What actually shortens when a muscle contracts?

Actin and myosin filaments slide past each other inside the sarcomere

The muscle fibre's membrane folds inward

The tendon at each end coils up

Individual fibres shrink in volume

What distinguishes a slow-twitch (type I) fibre from a fast-twitch (type II) one?

It relies on oxidative metabolism and resists fatigue, at the cost of peak force

It contracts harder but cannot sustain any activity at all

It is only found in cardiac muscle

Musculoskeletal System

Anatomy MCQs: Muscles

About 640 skeletal muscles — the body's only source of movement, each one a bundle of fibres that shortens on a nerve's command and lets go the instant the command stops.

5 questions

The essentials

Function: Moves the skeleton, holds every joint stable against gravity, generates most of the body's resting heat, and powers breathing, swallowing and speech
Location: Wrapped around every bone the skeleton provides, from the scalp to the sole of the foot

The questions

Roughly how many skeletal muscles does the body have?
1. 60
2. 260
3. 640
4. 1,600
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Answer: 640
About 640 named skeletal muscles, most of them paired left and right — the true count depends on where you stop splitting a muscle into its heads.
What actually shortens when a muscle contracts?
1. The muscle fibre's membrane folds inward
2. Actin and myosin filaments slide past each other inside the sarcomere
3. The tendon at each end coils up
4. Individual fibres shrink in volume
Show the answer
Answer: Actin and myosin filaments slide past each other inside the sarcomere
Myosin heads walk along actin without either filament changing length — the sarcomere shortens as the filaments overlap more, not because either one shrinks.
What distinguishes a slow-twitch (type I) fibre from a fast-twitch (type II) one?
1. It relies on oxidative metabolism and resists fatigue, at the cost of peak force
2. It contracts harder but cannot sustain any activity at all
3. It is only found in cardiac muscle
4. It has no mitochondria
Show the answer
Answer: It relies on oxidative metabolism and resists fatigue, at the cost of peak force
Type I fibres are packed with mitochondria and myoglobin for sustained, fatigue-resistant work — postural muscle; type II trade that endurance for speed and force.