What Actually Causes Muscle Growth — A Mechanistic and Practical Guide for Athletes
Table of Contents
Key Takeaways: What Actually Causes Muscle Growth
- Muscle growth is driven primarily by mechanical tension, not by trying to damage the muscle as much as possible.
- Mechanical tension is converted into cellular signals through mechanotransduction, with proteins and pathways such as mTORC1, FAK, and titin helping translate loading into muscle growth.
- Muscle soreness is not a reliable measure of training quality. A session can be productive even if it does not cause significant DOMS.
- Training close to failure can be useful for hypertrophy, but reaching absolute failure does not appear to provide a meaningful extra benefit over stopping one or two repetitions earlier.
- Strength gains appear to be less dependent on training to failure than muscle growth, meaning athletes may often build strength without taking every set to the limit.
- Loading muscles at longer lengths may provide an additional hypertrophic stimulus, which supports the practical value of full, controlled ranges of motion when appropriate.
- The goal of hypertrophy training is not to destroy the muscle. The goal is to apply enough high-quality mechanical tension, support recovery with nutrition and sleep, and repeat the process consistently.
Introduction: What Actually Causes Muscle Growth
Many athletes grow up with the idea that muscle growth happens because training tears muscle fibres apart, and the body then repairs them bigger and stronger. I heard this explanation throughout my own athletic career, and for a long time I believed it myself. I also still come across the same belief among patients, athletes, and training colleagues: if a workout is not taken close to failure, or if the muscles are not sore the next day, it probably was not effective enough to stimulate meaningful muscle growth.
This way of thinking has shaped strength training culture for decades. It has encouraged many athletes to judge the quality of a session by how exhausted they feel afterwards, rather than by the quality of the training stimulus itself. But does current evidence actually support this traditional model?
Over the past decade, our understanding of muscle hypertrophy has evolved considerably. Current evidence suggests that muscle damage is not the primary signal responsible for muscle growth, and soreness is not a reliable indicator of an effective hypertrophy stimulus. Instead, muscle growth appears to be driven primarily by mechanical tension and the molecular signalling pathways activated in response to loading.
This distinction is more than just an interesting physiological concept. In practice, it has important implications for how athletes should structure their resistance training. If comparable muscle growth can be achieved without routinely training to absolute failure, athletes may be able to accumulate the same hypertrophic stimulus while reducing unnecessary fatigue and preserving recovery for sport-specific training. From a clinical perspective, this is one of the most practically relevant aspects of the current evidence, particularly for athletes balancing strength training alongside technical practice, endurance work, or competition.
In this article, I review what current evidence says about the mechanisms responsible for muscle hypertrophy, why muscle damage is better understood as a consequence rather than the primary cause of muscle growth, and how these findings may influence practical resistance training programming for athletes.
The Old Model: Muscle Growth Through Damage and Repair
The classical framework held that eccentric exercise tears sarcomeres at the Z-disc, triggering an inflammatory cascade, satellite cell activation, and subsequent repair that results in a larger fibre. Delayed-onset muscle soreness (DOMS) became the surrogate marker of a productive session. If you weren’t sore, you hadn’t trained hard enough.
This model is intuitive, and not entirely wrong — muscle damage can accompany some forms of resistance training and may be part of the broader adaptive response. However, current evidence does not support positioning muscle damage as the primary driver of hypertrophy. Mechanical tension is now widely recognized as the primary stimulus for resistance training-induced muscle growth [1].
The repeated bout effect is one practical example of this dissociation. After an initial bout of eccentric exercise, repeating the same or a similar exercise typically produces less muscle damage and fewer symptoms, reflecting the body’s adaptation to the familiar stimulus [10]. In other words, the body can become better adapted to a training stimulus while experiencing less soreness over time.
Stretching-induced hypertrophy in passively loaded muscle provides another experimental example, with animal data suggesting that titin-mediated mechanosensing can contribute to fibre growth without active contraction [5].
Muscle damage is not considered the primary signal for muscle growth — it is a downstream consequence of certain loading conditions.
The most practical takeaway is simple: you do not need to break the muscle down as much as possible in order to make progress. A heavily damaged muscle is often slower to recover, whereas a well-controlled training session that stops slightly short of failure may allow the athlete to train again sooner and with better quality.
