How do your Muscles Grow?- Scientific Study Part 1

Lifting weights can make you to lose fat, build muscle, or gain strength. It depends on your goals.

You may be lifting to gain strength, build muscle, or lose fat.

Do you know the mechanism of fat loss or muscle building?

Read on:

There are three types of muscles in human body.

  • Cardiac muscles ( heart muscles)
  • Smooth muscles ( covering organs )
  • Skeletal Muscles in simple “Muscles”

An average adult male is made up of 42% of skeletal muscle and female is made up of 36%.

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Skeletal muscles are adaptable to training. You lift weights and your muscles adapt by increasing muscle mass or strength gain or both. 

In this article you can find out

  • The mechanism involved in muscle growth
  • The process which initiate muscle growth
  • The physical and chemical changes while you workout

Your muscle cells will recycle similar to every other cell in your body. Muscle growth starts with the activation on satellite cells. (Satellite cells can provide muscle fibers with extra nucleus during recovery process).

They are not activated during daily activities and are found beneath outer most membrane (sacrolemma) of muscle fiber.

Exercise, tension or overuse can cause muscle injury which will activate them to proliferate and differentiate. They will fuse with the damaged muscle fiber and donate their nucleus.

Your muscle need extra nucleus during muscle growth, because your nucleus content to fiber mass remains constant. In order to increase fiber size you have to add more nucleus.

The whole thing happens as a result of various mechanical and chemical means.

Muscle building process

Muscle growth happens via two mechanisms the first is hypertrophy and other is hyperplasia. Hypertrophy growth is an increase in muscle size which happens due to an increase in muscle fiber size.

Hyperplasia growth is observed with an increase in muscle fiber numbers. (the number of muscle fibers increases)

Hypertrophy

muscle hypertrophy

In hypertrophy the muscle fiber size increases. This increase in muscle fiber size is due to the increase in number of myofibrils (smallest functional unit of muscle fiber) and thus known as myofibrils hypertrophy.

There is another type of hypertrophy that is been pushed through various experts and publications called the sarcoplasmic hypertrophy. There is no scientific proof of this type of hypertrophy observed in humans.

The promoters credit this idea to Russian sports scientist Vladimir Zatsiorsky.

Sarcomere or Myofibrils hypertrophy

sarcomere hypertrophy

Sarcomere hypertrophy or myofibrils hypertrophy cause an increase in myofibrils (the actin and myosin filaments) number inside muscle fibers. This can cause an increase in muscle fiber density.

The myofibril consists of actin and myosin filaments. They are the contractile portion of muscle fibers. The sliding motion of actin over myosin filaments in response to motor neuron activation is responsible for muscle contraction.

The increase in these filament numbers causes an increase in muscle strength and performance.

Added myofibril demands more energy to function. This will add more mitochondria in muscle fiber.

Mitochondria are the power house of a cell. It produces chemical energy in the form of ATP.

Your muscles use this ATP synthase to power them during a workout.

During endurance exercise the size and strength of mitochondria increases.

This helps to produce more ATP in the sarcoplasm, which powers you to do more reps.

Sarcoplasmic hypertrophy VIA BROSCIENCE (myth busted)

sarcoplasmic hypertrophy

Note: Resistance training will cause an increase in your sarcoplasmic volume. But it we can’t call it as another type of hypertrophy.

Sarcoplasmic hypertrophy causes an increase in muscle volume due to an increase in non contractile elements of muscle fiber.

In sarcoplasmic hypertrophy the volume of sarcoplasm (the cytoplasm of muscle cell) increases that in turn increases the total volume of muscle.

The sarcoplasm contains glycosomes (granules of stored glycogen) and significant amounts of myoglobin, an oxygen-binding protein.

Sarcoplasmic hypertrophy does not have a big role in improving your strength as there is no change in your contractile portion(myofibrils) of muscle fiber( contractile portion cause your muscle fiber to contract during lifting)

An increase in sarcoplasm can enhance your endurance performance, as there is more glycogen and myoglobin.

Sarcoplasmic v/s Sarcomere hypertrophy – again myth busted

As you have read above your sarcoplasmic volume will increase e with resistance training.

