What Are Muscles?
Muscles are tissues made from specialised cells that can contract (shorten) and relax (lengthen). This ability to change length is what produces all movement in your body — from running and lifting to your heart beating and food moving through your digestive system.
Your body contains over 600 muscles, making up roughly 40% of your total body weight. Muscles require a constant supply of energy (from cellular respiration) and oxygen to do their work. When muscles are used regularly, they grow stronger and larger — this is the reason athletes train hard.
Three Types of Muscle
There are three types of muscle tissue in the human body. Skeletal muscle is attached to bones by strong cords called tendons and is responsible for all the deliberate movements you make — walking, writing, smiling. Skeletal muscle is under voluntary control, meaning you consciously decide to use it.
Smooth muscle is found in the walls of internal organs such as the stomach, intestines, and blood vessels. It moves food through your gut and controls blood flow, but you cannot control it consciously — it works involuntarily. Smooth muscle cells look different under a microscope: they have no striped pattern, which is why they are called "smooth."
Cardiac muscle is found only in the heart. It contracts rhythmically and tirelessly throughout your entire life without ever needing to rest. Like smooth muscle, it works involuntarily — you do not have to think about making your heart beat.
The strongest muscle in your body relative to its size is the masseter — the jaw muscle. It can exert a biting force of up to 200 kg! The largest muscle is the gluteus maximus (your buttocks), and the most active muscle is your eye muscle, which moves thousands of times per day.
How Muscles Produce Movement
Skeletal muscles can only pull — they cannot push. To move a bone in both directions, muscles are arranged in antagonistic pairs. One muscle (the agonist or prime mover) contracts to pull the bone in one direction, while the other (the antagonist) relaxes and lengthens. When the movement is reversed, the two muscles swap roles.
The classic example is the biceps and triceps in your upper arm. When you bend your elbow, the biceps contracts (shortens) and the triceps relaxes. When you straighten your arm, the triceps contracts and the biceps relaxes.
Remember: muscles can only pull, never push. When something in your body is being "pushed," it is actually being pulled from the other side by a muscle on the opposite side of the joint.
Muscles and Energy
Muscles need energy to contract. This energy comes from ATP (adenosine triphosphate), which is produced by cellular respiration in the muscle cells. During light activity, aerobic respiration provides enough ATP. During intense activity, muscles may switch to anaerobic respiration, which produces lactic acid — this is why muscles ache during hard exercise.
Regular exercise strengthens muscles by causing tiny tears in the muscle fibres, which then repair and grow back thicker and stronger. This is why rest between training sessions is just as important as the exercise itself.
A common confusion is mixing up tendons and ligaments. Tendons connect muscle to bone. Ligaments connect bone to bone. Both are tough, flexible cords, but they have different jobs. Spraining an ankle tears ligaments; pulling a muscle can damage tendons.
Real world: physiotherapy after injury
When a ligament is torn, the surrounding muscles may weaken from disuse. A physiotherapist designs progressive resistance exercises to rebuild the antagonistic muscle pairs around the joint — for example, strengthening both quadriceps and hamstrings after a knee injury. Understanding that muscles only pull and work in pairs directs the entire rehabilitation programme.
Real world: sports training and hypertrophy
A weightlifter performs bicep curls repeatedly, causing tiny tears in muscle fibres. During rest, the fibres repair and grow thicker (hypertrophy). Over weeks, the muscle becomes noticeably stronger. Coaches design training programmes with rest days because muscle growth happens during recovery, not during the workout itself.
Real world: cardiac muscle and heart disease
Cardiac muscle works continuously and involuntarily throughout life. In coronary artery disease, the blood supply to heart muscle cells is reduced, starving them of oxygen and glucose for respiration. Without energy, cardiac muscle cells die — causing a heart attack. Understanding cardiac muscle's unique properties informs every aspect of cardiology treatment.
Practice Problems
What is the difference between voluntary and involuntary muscle?
Show answer
Explain how antagonistic pairs of muscles work using the biceps and triceps as an example.