In my own experience, this is one of the most useful mindset shifts for athletes. If the same or a very similar hypertrophy stimulus can be achieved with less soreness and less recovery cost, then constantly chasing DOMS starts to make less sense. A session does not have to feel devastating to be productive.
This also matters psychologically. Many athletes feel that a workout only “counts” if it feels brutal. But if the goal is long-term development, a good session can also be one that feels controlled, repeatable, and recoverable. You can leave the gym feeling like you could have done a little more — and still have done enough.
What Actually Drives Muscle Growth: The Mechanistic Picture
Mechanical Tension Is the Primary Stimulus for Muscle Growth
Current evidence indicates that mechanical tension is widely recognized as the primary stimulus underlying the molecular mechanisms that influence muscle growth induced by resistance training [1]. Claims that acute hormonal responses, metabolic stress, cell swelling or “the pump” meaningfully contribute to hypertrophy are not supported by scientific evidence [1].
When a muscle contracts against an external load, mechanical stress is converted into biochemical signals — a process called mechanotransduction [4]. A large body of work shows that a mammalian target of rapamycin complex 1 (mTORC1)-mediated increase of muscle protein synthesis is the key, but not sole, mechanism by which resistance exercise causes muscle hypertrophy [2].
The pump can be misleading. Athletes who chase the pump often also do more total work — more sets, more repetitions, or more training close to fatigue. In that case, the benefit may come from the accumulated mechanical work, not from the pump itself.
The same applies to failure training. It may feel more effective because more work is being done, but the key question is not whether the muscle feels swollen or exhausted. The better question is whether enough high-quality work was completed — and whether the athlete can recover from it.
The mTORC1 Pathway: The Central Hub for Muscle Growth
mTORC1 is a central regulator of muscle protein synthesis in response to resistance training. In animal models, muscle hypertrophy and protein synthesis were accelerated when mTORC1 was activated by the upstreaming regulator protein kinase B (Akt), whereas rapamycin-inhibited mTORC1 blunted mechanical overload-induced hypertrophy in mice [3]. After activation by resistance exercise or amino acid ingestion, mTORC1 can localize with lysosomes and move toward the cell membrane, enhancing protein translation and accretion in skeletal muscle [3].
The hypertrophic response to mechanical load involves mechanotransduction, a process where mechanosensors convert musculoskeletal stress from mechanical loading into chemical signals that activate intracellular anabolic and catabolic pathways, ultimately leading to the enlargement of myofiber [4]. A key mediator of this load-induced mechanotransduction is focal adhesion kinase (FAK), a nonreceptor kinase that transduces skeletal muscle stress into signals transmitted across the cytoplasmic membrane, activating cell growth pathways [4].
IGF-1 and testosterone contribute to the anabolic environment that supports mTORC1 activity, but mechanical loading is considered the primary upstream stimulus for resistance exercise-induced muscle hypertrophy.
In practical terms: mechanical load → FAK and other mechanosensors → mTORC1 activation → increased muscle protein synthesis → myofibre growth. This cascade is active regardless of whether significant muscle damage occurs.
Titin: A Stretch Sensor Relevant to Muscle Growth
One of the most significant mechanistic advances of recent years concerns titin — a giant elastic protein that spans the length of the sarcomere and functions as a mechanosensor in its own right. This is directly relevant to the emerging evidence on loading at long muscle lengths and isometric training.
Titin is an elastic sarcomeric filament that has been proposed to play a key role in mechanosensing and trophicity of muscle [5]. In a mouse model using unilateral diaphragm denervation (UDD) — where the denervated hemidiaphragm is passively stretched by the contralateral innervated side — the denervated hemidiaphragm mass increased 48 ± 3% after 6 days of UDD, due to the addition of both sarcomeres in series and in parallel [5]. Crucially, muscle growth scaled with titin stiffness: mice with reduced titin stiffness showed a 20 ± 6% attenuated hypertrophy response, whereas mice with increased titin stiffness showed an 18 ± 8% exaggerated response [5]. The conclusion from this animal model: titin functions as a mechanosensor that regulates muscle trophicity [5].
These findings suggest that passive mechanical stretch itself may provide a hypertrophic stimulus through titin-mediated mechanosensing, without requiring active contraction, eccentric loading, or muscle damage. Whether this mechanism operates similarly in exercising humans remains an active area of research.