The magnitude of sarcoplasmic hypertrophy is small. As your muscle fiber is 70- 90% occupied by myofibrillar protein (Macdougall et al, 1982) and the remaining by sarcoplasm, blood vessel, and other non contractile elements.

Think about 10% increase in your sarcoplasm and the total change in muscle fiber will be around 1% (not a math’s geek).

So the changes in sarcoplasm have a less chance to affect the total muscle volume.

An increase in your myofibrils cause an increase in non contractile elements, Due to the increased energy demands (as we discussed earlier on mitochondria), they work hand in hand.

How sarcoplasmic theory got this mileage?

Those who support sarcoplasmic hypertrophy compare bodybuilders and power lifters.

Bodybuilder

Powerlifter

Muscles

Strength

Bodybuilder muscles
powerlifter strength

Bodybuilders have more bulk and less strength than their power lifting counter parts.

Power lifters have more strength than bodybuilders in similar weight category.

How?

Power lifters train in the 1-5 reps with 85% of 1RM. Their motor neuron will be trained for maximum force for small duration.

They have the ability to produce more power per pound of muscle, enables them to lift more.

The next is focus on muscles and choice of exercise:

Bodybuilders focus on muscle growth and add various isolation exercises to hit those muscles, while power lifters care about strength.

Hyperplasia

muscle hyperplasia

Hyperplasia happens when your muscle fiber number increases. The new fiber will be in the same size as originals.

But you will have the same number of muscle fibers you have at birth.

Then how?

There are few evidences that hyperplasia is possible in animals.

This study shows that mechanical overload on muscles increases the muscle fiber number in animals.

Which mechanism:

Satellite cells get activated when you injure a muscle through stretch or lifting. These cells undergo cell division and give rise to immature muscle cells (myoblastic cells).

These immature muscle cells (myoblastic cells) can either fuse with existing muscle fiber or combine with similar cells to form a new muscle fiber.

Hyperplasia in humans is lacking evidence and supporting studies.

Workout related stimulus for muscle growth

We have seen the process by which the muscles actually grow.

But do you know what fires this growth process?

“Resistance training will cause micro tears in muscle fiber, this will ignite the muscle building process that results in hypertrophy”

Let’s examine muscle building as you start lifting in the gym. (The exercise can be bench press)

First step: You start lifting weights.( what happens when you lift weights)

You will start the workout in the gym. Before workout don’t forget to warm up your body and muscles.

bench press muscle growth

Do 2- 3 sets of bench press as warm up. Now add weight that allows you to do only 8 reps.

As you have finished the set increase the weight and do 6-8 reps.

Do 1 or 2 additional sets as per your workout routine.

Finish and move to next exercise.

During this workout your muscles have undergone tension, damage and metabolic stress.

These three factors are the stimuli for muscle growth.

# Let’s look at muscle tension:

muscle tension

How does muscle tension act as a growth promoter?

While workout your muscles will be under mechanical tension. Stretches also contribute to add tension in muscle.

Tension disrupts the integrity of muscle fibers, it cause chemical and mechanical changes in muscle fiber.

The big thing happens during eccentric portion of workout (i.e. when you were lowering weights eg, in bench press).

active passive and total tension

During eccentric portion passive muscular tension develops in your muscles. This happens due to the stretching of collagen content and other elements in your muscle fibers.

This will further increase the tension on the myofibril (smallest unit of muscle fibers), thus multiplying the hypertrophy benefits. 

But don’t increase the tension to a large degree. Some studies have also shown that large muscular tension will induce neural adaptations rather than hypertrophy. 

Do you heard of time under tension?

This time under tension is the total time that a working muscle is under strain during a set.

This study shows that the time under tension is an important factor in optimizing muscle growth.

Your workout must provide enough TUT so that it will get enough stimuli for growth.

Our take:

To increase the tension in muscle you can apply these things.

Slow down during your eccentric movement ( during lowering of weight as in case of barbell press) take more time to lower the weight, that ensure enough tension on your muscles.

