Increasing muscle mass is a goal widely pursued by those who resistance train. The way in which our muscles respond to training, and increase in size is complex, yet growing muscle can be surprisingly simple, just not that easy!Around 5 years ago I wrote an article titled “Muscular Hypertrophy – Using the Science of Set-Rep Schemes”, accessible (here). While the majority of the information that I presented in the article is still true according to current scientific beliefs; along with some personal growth as a fitness professional since its publishing, research has also managed to build a bigger and clearer picture as to how muscular hypertrophy occurs, the events that take place leading to increased muscle size, and how certain training variables (sets, repetitions, exercises and so on) can be manipulated in order to achieve the greatest results.


Skeletal muscle hypertrophy can be broken down in to two different “types”; sarcoplasmic and myofibrillar. Sarcoplasmic hypertrophy can be considered an increase in size of the non-contractile elements and fluid within the muscle. It’s sometimes considered “non-functional” hypertrophy as increases in muscle size can be seen without paralleled increases in strength. “Non-functional” should however be considered an incorrect term, as increases in non-contractile components may include an increase in other elements that contribute towards muscle function, such as those related to energy storage. Furthermore chronic adaptations leading to cell swelling may contribute to an increase in size of contractile components. Myofibrillar hypertrophy on the other hand is an increase in size and amounts of the myofibrillar contractile proteins actin and myosin, and number of sarcomeres in parallel.

When using the analogy of a sports car; Myofibrillar hypertrophy could be considered as increasing the size of your engine, whereas sarcoplasmic hypertrophy is more like an increase in the size and efficiency of the air and fuel pipes, valves and pumps surrounding the main engine block.


It is suggested that there are three primary factors that are responsible for initiating the muscle hypertrophic response. Understanding these is of particular importance for anyone looking to add lean mass, as these can be manipulated and targeted through different forms and styles of training.

Mechanical tension

When a muscle passively stretches (stretching the muscle without actively contracting it) this is what’s referred to as passive elastic tension. On the other hand if you place tension on a muscle by isometrically flexing it as hard as possible, this is referred to as active tension. During both cases high amounts of mechanical tension are formed. High mechanical tension appears to be a potent growth stimulus and not only increases hypertrophy but unloads the effects of atrophy – So more muscle growth and less tissue breakdown! Performing some high tension isometric actions as part of your training could therefore be of some use. In addition stretching can likely play an important role in optimizing muscle growth, and should form part of any exercise programme. Taking a slightly holistic approach to training by stretching the targeted muscle between sets of exercise may also be of some use when trying to maximize growth, and in fact a fairly recent review published in the Journal of Strength and Conditioning Research(Here) suggested some efficacy. It should be noted however that research has shown that both isometric contractions and stretching alone will not maximize muscle growth on their own, and dynamic full-range motions are superior.

As well as the amount of tension being produced in the muscle; time under tension (TUT) is also an important factor to consider, as there will undoubtedly be an optimal balance that exists between both. For example, you cannot produce high tension and maximal force within a muscle for a long duration of time, therefore varying between very high tension/force exercises for shorter durations, and moderate tension exercises for moderate durations may be the best approach to consider.

Taking a muscle through its full range of motion and lengthening, or “stretching” it under load has also been shown in the real world time and time again to produce a significant hypertrophic response. For example stretching the latissimus muscles fully at the bottom range of a pull-up, and performing what has sometimes been referred to as a “loaded stretch”, has been shown to trigger a muscle growth response. This isn’t surprising as high amounts of mechanical tension are produced during these actions, as high eccentric force is being produced by the muscle in a lengthened state.

Metabolic stress

A number of studies have shown metabolic stress to play an important role within the hypertrophic response of muscle tissue. Some have even suggested high metabolic stress and metabolite production to be even more important than high mechanical tension. Metabolic stress is a result of exercise that relies on anaerobic glycolysis for energy production, and is typically achieved with the type of training associated with high lactate and hydrogen ion production, increased blood acidity, and the subsequent “burning” sensation experienced. This build-up of metabolic by-products can trigger an anabolic hormonal surge, including growth hormone (hGH) and insulin-like growth factors, among other things. The occlusion of veins by repeated muscle contractions also prevent blood from escaping, and cell swelling to occur, as well as hypoxia or a lack of oxygen supply to the muscles. This is the “pump” that is often experienced, and even chased by many.

When talking about the pump I’m always weirdly reminded of this hilarious Arnold Schwarzenegger interview out of Pumping Iron:

“The greatest feeling you can get in a gym or the most satisfying feeling you can get in the gym is the pump. Let’s say you train your biceps, blood is rushing in to your muscles and that’s what we call the pump. Your muscles get a really tight feeling like your skin is going to explode any minute and it’s really tight and it’s like someone is blowing air into your muscle and it just blows up and it feels different, it feels fantastic! It’s as satisfying to me as cumming is, you know, as in having sex with a woman and cumming. So you can believe how much I am in heaven? I am like getting the feeling of cumming in the gym; I’m getting the feeling of cumming at home; I’m getting the feeling of cumming backstage; when I pump up, when I pose out in front of 5000 people I get the same feeling, so I am cumming day and night. It’s terrific, right? So you know, I am in heaven.”