Although these findings are mechanistically fascinating, they should be interpreted with caution when applied to resistance training in humans. The primary stimulus for muscle hypertrophy still appears to be mechanical tension generated during appropriately loaded resistance exercise, not passive stretching alone.
In my view, the practical value of the titin research is not that athletes should replace strength training with stretching. Rather, it provides another piece of evidence that muscle damage itself is not essential for hypertrophy. Stretch-induced mechanosensing helps explain how muscles can respond to mechanical loading without extensive tissue disruption, but it does not change the fundamental role of progressive resistance training as the cornerstone of muscle growth.
The Muscle Damage Myth: What It Means for Muscle Growth in Athletes
Training to Failure and Muscle Growth: What the Evidence Shows
If mechanical tension — rather than muscle damage — is the primary stimulus for muscle growth, then soreness should not be treated as a reliable measure of training quality. In practice, DOMS often reflects how novel, unfamiliar, or disruptive a session was to the muscle, rather than proving that the session produced a superior hypertrophic stimulus.
This is where the training to failure debate becomes practically important for athletes. If soreness and exhaustion are not reliable markers of growth, then the key question is not how much discomfort a session produces, but whether taking sets all the way to failure actually improves hypertrophy enough to justify the additional recovery cost.
A systematic review and meta-analysis of 15 studies found no evidence to support that resistance training performed to momentary muscular failure is superior to non-failure resistance training for muscle hypertrophy, with a trivial standardised effect size of 0.12 (95% CI −0.13, 0.37, p=0.343) for the momentary failure subgroup [6].
A subsequent 8-week randomised controlled trial in resistance-trained males and females found that increases in quadriceps thickness were similar for FAIL [0.181 cm (HDI: 0.119 to 0.243)] and RIR [0.182 cm (HDI: 0.115 to 0.247)] conditions [7]. The practical implication: stopping 1–2 repetitions short of failure produces equivalent muscle growth to grinding out the final rep.
Where proximity to failure does matter is in the dose-response relationship. A series of meta-regressions found that the dose-response relationship between proximity to failure and strength gain appears to differ from the relationship with muscle hypertrophy, with only the latter being meaningfully influenced by RIR, while strength gains were similar across a wide range of RIR — though the authors note this analysis is exploratory in nature and exact relationships remain to be clarified [8]. Training somewhat close to failure (approximately 1–3 repetitions in reserve) appears to provide a muscle growth advantage over training with large reserves — but reaching absolute failure provides no additional benefit over stopping 1–2 reps short.
For athletes, this distinction matters. Performing resistance training to momentary muscular failure consistently induces higher levels of acute neuromuscular fatigue versus performing sets with 1- to 2-RIR [7]. Routine failure training risks overreaching in athletes carrying concurrent technical and physical training loads. Leaving 1–3 repetitions in reserve preserves the muscle growth stimulus while limiting fatigue accumulation.
From a practical perspective, the takeaway is straightforward. If the goal is muscle hypertrophy, training reasonably close to failure appears to be sufficient, while pushing every set to absolute failure is unlikely to provide meaningful additional benefit. Strength, however, seems to behave differently, with current evidence suggesting that comparable strength gains can often be achieved without training to failure [8].
For athletes, this may provide greater flexibility in programming. Sets can often be stopped earlier to reduce fatigue, provided that sufficient training volume and intensity are maintained over time. The key is not simply staying further away from failure, but ensuring that enough high-quality work is still accumulated to drive adaptation.
The Long Muscle Length Advantage for Muscle Growth
If mechanical tension is the primary signal for muscle growth and titin functions as a stretch sensor, it follows that training at longer muscle lengths — where titin is under greater passive stretch — may produce a stronger hypertrophic stimulus. The emerging evidence supports this.
A systematic review and meta-analysis found that resistance exercise performed at longer muscle length produced significantly greater muscle growth compared to shorter length (ES = 0.283; 95% CI 0.04–0.52; p = 0.036) [9]. This finding covered multiple muscle groups including the vastus lateralis, rectus femoris, vastus medialis, vastus intermedius, gastrocnemius, and biceps brachii [9].
