proteins, and free-form EAAs can act as a direct nutritional stimulus for muscle and whole-body protein synthesis when they produce a strong EAA signal in the blood.²

How does resistance training stimulate muscle growth?

Resistance training provides the mechanical signal that increases muscle remodeling and can elevate muscle protein synthesis.¹ ²

Is it better to take amino acids before or after a workout?

In one controlled study, amino acids + carbohydrate taken immediately before resistance exercise produced a greater net anabolic response than the same dose taken immediately after.³

Why do different proteins build muscle differently?

Different proteins create different blood EAA patterns (how fast and how high EAAs rise), which helps explain differences in anabolic responses.²

Do you need large EAA doses (10–15 g) to stimulate MPS?

Not always. Multiple studies show meaningful MPS responses with much smaller EAA doses in certain contexts (including comparisons against larger whey doses).⁵⁶ The blood EAA response and EAA composition help explain why.²

Scientific Research

1. Smith GI, Patterson BW, Mittendorfer B. Human muscle protein turnover—why is it so variable? J Appl Physiol (1985). 2011;110(2):480–491. doi:10.1152/japplphysiol.00125.2010.
2. Church DD, Hirsch KR, Park S, Kim I-Y, Gwin JA, Pasiakos SM, Wolfe RR, Ferrando AA. Essential Amino Acids and Protein Synthesis: Insights into Maximizing the Muscle and Whole-Body Response to Feeding. Nutrients. 2020;12(12):3717. doi:10.3390/nu12123717.
3. Tipton KD, Rasmussen BB, Miller SL, Wolf SE, Owens-Stovall SK, Petrini BE, Wolfe RR. Timing of amino acid-carbohydrate ingestion alters anabolic response of muscle to resistance exercise. Am J Physiol Endocrinol Metab. 2001;281(2):E197–E206. doi:10.1152/ajpendo.2001.281.2.E197.
4. Paddon-Jones D, Sheffield-Moore M, Urban RJ, Sanford AP, Aarsland A, Wolfe RR, Ferrando AA. Essential amino acid and carbohydrate supplementation ameliorates muscle protein loss in humans during 28 days bed rest. J Clin Endocrinol Metab. 2004.
5. Wilkinson DJ, Bukhari SSI, Phillips BE, et al. Effects of leucine-enriched essential amino acid and whey protein bolus dosing upon skeletal muscle protein synthesis at rest and after exercise in older women. Clin Nutr. 2018;37(6 Pt A):2011–2021. doi:10.1016/j.clnu.2017.09.008.
6. Apicella MCA, Jameson TSO, Monteyne AJ, et al. Post-exercise myofibrillar protein synthesis rates do not differ following 1.5 g essential amino acids compared to 15 and 20 g of whey protein in young females. Am J Physiol Endocrinol Metab. 2025;328(3):E420–E434. doi:10.1152/ajpendo.00365.2024.
7. Gwin JA, Church DD, Hatch-McChesney A, et al. Essential amino acid-enriched whey enhances post-exercise whole-body protein balance during energy deficit more than iso-nitrogenous whey or a mixed-macronutrient meal: a randomized, crossover study. J Int Soc Sports Nutr. 2021;18:4. doi:10.1186/s12970-020-00401-5.
8. Areta JL, Burke LM, Ross ML, et al. Timing and distribution of protein ingestion during prolonged recovery from resistance exercise alters myofibrillar protein synthesis. J Physiol. 2013;591(9):2319–2331. doi:10.1113/jphysiol.2012.244897.
9. Moore DR, Areta J, Coffey VG, et al. Daytime pattern of post-exercise protein intake affects whole-body protein turnover in resistance-trained males. Nutr Metab (Lond). 2012;9:91. doi:10.1186/1743-7075-9-91.