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Human Physiology Unit. Muscle Structure and Function. Muscle Function. Muscles are for: contraction for locomotion and skeletal movement contraction for propulsion contraction for pressure regulation. Types of Human Muscle.

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muscle function
Muscle Function
  • Muscles are for:
    • contraction for locomotion and skeletal movement
    • contraction for propulsion
    • contraction for pressure regulation
types of human muscle
Types of Human Muscle
  • Based on their structures, contractile properties, and control mechanisms, there are three types of muscle in the human body:
  • skeletal muscle,
  • smooth muscle, and
  • cardiac muscle.
smooth muscle
Smooth Muscle
  • Smooth muscle forms the walls of blood vessels and body organs
  • Control of is involuntary & by the autonomic nervous system
  • Contraction is slow & uniform; this muscle is relatively fatigue resistant
cardiac muscle
Cardiac Muscle
  • Cardiac muscle makes up your heart
  • Has characteristics of both skeletal muscle and smooth muscle
  • Is fatigue resistant & contractions are involuntary
skeletal muscle
Skeletal Muscle
  • Skeletal muscle is attached to bone & is responsible for supporting & moving the skeleton
  • Contraction is under voluntary control by motor neurons.
skeletal muscle1
Skeletal Muscle
  • Skeletal muscle is a group of muscle fibres bound together by connective tissue
  • Skeletal muscles are held to the bones with the help of tendons
    • Tendons are cords made of tough collagen tissue.
    • The tendons are attached so well that when you contract one of your muscles, the tendon and bone move along with it.
skeletal muscle2
Skeletal Muscle
    • During contraction, the muscle shortens and moves the attached bone in a certain direction.
  • Skeletal muscles are capable of rapid contraction and relaxation.
  • The higher the intensity of an activity, the faster the muscle will fatigue.
skeletal muscle3
Skeletal Muscle
  • In order for a muscle to cause a movement, it crosses a joint.
  • A muscle is attached to two bones, which form the joint, by tendons. Just one of these bones will move when the muscle contracts.
  • For example, when the quadriceps muscles contract, the tibia of the lower leg is pulled forwards to straighten the knee.
skeletal muscle4
Skeletal Muscle
  • Muscles have two ends
    • The origin is the end that attaches to the stationary bone (in the example of the quadriceps - the end attaching to the femur);
    • the other end is called the insertion and is attached to the moving bone (the tibia).
muscle pairs
Muscle Pairs
  • Muscles work together in perfect synchrony
  • Muscles only pull, they cannot push, they work in pairs
  • These pairs are called antagonistic pairs.
muscle pairs1
Muscle Pairs
  • As one muscle contracts (shortens), the other relaxes (lengthens)
    • For example, when you sit down

your hamstrings contract, while your

quadriceps relax

muscle pairs2
Muscle Pairs
  • The muscle which is contracting is called the agonist or prime mover.
  • The relaxing muscle is called the antagonist.
  • The third muscle type within this model is called a synergist or stabilizer. These muscles help to stabilize the bone which isn't moving.
muscle pairs3
Muscle Pairs

Practice example:

  • When performing a bicep curl:
  • Agonist - 
    • Biceps brachii
  • Antagonist - 
    • Triceps brachii
  • Synergist -
    • Trapezius & rhomboids
muscle pairs4
Muscle Pairs

Primary opposing muscle groups

tibialis anterior




upper back (trapezius, rhomboids)


triceps brachii

  • calves
  • quadriceps
  • hip flexors
  • erector spinae
  • pectoralis major and minor
  • latissimusdorsi
  • biceps brachii
muscle pairs5
Muscle Pairs
  • Think of a few simple exercises that you might do in the gym or at home.
  • Which muscle is the agonist and which is the antagonist?
muscle contractions
Muscle Contractions
  • Muscles can contract in two different ways:
    • Isometric
      • the muscle length does not change and there is no movement.
      • E.g. carrying a bucket of water
    • Isotonic
      • the muscle length changes, causing movement at a joint.
      • E.g. a bicep curl
muscle contractions1
Muscle Contractions


  • There are 2 types of isotonic contractions:
    • Concentric contraction
      • the muscle decreases in length (shortens) against an opposing load, such as lifting a weight up.  
    • Eccentric contraction
      • the muscle increases in length (lengthens) as it resists a load, such as returning a weight to starting position, or resisting a stretch.  During an eccentric contraction the muscles that are lengthening serve as the agonists (and do all of the work).
muscle contractions2
Muscle Contractions
  • Performing exercises and being active in our daily life can cause our muscles to get stronger.
    • As you may expect, strong people have larger muscle fibres. This growth in muscle size is called hypertrophy.
  • If we do not use our muscles regularly, the opposite can happen and the muscles reduce in size. This is called atrophy.
  • Muscles are always slightly under tension to enable us to hold a position, such as sitting upright. This small amount of muscle tension, is known as muscle tone. Exercise improves muscle tone.
skeletal muscle structure1
Skeletal Muscle Structure
  • Skeletal muscle consists of connective tissues and bundles of muscle fibres
  • A skeletal muscle is surrounded by connective tissue called EPIMYSIUM
  • A muscle is made up of several bundles of muscle fibres called FASICULI
  • PERIMYSIUM is connective tissue that surrounds each muscle bundle (fasiculi)
  • Each muscle fibre in the fasiculi is covered by connective tissue called ENDOMYSIUM
  • Each muscle fibre is composed of several MYOFIBRILS
skeletal muscle structure2
Skeletal Muscle Structure
  • Each muscle fibre is composed of several MYOFIBRILS
  • Within each muscle fibre is sarcoplasm. Sarcoplasm contains glycogen, fat particles, enzymes and the mitochondria.
  • Each myofibril contains several SARCOMERES (the smallest unit of muscle contraction)
    • Within each contractile unit (sarcomere) there are two major proteins:
      • ACTINthin filament lighter colour
      • MYOSIN thick filament darker colour
skeletal muscle structure3
Skeletal Muscle Structure
  • Myosin and actin filaments run in parallel to each other along the length of the muscle fibre.
    • Myosinhas tiny globular heads protruding from it at regular intervals. These are called cross bridges and play a pivotal role in muscle action.
  • Each myofibril is organized into sections along its length. Each section is called a sarcomere and they are repeated right along the length of a muscle fibre.
  • It's similar to how a meter ruler is split into centimetres and millimetres. Just as the millimetre is the smallest function of a ruler, the sarcomere is the smallest contractile portion of a muscle fibre.
skeletal muscle structure5
Skeletal Muscle Structure
  • The sarcomere is often divided up into different zones to show how it behaves during muscle action.
skeletal muscle structure6
Skeletal Muscle Structure
  • The Z-lineseparates each sarcomere. The H-zone is the center of the sarcomere and the M-line is where adjacent myosin filaments anchor on to each other.
  • The darker A-bands are where myosin filaments align and the lighter I-bands are where actin filaments align.
skeletal muscle structure7
Skeletal Muscle Structure
  • When a muscle contracts the H-zone and I-band both decrease as the z-lines are pulled towards each other.