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Lesson 4 - Muscles - Part 2


The origin of a muscle is the place where the muscle is anchored to a fixed point on the skeleton by a tendon.

The insertion of a muscle is where the tendon is attached to a bone that is to be moved, eg the biceps has two origins - the top of the scapula and the glenoid cavity. These are fixed. The insertion is into the radius. The radius is moved by the biceps when we do a biceps curl.

Antagonistic muscles - skeletal muscles that work in pairs eg biceps / triceps.

Agonist - muscle that causes a particular movement. Also known as the prime mover.

Antagonist - muscle that causes the opposite movement to the agonist (eg in above example, triceps) For the hamstring curl, the agonist is the hamstring, the antagonist is the quadriceps.

Synergist - Helps the agonist by adding extra force or removing unwanted movement.

Fixator - Holds the origin of the agonist stable, eg muscles in neck that hold the head up.The role of muscles is not fixed, ie they can be agonists in one movement, synergists in a second and fixators in a third.

Muscle Fibres

Muscle fibres differ in composition and function.

The main types of muscle fibre are:

  • Slow twitch fibres - Type 1 (Slow Oxidative)
  • Fast twitch fibres, which are subdivided thus:
    • Fast Oxidative Glycolytic (FOG) - Type IIA
    • Fast Twitch Glycolytic (FTG) - Type IIB

Myoglobin - A protein in muscle similar to haemoglobin in blood that stores oxygen until it is needed by the mitochondria.
Mitochondria - Organelles within muscles that are the muscle's power source supplying them with energy.

Type of Fibre Type I Type IIA Type IIB
Colour Red Pink White
Myoglobin content High Intermediate Low
Mitochondrial Density High High (but less than Type 1) Low
Oxidative Capacity High High (but less than Type 1) Low
Glycolytic Capacity Low High High
Force Produced Low Intermediate High
Endurance High Intermediate Low
Type of activity Aerobic Anaerobic - long term Anaerobic - short term
ATP Produce ATP aerobically, split ATP at a slow rate Produce ATP aerobically, split ATP rapidly Produce ATP anaerobically, split ATP rapidly
Contraction Time Slow Fast Very Fast


Notes on above:

  • Colour is dependent on the myoglobin content - the redder the fibre, the more myoglobin it contains.
  • The different density of fibre types is genetically determined.
  • Long term aerobic activity and endurance uses mostly slow twitch fibres.
  • Fast twitch fibres are associated with anaerobic activity.
  • Skeletal muscles are a mixture of all three types of fibre, but the proportions depend on what the muscle is used for - arm and shoulder muscles having a higher proportion of fast twitch fibres and leg muscles having a higher proportion of slow twitch fibres.
  • The most powerful muscle contractions are powered by all the muscle fibres. For a moderate contraction, fast twitch red and slow twitch fibres are activated. Weak contractions require only the contraction of slow twitch fibres.

Muscle Contractions

Myofibrils - many thousands contained in muscle fibres.
Thin Myofilaments and Thick Myofilaments - smaller structures which make up the myofibrils. It is the arrangement of the thick and thin myofilaments, Myosin and Actin, that give skeletal muscle its striated appearance.
Sarcomere - the contractile unit (distance between z lines in adjacent picture) and the smallest unit of a muscle. Sarcomeres are subsections of myofibrils - in other words, myofibrils contain chains of sarcomeres and when a muscle contracts, each individual sarcomere contracts.

Sliding filament theory of contraction

The sarcomere shortens but thick and thin myofilaments stay the same length, sliding over one another. Thin myofilaments slide inwards towards the H zone, causing it to disappear.

The thin myofilaments are called Actin and the thick myofilaments are called Myosin. Calcium is necessary to enable the Myosin to bind to the Actin to enable the muscle to contract. When the muscle is relaxed, the Myosin cannot bind to the Actin because the binding sites on the Actin head are blocked by trypomyosin. When calcium binds to troponin, it pulls the trypomyosin from the Actin heads, allowing the binding and the muscle contraction.

Muscle Tone

Different fibres may be activated at different times, enabling partial contractions to be sustained for long durations.

flaccid - muscles with less than normal tone (caused by illness, injury or lack of use).
atrophy - occurs when muscle is not used over an extended period of time. The muscle cells themselves reduce in size.
hypertrophy - increase in muscle size due to exercise.

Effects of exercise on muscles

Short Term:

  • Capillary dilation
  • Increased pliability

Long Term:

  • Hypertrophy
  • Increased metabolic activity
  • Increased capillarisation
  • Increase in number of mitochondria
  • Increase in muscular strength
  • Increase in muscular endurance