Practically, this means exercise selection and range of motion matter for muscle growth in a specific way: prioritise the stretched position. Exercises that load muscles at their longest point — incline curls for the biceps, Romanian deadlifts for the hamstrings, deep squats for the quadriceps — provide a mechanical environment suited to driving the mechanosensing cascades described above.
In my own training experience, exercises performed through a larger range of motion often feel more demanding. A deep squat, a full-range split squat, or a Romanian deadlift that loads the hamstrings in a lengthened position creates a very different training stimulus than a shortened-range version of the same movement.
The titin findings are interesting because they offer a possible physiological explanation for something many athletes may already recognise in practice: the stretched position can feel like more than just “extra range.” Mechanical tension remains the foundation, but loading a muscle at longer lengths may also have unique relevance for hypertrophy through stretch-related mechanosensing.
Practical Recommendations for Muscle Growth in Athletes
The mechanistic shift from the damage model to the mechanical tension model translates into several concrete training principles:
Train with intention, not destruction. The goal of a muscle growth session is to accumulate sufficient mechanical tension through controlled loading — not to generate maximal soreness or fatigue. Quality of stimulus matters more than quantity of discomfort.
Leave 1–3 repetitions in reserve on most sets. Non-failure training produces equivalent muscle growth to failure training when volume is equated [6][7]. For athletes managing concurrent technical and physical training loads, this is particularly relevant — failure training consistently induces higher acute neuromuscular fatigue than stopping 1–2 reps short [7].
Prioritise range of motion and the lengthened position. Resistance exercise performed at longer muscle length produced significantly greater muscle growth [9]. Wherever structurally appropriate, favour exercises and joint angles that load the muscle in its stretched position.
Understand that soreness is not a reliable measure of muscle growth quality. If mechanical tension is the primary stimulus for hypertrophy, then DOMS should not be treated as proof that a session was more productive [1]. A session that produces little or no soreness can still provide a meaningful hypertrophic stimulus if sufficient mechanical tension and training volume are present. Mechanistically, resistance exercise-induced hypertrophy is largely mediated through anabolic signalling pathways such as mTORC1 [2].
Support the nutritional environment. Resistance training provides the mechanical stimulus, but adaptation also depends on adequate recovery and nutrient availability. Protein intake is especially relevant for supporting muscle protein synthesis, while creatine can help support repeated high-effort work through the phosphocreatine system.
Prioritise sleep and recovery. For many athletes, the limiting factor is not willpower. In my experience, motivated athletes can often train hard enough to reach the limits of their recovery capacity, and if this continues unchecked, they may drift toward overreaching. Sleep is one of the simplest recovery variables an athlete can influence. When sleep is consistently neglected, the athlete may have less capacity to adapt to strength training and other training stressors. When sleep is protected, it can support the recovery environment needed for productive training adaptation.
Conclusion: What Actually Causes Muscle Growth
Muscle growth is not simply a process of damaging muscle fibres and waiting for the body to repair them bigger. Current evidence points toward a more useful model: mechanical tension is the primary stimulus, and muscle damage is better understood as a possible consequence of certain types of loading rather than the main driver of hypertrophy.
For athletes, this distinction matters in practice. A productive training session does not need to leave the muscles severely sore, and every set does not need to be taken to absolute failure. Training sufficiently close to failure, using appropriate volume, and loading muscles through meaningful ranges of motion can provide a strong hypertrophic stimulus while limiting unnecessary fatigue.
In my view, this is one of the most important mindset shifts in strength training. The goal is not to destroy the muscle. The goal is to apply enough high-quality mechanical tension, recover from it, and repeat that process consistently over time. When training is controlled, repeatable, and recoverable, athletes can often make progress without constantly chasing soreness, exhaustion, or the feeling that every session must be brutal.
Bibliography
[1] https://doi.org/10.1016/j.jshs.2025.101104
[2] https://pubmed.ncbi.nlm.nih.gov/30335577/
[3] https://www.ncbi.nlm.nih.gov/pmc/articles/PMC12426163/
[4] https://doi.org/10.1152/ajpcell.00266.2024
[5] https://pubmed.ncbi.nlm.nih.gov/29978560/
[6] https://doi.org/10.1007/s40279-022-01784-y
[7] https://doi.org/10.1080/02640414.2024.2321021
[8] https://pubmed.ncbi.nlm.nih.gov/38970765/