Don’t lockout, as you lockout the tension on your muscle reduces. This can reduce the effectiveness of workout.

Keep the workout intensity high so that the muscles will get enough tension. Use weights that are 60-80% of 1 RM.

# Second stimulus for muscle growth: muscle damage

muscle damage

Exercise will cause localized damage to muscle fibers and is one factor that causes the muscle growth. 

The myofibrils in your muscles don’t have uniform strength, this cause in non uniform stretches during workout.

This strength difference will cause non uniform lengthening of myofibrils. This leads the weaker ones to tear.

Workout also causes micro ruptures in your plasma membranes which cause the calcium in blood to leak into muscle cells. http://www.ncbi.nlm.nih.gov/pmc/articles/PMC1748424/figure/i1062-6050-41-4-457-f03/ (figure)

Increased calcium concentration cause an increase in cytokine production which in turn increase neutrophil recruitment.

muscle damage recovery cycle

When a muscle fiber is damaged your body will start its inflammatory response.

Neutrophil will migrate to the affected area and initiates the repair process, neutrophil can cause muscle damage. 

But they are important in muscle repair and recovery, as they make way for macrophages.

The actual mechanisms by which the macrophages are attracted to the damage area are unknown.

The role of macrophages is to remove the damaged muscle fiber. They release toxins which breakdown and removes the damaged muscle fibers.

Macrophages release cytokines and growth factors, of them important in muscle hypertrophy are fibroblast growth factor (FGF), insulin-like growth factor (IGF-1), and transforming growth factor–β1 (TGF-β1).

These growth factors activate fibroblast that secretes collagen and start the repair process. This fibroblast may stop the inflammation process by secretion of PGE 2.

The next stage is satellite cell proliferation and muscle fiber repair, which we will discuss later.

The satellite cell proliferation has been enhanced in the presence of macrophages. 

Thus we have seen the muscle damage and the mechanism by which the debris are removed from muscle fiber.

# The third muscle growth factor: metabolic stress

metabolic stress

The pump you feel while workout shows that your muscles are under metabolic stress.

Don’t feel bad about this metabolic stress, as it is shown to increase your muscle growth. 

This study shows that the metabolic stress will cause an increase in muscle cross sectional area.

What cause metabolic stress?

Metabolic stress is caused by the accumulation of byproducts of various metabolic processes that happens during workout.

How?

When your muscles use anaerobic glycolysis for ATP production, these metabolites are released as by product.

The metabolites are muscle lactate, hydrogen ion, inorganic phosphate, creatine and others.

Is it necessary to feel the pump?

No, the pump you feel during workout is not necessary for muscle growth.

Shows the muscle hypertrophy attained with high intensity training can be achieved with low intensity when trained with blood flow restrictions.

How?

This happens due to increase in metabolic stress as you are restricting blood flow. Further research has to be conducted to optimize this training method.

The growth mechanisms are interrelated, the combined effect of three will activate satellite cells and mTor pathways (communication channel in cells).

Macrophages release various cytokines and growth factors. Let’s look at the growth factors and their role in muscle growth.

The role of Hormones in Muscle growth

Hormones stimulate the cells to divide and grow into mature cells.

When we talk about skeletal muscles the major Hormones are insulin growth factor (IGF), Fibroblast Growth Factor (FGF), Hepatocyte Growth Factor (HGF) , Growth Hormone, and Testosterone.

What do these hormones do?

Insulin Growth Factor (IGF-1):

IGF-1 increases the protein synthesis in muscle cells, and the uptake of amino acids.

IGF-1 activates the cell multiplication and growth of satellite cells.

It enhances the fusion of satellite cells with muscle fiber, which in turn facilitate the transfer of nucleus from satellite cell to muscle fiber. 

IGF-1 will increase slightly after exercise, and High intensity exercise can lead to increased levels. 

Training to failure over longer period of time will increase your endurance, but it at the same time reduce your IGF-1 levels. Non-failure training method will be beneficial for IGF-1 and testosterone.

Fibroblast growth factor (FBF):

Fibroblast Growth Factor plays a critical role in the recruitment of satellite cells into proliferation.

