The Mechanisms of Muscle Hypertrophy: What Causes Muscles to Grow?

bodybuilder 7 wcAs we all know, when you lift weights, your body responds to the challenge by increasing the size and strength of the muscles involved in the lift, so that you will be able to do the job more easily next time. But what is it that actually causes this to happen? What are the specific mechanisms involved that produce muscle hypertrophy?

Well, there are three underlying mechanisms that appear to trigger a growth response in skeletal muscle. These are mechanical tension, metabolic stress and muscle damage. So, if you want to achieve maximum muscle growth, you should really have a basic understanding of each of these, so that you’ll be able to structure your training in a way that will optimize the effects of all of them.

So, in this article, I’ll give you an outline of these mechanisms of muscle hypertrophy, and I’ll also explain how best to train in order to get the full benefits of all three of them.

Mechanical Tension

Tension is produced in a muscle when it is put under load, i.e. when you lift a weight. And the heavier the weight is (in relation to your one rep max) the greater the tension will be. So muscle tension is maximized with heavy loads, i.e. those that can only be lifted for 1–5 reps.

However, although mechanical tension is the primary driver of muscle growth, its effect is limited. So, while heavy, low rep training can produce good size gains for a while, after a certain point most trainees find that training with near maximal loads does not produce much in the way of continued gains in size, although it remains very good for increasing strength.

More moderate loads (i.e. those that can be lifted for 6–12 reps) also produce a good level of muscle tension, however, and training in this rep range tends to lead to greater increases in muscle size over the longer term.

However, the heavier the load you can lift in the 6–12 rep range, the better the muscle building response you will get. So training with heavier relative loads (1–5 RM), to facilitate additional strength gains, will help to yield greater benefits when training with more moderate loads, as the actual weight you will be lifting for that rep range will be heavier. And remember that progressive overload (lifting progressively more weight for the same number of reps over time) is the number one requirement for continually increasing muscle size.

Also, it’s important that tension be applied through a full range of motion (as appropriate for the exercise concerned, of course) in order to get the full benefits. Partial reps do not fully engage the target muscles, and lead to sub-optimal results.

Metabolic Stress

When you experience a “pump”, or a burning sensation in the muscle, while training, you are experiencing the effects of metabolic stress. This is caused by the accumulation of metabolites (e.g. lactic acid, H+ ions, inorganic phosphate etc.) in the muscles, and this too is a driver of muscle hypertrophy.

A good amount of metabolic stress is achieved when training with moderate loads, but it is maximized by using higher reps (>12) and/or having shorter rest periods between sets. But if your rest periods are too short (30 seconds or less), you will not recover sufficiently to enable you to use an effective load for all of your sets. Therefore, rest periods of 1–2 minutes are about ideal when trying to achieve maximum muscle growth via this mechanism.

Muscle Damage

The final driver of muscle growth is muscle damage. When you lift weights, it causes small tears in the muscle fibers, and the resulting inflammatory response triggers the repair mechanism, which causes these fibers to grow back bigger and stronger.

However, it is not true that the amount of muscle damage is directly proportional to the amount of muscle growth that occurs, as you can only recover from so much. So if you overdo it, you could easily compromise your gains, or even stop them completely.

But, by lifting to the point of (but not beyond) muscular failure, when using moderate and lighter loads, and also by emphasizing the negative (eccentric) aspect of the movement (i.e. lowering the weight slowly), you can cause increased muscle damage, which may result in improved gains, providing you don’t use these techniques too often.

At this point, it should be noted that metabolic stress and muscle damage do not produce muscle hypertrophy by themselves. But they provide an additive effect, alongside mechanical tension. For example, if you induce muscle damage by trauma or compression of the muscle, that will not result in hypertrophy. In fact, the opposite may occur. But when some degree of muscle damage occurs while lifting a load, the combined effect of tension and damage will result in a greater level of hypertrophy than tension alone would provide.

In Conclusion

Taking into account all of the above points, in order to maximize muscle hypertrophy you should do all of the following:

  • Train with heavier loads (1–5RM), to facilitate maximal strength development
  • Train with moderate loads (6–12RM) most of the time
  • Train with lighter loads (>12RM) to induce higher levels of metabolic stress
  • Keep rest periods at 1–2 minutes when using moderate and lighter loads
  • Use an appropriately full range of motion for each exercise
  • Train to the point of failure occasionally, when using moderate and lighter loads
  • Emphasize the eccentric part of the movement when using moderate and lighter loads
  • Do supra-maximal eccentric-only training occasionally

You don’t have to do all of these all of the time, and in fact it would be counter-productive if you did, but if you make sure you incorporate most of these principles into your workouts most of the time, you will be doing everything you need to be doing, from a training perspective, in order to facilitate maximum gains in muscle size.

So, now you understand the underlying mechanisms of muscle hypertrophy, you will be better able to structure your workouts in a way that will produce the greatest gains in muscle size over time. As always, best of luck in your training.

Photo credit: Арина Варская, CC BY-SA 3.0, via Wikimedia Commons

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