Anatomy & Physiology
A practical map of the body's structures and systems
Lesson 10 of 20
Muscle fibers & contraction
Skeletal muscle produces movement and helps maintain posture and heat. Its fibers contain myofibrils built from repeating contractile units. Contraction occurs when actin and myosin interact, with calcium and ATP supporting the cycle. A muscle can generate tension while shortening, lengthening, or holding its length.
Key points
- Slow oxidative fibers resist fatigue and support sustained activity; fast fibers can generate force rapidly.
- A motor unit is a motor neuron and the muscle fibers it activates.
- Calcium exposes binding sites on actin; myosin cross-bridges pull using energy from ATP.
- Concentric contraction shortens while producing force; eccentric contraction lengthens under tension.
- Isometric contraction produces tension without noticeable change in muscle length.
- Muscle force is graded through motor-unit recruitment and firing rate.
Key terms
- Sarcomere
- Repeating contractile unit within a striated muscle fiber.
- Actin
- Thin filament that interacts with myosin during contraction.
- Myosin
- Motor protein whose heads bind to actin and generate force.
- Motor unit
- One motor neuron and all muscle fibers it innervates.
- Eccentric contraction
- Force-producing contraction while a muscle lengthens.
- Isometric contraction
- Force production without appreciable change in muscle length.
Memory tip
Concentric shortens, eccentric lengthens under load, isometric holds.
Clinical note
The sliding-filament model describes microscopic interaction; it does not mean filaments themselves shrink.
Unit checkpoint
Lock it in: match the terms, then test yourself.