How our bodies build muscle

Muscles are often taken for granted. Responsible for every move you make, the primary goal of a muscle is to turn energy into motion.


Muscles are broken down into three categories. Skeletal are the type that people in the gym train and what individuals are most commonly aware of. Smooth are the involuntary muscles such as blood vessels, airways and your bladder. The final category is cardiac, the muscles of the heart. It is skeletal muscle, however, that allows humans to both shape their bodies and increase their strength.


Skeletal muscles are complex, designed to contract when asked to perform any action. If you performed a bicep curl, for example, your brain will send a signal to the nerve cells indicating that it’s time for the biceps to engage. It’s the same process for each muscle that’s within the skeletal category, but it’s the way these are constructed that allows us to develop them.


A muscle is made up of fibres – each muscle will boast a higher or lower amount – that fall into two distinct groups: slow twitching (type I fibres) and fast twitching (type II fibres). Type I muscles utilise the oxygen in your body better to generate more fuel, also known as adenosine triphosphate (ATP). They can take extra strain and, more often than not, fatigue slower. Type II muscles, on the other hand, are the opposite. Not needing oxygen to generate fuel, they create spurts of strength and exhaust far quicker. The distinctions are similar to that of a marathon runner and a sprinter, with the former relying on their muscles taking longer to break down and the latter using the intensity and force of the faster twitching fibres to peak quickly.

The muscle mass bodybuilders aim to build in the gym is made up of fast-twitch fibres


It’s these processes that allow us to both manipulate a muscle and make it stronger. Every time you lift a weight you’re tearing these muscle fibres apart, forcing the body to repair them. Once healed, the fibres are thicker than before, a process that can be manipulated with the right diet.


Bodybuilders get protein into their system as soon as possible after a workout, as the substance is broken down into amino acids that are used to produce and repair muscles. Your diet can even influence how effective this is: fast-acting carbohydrates play an important role in spiking insulin levels, which in turn replace muscle glycogen (reserve source of glucose) used during training. Such a process will also filter protein where it’s needed, for maximum recovery and growth.


Bodybuilding powders often contain a blend of protein and carbohydrate to restore energy and repair muscle tissue


This is why muscles get bigger and stronger with rest, and not at the gym where you’re in fact breaking and destroying them.
These principles shift across in terms of how muscles get stronger, too. The notion that lifting heavy weights at a lower rep range will increase body mass, whereas doing the opposite will make the muscle more visible, is a myth.


Instead, when you train with heavy weights and force your muscles to expend all their ATP, you put the body in a state to recruit more muscle fibres and stimulate those that are missed when focusing on lighter weights. You’re essentially teaching your muscles that they can become stronger. It won’t suddenly make them bigger, but it will activate more fibres that in turn help you lift more. This type of training produces a form of muscle hypertrophy, which, in this instance, is increasing the size of your muscle cells. Hypertrophy can be manipulated to both boost muscular strength or simply focus on increasing body mass.


Bodybuilders consume foods that are high in protein and limited in fat content to keep their body fat percentage to a minimum


Your body will also remember how strong you are, even if you stop training. Although you’d have to work back up to your previous level, it would take half the time thanks to muscle memory. Following the same approach as how we remember to perform everyday tasks, your muscles get used to the same movement and adapt accordingly.


Inside your muscle
A muscle contracts when a fibre is kicked into gear as tension is put upon it. Myofibrils, which are found within a fibre, are made up of actin and myosin. When these two threads join the myofibril shortens, or contracts. If you straighten your arm, keeping your palm facing up, and measure the length of your bicep muscle, you’ll find it will become a lot smaller if you curl your hand towards the shoulder.


Click on the image to see a closer view of our muscular components


There are multiple types of contractions, though. Isometric contractions occur when the angle of the joint or length of muscle do not change, for example holding a dumbbell at arm’s length and fighting the resistance. Such exercises are usually adapted to try and increase strength. Isotonic contractions are the opposite and more common, such as traditional weightlifting where, as mentioned, the muscle shortens with contraction. Although this can also be used for strength training, it’s beneficial for expanding muscle size.


This article was written by the How It Works team and first appeared in issue 20 of How It Works.

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