What Is the Muscular System?
The muscular system is the collection of all the muscles in the body — more than 600 in total. Muscles are specialised tissues that can contract (shorten) and relax to produce movement. Without muscles, we could not walk, breathe, pump blood, or even blink.
Muscles make up about 40% of body weight in a typical healthy adult.
Three Types of Muscle
Skeletal muscle (voluntary) — Attached to bones by tendons. These muscles are under conscious control — you decide when to contract them. They are responsible for all body movements: walking, lifting, writing, and facial expressions. Under a microscope, skeletal muscle fibres appear striped (striated).
Smooth muscle (involuntary) — Found in the walls of internal organs such as the stomach, intestines, blood vessels, and bladder. It works automatically without conscious control, moving food through the digestive system and controlling blood vessel diameter.
Cardiac muscle — Found only in the heart. It is involuntary (always works automatically), striated like skeletal muscle, and never fatigues — it contracts rhythmically about 70 times per minute throughout life.
The diaphragm is a dome-shaped skeletal muscle just below the lungs. When it contracts, it flattens and increases the volume of the chest cavity, causing you to inhale. It is a skeletal muscle but is mostly under automatic (involuntary) control.
How Muscles Work
Muscles can only pull — they cannot push. To move a limb in two directions, muscles work in antagonistic pairs: when one contracts, the other relaxes.
The best example is the arm:
- The biceps (front of upper arm) contracts → the arm bends (flexes) at the elbow.
- The triceps (back of upper arm) contracts → the arm straightens (extends) at the elbow.
When the biceps contracts, the triceps relaxes, and vice versa. This is why you can both bend and straighten your arm smoothly.
The word flexion means bending a joint; extension means straightening it. Muscles that bend joints are called flexors; muscles that straighten them are called extensors.
Muscles and Energy
Muscles need energy (ATP) to contract. This energy comes from cellular respiration in mitochondria. During intense exercise, when oxygen delivery cannot keep up, muscles switch to anaerobic respiration, which produces lactic acid — this causes the burning feeling and muscle fatigue.
After exercise, muscles need extra oxygen to clear the lactic acid — this is the "oxygen debt," which is why you continue breathing heavily after stopping exercise.
Keeping Muscles Healthy
Regular exercise makes muscles larger (hypertrophy) and stronger. Muscles that are not used shrink over time (atrophy). Protein from food is essential for building and repairing muscle tissue. Warm-up exercises before activity increase blood flow and reduce injury risk.
Real world: Muscle soreness after exercise (DOMS)
Delayed onset muscle soreness (DOMS) — the aching you feel 1–2 days after unfamiliar or intense exercise — is caused by microscopic tears in skeletal muscle fibres. The body repairs them and builds new protein, making the muscle slightly larger and stronger. This is the biological mechanism behind how exercise builds muscle — and why adequate protein intake and rest matter after training.
Real world: Arm wrestling and antagonistic muscles
In arm wrestling, both competitors' biceps (flexors) are contracting to pull the arm toward them, while their triceps (extensors) resist being stretched. The winner is the person whose biceps generates more force. This is a dramatic, real-world demonstration of antagonistic muscle pairs competing against each other — making the concept immediately vivid.
Real world: Physiotherapy after injury
After a broken bone or muscle injury, physiotherapists prescribe specific exercises to rebuild muscle strength — because muscles atrophy (shrink) rapidly when not used. Patients who remain immobile lose measurable muscle mass within days. The physio exercises target specific antagonistic pairs to rebuild balance and function — applying the biology of muscle use and disuse to medical recovery.
Where do you see this in real life? Put one hand on your biceps and slowly bend and straighten your elbow. You'll feel the biceps harden as you flex and soften as you extend — that's the muscle contracting and relaxing. Put your other hand on the back of your upper arm (triceps) and repeat: it works the opposite way — showing antagonistic pairs in action.
Practice Problems
answer="When the biceps contracts (shortens), it pulls the forearm up, bending the elbow (flexion). At the same time, the triceps relaxes. To extend the arm, the triceps contracts and the biceps relaxes. Because muscles can only pull (not push), two muscles working in opposition are needed for two-directional movement." />
Name the three types of muscle and give the location of each.