Types of muscle and how skeletal muscle is built
The body has three kinds of muscle:
- Skeletal muscle: moves bones; we control it; looks striped (striated) under a microscope.
- Smooth muscle: in the gut and blood vessels; works by itself; no stripes.
- Cardiac muscle: only in the heart; striped; never tires.
A skeletal muscle is joined to bones by tendons. It is a bundle of long cells called muscle fibres. Each fibre has many nuclei and is packed with myofibrils. A myofibril is a chain of sarcomeres, the units that contract. Each sarcomere runs from one Z line to the next and holds thin actin and thick myosin filaments. Their overlap makes the stripes.
Muscle groups and antagonistic pairs
Muscles can only pull, never push. So they work in pairs: the biceps bends the elbow, the triceps straightens it. When one contracts, the other relaxes.
Contraction and relaxation: the sliding filament model
- A motor nerve sends a signal to the fibre.
- The fibre releases calcium ions from an internal store.
- Calcium uncovers binding sites on actin.
- Myosin heads attach to actin, tilt and pull it to the centre (the power stroke).
- ATP binds to the head so it lets go; splitting ATP re-cocks the head for the next pull.
The filaments do not get shorter; they slide past each other, so the Z lines come closer.
Relaxation: when the nerve signal stops, calcium is pumped back into the store (this also uses ATP), the binding sites close and the muscle can be stretched back by its partner.
Static and dynamic work
In dynamic work the muscle changes length and moves something (walking, lifting). In static work it stays tense without moving (holding a bag still). Static work tires muscles faster because squeezed blood vessels bring less oxygen.
Energy for contraction: ATP, respiration and fermentation
A muscle holds only enough ATP for about 2 seconds of hard work. It refills ATP in three ways:
- Creatine phosphate: gives its phosphate to ADP. Very fast, lasts about 10 seconds.
- Lactic fermentation (anaerobic): glucose → lactate. No oxygen needed, fast, but only 2 ATP per glucose. Works for about 10 s to 2 minutes.
- Aerobic respiration: glucose + oxygen → carbon dioxide + water, giving about 30–32 ATP per glucose in the mitochondria. Slower to start but lasts for hours. Fats can be used too.
Glucose comes from the blood and from glycogen stored in muscles and liver.
Blood glucose and diabetes
Muscles take glucose from blood. The hormone insulin (from the pancreas) helps cells take it in after a meal; glucagon makes the liver release glucose between meals. In diabetes this control fails: in type 1 the body makes no insulin, in type 2 cells respond poorly to it. Regular exercise helps muscles use glucose and lowers the risk of type 2 diabetes.
Fatigue, injuries, first aid and doping
Fatigue
During hard work ATP and creatine phosphate fall, lactate and acid build up, and calcium handling slows. The pulls get weaker. Rest, oxygen and food let the muscle recover. Training adds mitochondria and capillaries, so the muscle tires later.
Common injuries and first aid
- Cramp: a sudden painful contraction. Gently stretch and massage.
- Strain (pulled muscle) or sprain (ligament at a joint): use RICE: Rest, Ice (wrapped, 15–20 min), Compression, Elevation.
- Dislocation or fracture: do not move the part; support it and get medical help.
Posture and prevention
Warm up, stretch, build strength slowly, sit and lift with a straight back, and carry bags on both shoulders.
Doping
Doping means using banned drugs or methods to boost performance, for example anabolic steroids (more muscle), EPO (more red blood cells) or stimulants. It is unfair and dangerous: it can damage the heart, liver and hormones. Anti-doping agencies test athletes.
Key formulas and definitions
- Sarcomere: Z line → actin + myosin → Z line
- Contraction: nerve signal → Ca²⁺ → myosin binds actin → power stroke (ATP)
- ATP → ADP + phosphate + energy
- Creatine phosphate + ADP → creatine + ATP
- Glucose → 2 lactate + 2 ATP (fermentation)
- Glucose + 6 O₂ → 6 CO₂ + 6 H₂O + about 30 ATP (aerobic)
Worked examples
1. Why does a sarcomere get shorter while actin and myosin stay the same length?
The filaments slide past each other. Myosin pulls actin towards the middle, so the overlap grows and the Z lines come closer.
2. A sprinter runs 100 m in 11 s. Which energy sources does she mainly use?
Stored ATP and creatine phosphate for the first seconds, then lactic fermentation. There is no time for aerobic respiration to take over.
3. Fermentation gives 2 ATP per glucose and aerobic respiration about 30. How many glucose molecules does fermentation use to make as much ATP as one glucose by aerobic respiration?
30 ÷ 2 = 15 glucose molecules. That is why fermentation empties glycogen stores fast.
4. Why does holding a heavy bag still tire your arm faster than swinging it?
In static work the muscle stays squeezed, which presses on blood vessels. Less oxygen arrives, so the muscle relies on fermentation and tires quickly.
Common mistakes
- Saying the actin and myosin filaments shrink. They keep their length and slide.
- Thinking ATP is only needed to pull. ATP is also needed for myosin to let go and to pump calcium back during relaxation.
- Saying muscles can push. Muscles only pull; a partner muscle pulls the bone back.
- Calling lactate the only cause of soreness days later. Next-day soreness comes mostly from tiny tears in fibres; lactate is cleared within an hour or so.