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 are the specific mechanisms involved that actually cause this to happen?

Well, there are three main underlying mechanisms that 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 need to train in a way that will optimize the effects of all of these.

So in this article I’ll give an outline of each 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 whilst heavy low rep training can produce size gains for a while, after a certain point many trainees find that training with near maximal loads, whilst useful for increasing strength, does not produce much in the way of continued gains in size.

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.

Additionally it’s important for tension to be applied through a full range of motion (as appropriate for the exercise concerned of course) in order to get the full benefits.

Metabolic stress

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

Metabolic stress is maximized by using higher reps (>12) and/or 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. Therefore rest periods of 1 – 2 minutes are about ideal when trying to achieve maximum muscle growth.

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 causes these fibers to grow back bigger and stronger.

However it is not true that the amount of muscle damage is directly related 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 aspect of the movement, you can cause increased muscle damage, which can result in improved gains, at least in the short term.

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 (negative) part of the movement when using moderate and lighter loads

You don’t have to do all of these all of the time of course, 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 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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