Muscle Groups (Part 1)

Joint Actions

Make sure you understand the following terms:

  • flexion
  • extension
  • abductions
  • circumduction
  • rotation
  • plantar flexion
  • dorsi flexion
  • supination
  • pronation


Movement Definition/Example
Flexion Involves decreasing the angle between two bones, e.g. bending the elbow or leg
Extension Increasing the angle between two bones, e.g. bending backwards at the trunk
Abduction Moving body parts away from the body’s midline
Adduction Moving bones towards the body’s midline
Pronation Turning the hand to have it palm downwards (radius and ulna are crossed)
Supination Palms facing upwards (radius and ulna are uncrossed and parallel)
Inversion Moving the sole of the foot inwards at the ankle
Eversion Moving the sole of the foot outwards at the ankle
Dorsiflexion Pointing the toes upwards towards the tibia
Plantar flexion Pointing the toes away from the tibia
Circumduction Moving the end of a bone so that it makes a circle
Rotation Moving the bone around a central axis

Major Muscle Groups & the Microscopic Structure of Muscle

This resource:

  • provides an understanding of muscle action fibre types and their influence on activities
  • Identifyies characteristics of fast and slow twitch muscle fibres.
  • provides an understanding of the significance of fibre types on performance in different activities.
  • provides an understanding of microscopic structure, sliding filament theory; agonist/antagonist relationship; joint actions (flexion/extension, etc.); contraction types (eccentric/concentric/isometric)

Summary of Muscles & Their Actions

There are three types of muscles in the body:

  1. smooth (intestinal wall and arteries)
  1. cardiac
  1. skeletal

Skeletal muscles are labelled according to the way the muscle fibres lie with respect to the tendon.

Fusiform muscles

Fibres run in the same direction as the tendon. There aren’t a great number of these muscle types because they only produce low muscle force. However, they have great contractile range and so are found in muscles that require a great range of movement (e.g. gracilis muscle).

Penniform muscles

In these muscles, the fibres run at an angle to the tendon. They make up 75% of all the skeletal muscles in the body. They produce great force, but don’t have great range.

There are three types of penniform muscles:

  • unipenate – branch to one side only (e.g. Semimembranosus)
  • bipenate – branch both sides of the tendon (e.g. Rectus Femoris)
  • multipenate – branch repeatedly (e.g. deltoid)

Muscle Fibre Types

There are two types of muscle fibres. Each has unique characteristics:

Fast twitch (white) fibres

These fibres are associated with explosive activities requiring speed and power.

They produce a great deal of force; however, they fatigue more rapidly than slow twitch.

View the video on sprinting, which depends on fast-twitch fibres.

Slow twitch (red) fibres

These fibres are smaller than their FT counterparts and are best suited to aerobic activities. They are associated with oxygen, hence the term “red” fibres. All muscles have a percentage of both types of fibres. This is genetically determined to a large extent.

For example, distance runners typically may have:

79% ST fibres

21% FT fibres

100 metre sprinters may have:

75% FT fibres

25% ST fibres

Muscle Fibre Types: Features

Characteristics of FT and ST fibres that you need to remember:


Characteristic Slow twitch fibres Fast twitch fibres
Capillary density high low
Force of contraction low high
Fatigability low high
Peak force low high
PC stores low high
Mitochondria high low
Triglyceride stores high low


Training – it is possible to improve the endurance potential of FT fibres (i.e. give FT fibres more aerobic tendencies). It is, however, hard to develop FT characteristics with people who have predominantly ST fibres.

Preferential treatment – the intensity of the task determines which muscle fibre type is preferentially recruited. If immediate and rapid response to a stimulus is required, then FT fibres will contract first, then ST fibres. However, ST fibres get preferential treatment if the task is of low intensity.