– Arnold Schwarzenegger on “the pump”!!!

Simply, any training focused on producing metabolic stress and achieving a pump will trigger a potent anabolic hormonal surge and activate some of the cellular mechanisms associated with muscle hypertrophy.

Muscle damage

Most of us are aware that muscle damage, or tissue breakdown occurs during resistance training. It is theorized that localized damage to muscle tissue can activate a hypertrophic response. In short the response to localised damage, or myotrauma, is one that can be compared to muscle injury and the inflammatory response to infection. This response is believed to release various growth factors that may ultimately lead to hypertrophy, providing muscle protein synthesis exceeds degradation or tissue breakdown during the repair phase. A greater rate of tissue breakdown is achieved at higher intensities (i.e., the higher the %1RM the greater the rate of tissue breakdown), however time under tension can be very short at these intensities, as sets of exercise do not last very long. Therefore although the rate of tissue breakdown is high, the duration is very short. It has therefore been suggested that an optimal balance between both intensity and time under tension is achieved around 6-10 repetitions per set. Furthermore accentuating the eccentric component of the exercise, performing a stretch of the muscle while it is being activated, or overloading the eccentric portion of the exercise (i.e., using a greater eccentric load versus that used during the concentric portion of the exercise), can form high amounts of mechanical tension and tissue breakdown.


It’s clear to see that there’s an obvious crossover between mechanisms, and which of those are being activated at any given time of exercise, or during a resistance training session. So to help better understand what type of training can be considered as a tool for targeting specific mechanisms to a greater degree, I’ve constructed a table of some commonly used training styles and techniques to better describe each of their merits in a hypertrophy programme:

Training Method Mechanical Tension Metabolic Stress
High Intensity (≥85% 1RM) HIGH LOW HIGH
Mod Intensity (70-80% 1RM) MODERATE HIGH MODERATE
Mod/Low Intensity (≤65% 1RM) LOW HIGH LOW
Eccentric Training HIGH LOW HIGH
Isometric Training HIGH Higher for longer duration isometrics HIGH
Concentric Training LOW HIGH LOW
Targeted Inter-set Stretching (e.g., Fascia Stretch Training) HIGH MODERATE LOW
“Loaded stretch” Training HIGH Depends on duration of set HIGH
Emphasising the “Peak Contraction” Depends on the intensity/load HIGH Depends on the intensity/load
Slow Repetition Training (Long Time Under Tension) LOW HIGH LOW
Explosive Training HIGH LOW HIGH
Intensity Techniques (e.g., Drop Sets, Back-off Sets) Higher with high intensity/load methods HIGH Higher with high intensity/load methods



In summary, as mentioned at the beginning; the way in which our muscles respond to training, and increase in size are complex, yet growing muscle can be surprisingly simple, just not that easy! A complete hypertrophy programme should consist of the following components, but it’s up to you to take action to turn knowledge in to results:


  • Perform full range of motion exercises.
  • Perform some isometric exercises.
  • Stretch, before/during/after training.
  • Include some exercises that emphasize the “loaded stretch”.
  • Include some exercises that focus on the peak contraction or “squeeze”.
  • Perform some high intensity/load exercises (4-8 sets of 3-6 reps with moderate to high rest intervals).
  • Perform some moderate intensity/load exercises, with the odd foray in to higher repetition/time under tension exercises towards the end of the session (3-5 sets of 8-12 reps with moderate rest intervals, and on occasion 2-3 sets of 15-25 reps with shorter rest intervals).
  • Consider using a slower repetition speed when trying to achieve more time under tension, and metabolic stress.
  • Employ some special intensity techniques such as drop sets, back-off sets, 1 ½ repetition sets, 21’s, 28’s, mechanical drop sets, pre-fatigue supersets, post-fatigue supersets, and so on to hit a variety of mechanisms.
  • You can also structure training so that either all of these mechanisms are targeted within the same training session or cycle, of periodise them so that, for example for the first 4 weeks muscle damage and mechanical tension are targeted with a higher intensity of training, heavier weights and longer rest intervals, and then the next 4 weeks the focus can be on producing a high amount of metabolic stress with a mixture of moderate and moderate-light weights, longer TUT and slightly shorter rest intervals, and more training focused on achieving a muscle “pump”.



Gareth Sapstead (MSc CSCS) is a Strength and Conditioning Coach, sports performance and conditioning coach sports performance and conditioning coach and one of the UK’s leading Personal trainers.

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