The presence of FBF receptors in satellite cells plays an important role in the activation.

This study shows the function of FBF is related to satellite cell proliferation, muscle hypertrophy and regeneration.

Hepatocyte Growth Factor (HGF)

Hepatocyte Growth Factor activates the satellite cells.

Even mechanical stretch can cause synthesis of NO and which activate HGF leading to satellite cell activation.

Study was conducted using HGF and IGF-1, the preinjection with HGF augments the treatment effect of IGF-1. This is due to the activation of satellite cells by HGF.

Growth Hormone

growth hormone and igf-1

Growth hormone regulates the absorption of amino acids into muscles and aids in fat metabolism.

Growth hormone had been widely used as a performance enhancer drug.

Growth hormone increases the level of circulating IGF-1 in blood. The anabolic effect of growth hormone is due to IGF-1.

But this study shows that GH has independent anabolic effects.

Growth hormone regulates your postnatal growth. Its deficiency can cause reduced growth and dwarfism.

Higher levels of GH will cause gigantism in young and acromegaly in adults characterized by increased bone size and jaw.

Exercise increases the levels of Growth Hormone, and similar to IGF-1 levels. Higher the intensity of exercise higher will be GH levels. 

Testosterone

testosterone-

Testosterone is an anabolic hormone which stimulates your muscle protein synthesis and inhibit protein breakdown.

It stimulates the secretion of growth hormone, thus indirectly promotes the protein turnover (protein synthesis and breakdown).

Testosterone will enhance the activation and replication of satellite cells.More satellite number means hypertrophy.

Your response to resistance training will be limited if you suppress your Testosterone production.

Shows that your testosterone levels will be elevated if you train for long periods (months).

What happens after satellite cell activation?

muscle growth satellite cell

The growth factors and hormones activate the satellite cells. Then it will multiply in number (proliferation) and it transforms to an immature muscle cells then migrate to the damaged site.

The immature muscle cells then fuse with damaged muscle fiber and its nucleus get transferred to muscle fiber.

muscle growth robosome

This nucleus will send out mRNA (messenger RNA) to ribosome. Ribosome will initiate protein synthesis according to the instructions of mRNA.

Ribosome constructs the contractile (actin and myosin) filaments and other structural elements of the muscle fiber from the amino acids.

Muscle breakdown, recovery and growth can take anywhere from 2 to 5 days to complete. It greatly depends on your training intensity and experience.

Muscle growth step by step:

  • You start lifting weights and your muscles will undergo
  • Mechanical tension( the pain you felt, like your muscles are going to rip off)
  • Metabolic stress(the pump)
  • Muscle damage( contractile elements will break(smallest units in muscle fiber))
  • The three growth process will initiate an inflammatory response in damaged fibers
  • Calcium concentration increase in cells and make way for neutrophil and macrophages
  • Macrophages remove the muscle fiber debris and it is believed to release various growth factors and cytokines
  • Satellite cell activation
  • Growth factors and hormones (IGF-1, HGF, FGF, Testosterone, GH) activates the satellite cells
  • Satellite cell then grow and migrate to the damaged area
  • It will then fuse with damaged muscle fiber and donate their nucleus
  • Then nucleus will sent out mRNA(messenger RNA) to ribosome
  • Ribosome synthesize contractile (actin and myosin) and structural elements of muscle fiber from amino acids.

Growth Concludes:

Muscular hypertrophy is a complex multidimensional process. Muscle hypertrophy involves the satellite cell activation, inflammatory response, growth factors, cytokines, and hormones.

The whole process causes the muscles and nerve system to adapt with the training. So it’s advisable to cycle your workout.

We have seen the physiological and chemical changes that cause muscle hypertrophy.

In next article you will read about the relation between training principles and hypertrophy. We will also reveal the how to be in a hypertrophy mode, i.e. maximum muscle growth.

What's your thought on muscle growth? Share it in comments

Image Credits

Arun Kumar T
 

Arun is the founder @workoutable. He is Learning different ways to transform his body from fat to fit. Himself as a crash test dummy, experimenting with his body. He is trying to help people to transform their body.

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