The Muscular System
Your muscular system is the network of approximately 640 muscles that moves every part of your body. Muscles pull on your bones to make you walk, run, smile, blink and lift things. Other muscles work without you thinking about them, pumping your heart, pushing food through your gut, and squeezing your blood vessels. There are three different kinds of muscle in your body, each doing a different job, but all working in the same basic way: by contracting (shortening) when their cells receive an electrical signal.
- Number of musclesapprox. 640Plus three special heart muscles
- Three typesSkeletal, smooth, cardiacVoluntary, involuntary, heart
- Largest muscleGluteus maximusThe big muscle in your bottom
- Strongest by weightMasseter (jaw)Can bite with 90+ kg of force
- Hardest-workingThe heartBeats approx. 100,000 times a day
- % of body massapprox. 40%In an adult man
The three types of muscle
- Skeletal muscle: attached to bones and moves them. Under your conscious control (voluntary). Examples: biceps, quadriceps, calf muscles. Looks striped under a microscope.
- Smooth muscle: lines internal organs and blood vessels. Works without your control (involuntary). Pushes food through your gut, controls the width of your blood vessels, makes your eyes change focus. Looks smooth (unstriped) under a microscope.
- Cardiac muscle: only found in your heart. Works on its own (involuntary) but looks striped like skeletal muscle. Built to keep contracting steadily without ever resting.
How muscles work
Every muscle works by contracting (shortening) when it receives an electrical signal from a nerve. Muscles cannot push; they can only pull. So muscles work in pairs: one to bend a joint, another to straighten it. The most famous example is your upper arm:
- Biceps (on the front of your upper arm): contracts to bend your elbow up.
- Triceps (on the back of your upper arm): contracts to straighten your elbow back down.
This pair-wise design (called antagonistic muscles) means every joint is moved by at least two opposing muscles, ready to pull either way.
How a muscle contracts
The contraction itself happens at a microscopic level inside muscle fibres. Each fibre contains thousands of thin parallel threads made of two proteins, actin and myosin. When the muscle gets a signal from a nerve, the myosin threads grab the actin threads and pull on them, shortening the muscle. This is called the sliding filament theory.
Every muscle contraction uses energy in the form of a molecule called ATP, which comes from the food you eat (broken down in your digestive system) and the oxygen you breathe (delivered by your circulatory system). Hard-working muscles use a lot of ATP, which is why you breathe faster and your heart beats harder during exercise.
The biggest, strongest and busiest muscles
- Largest muscle: gluteus maximus, the big muscle in your bottom. It powers your hips and keeps you upright when you walk and stand.
- Strongest muscle by weight: the masseter, your jaw muscle. It can clench your teeth together with over 90 kg of force.
- Hardest-working muscle: your heart. It contracts around 100,000 times a day and never gets to rest for more than a fraction of a second.
- Fastest-moving muscles: the muscles that move your eyes. They are also among the busiest, making about 100,000 tiny movements every day.
- Smallest muscles: the stapedius, attached to the stapes bone in your ear. Just over 1 mm long.
How muscles grow stronger
When you exercise a muscle heavily (lifting weights, sprinting, climbing), tiny microscopic tears form in the muscle fibres. While you rest afterwards, your body repairs the tears and adds extra muscle protein, making the fibres slightly bigger and stronger than before. Repeat over weeks and months and the muscle grows visibly larger and stronger. This is called hypertrophy.
The same process works in reverse: muscles that are not used (because you have been ill, injured, or simply inactive) gradually shrink and weaken in a process called atrophy. Astronauts on the ISS lose significant muscle mass within weeks of being weightless and have to exercise for 2 hours every day to limit the damage.
Deeper dive: fast-twitch vs slow-twitch muscle fibres
Your skeletal muscles are not all the same. They are actually made of two main types of muscle fibre, and the mix in any particular muscle (and in any particular person) shapes what that muscle is good at.
- Slow-twitch fibres (Type I): contract slowly but can keep going for a long time without getting tired. They use oxygen efficiently and are red in colour because they contain lots of an oxygen-carrying protein called myoglobin. They are best for endurance: walking, jogging, long-distance running, swimming, maintaining posture.
- Fast-twitch fibres (Type II): contract quickly and powerfully but tire fast. They are pale in colour because they use less oxygen and more glucose. They are best for short bursts of intense effort: sprinting, jumping, lifting heavy weights.
Most people have roughly 50/50 of each type, but the mix varies. Elite marathon runners often have around 80% slow-twitch fibres; elite sprinters often have around 80% fast-twitch fibres. This is partly born and partly trained: training can shift the balance to some extent, but you cannot completely transform your muscle type.
Different muscles also have different default mixes. The soleus (a deep calf muscle that helps you stand) is over 80% slow-twitch, perfect for hours of standing. The biceps are roughly 50/50. The eyelid muscles are nearly all fast-twitch, perfect for super-quick blinks. Even chickens demonstrate this beautifully: chicken breast meat (mostly fast-twitch, used for short bursts of flapping) is white, while chicken leg meat (mostly slow-twitch, used for hours of walking around) is dark.
For the bones the muscles move, see the skeletal system. For how nerve signals tell muscles to contract, see the nervous system.