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Anatomy and Physiology - Lesson 3 - Muscles - Part 1


Muscles can be classified in the following ways:

  • Skeletal muscle - this muscle is called striated or stripey muscle, because it looks striped under a microscope. Skeletal muscle is a voluntary muscle.
  • Cardiac muscle - situated in the walls of the heart. It is striated and involuntary.
  • Smooth muscle - found in the walls of internal organs, blood vessels and the digestive, reproductive and urinary systems. Does not have the stripes found in skeletal muscle. This type of muscle is important in veins as it pushes blood back towards the heart. It is non-striated and involuntary.

Muscle comprises 40-50% of a person's total body weight.


  • Excitability - the ability of the muscle to react to stimulus eg reacting when you touch something hot.
  • Contractility - the ability to become shorter and thicker.
  • Extensibility - the ability to become longer and thinner.
  • Elasticity - the ability to return to its original shape after extending or contracting.


  • Motion
  • Maintaining posture
  • Producing heat (approx 85% of body heat)

Note: Superficial muscle lies closer to the surface, can be seen when contracted and causes movement. Muscles that lie below the superficial muscles are called deep muscles.

Types of muscle contractions

When a muscle is stimulated, the fibres tighten. The muscle can either shorten (concentric), stay the same length (isometric) or lengthen (eccentric).

  • Concentric - muscle shortens but tension remains the same, eg contracting biceps causes flexion at elbow.
  • Eccentric - increase in muscle tension when a muscle lengthens (eg a controlled lengthening - such as extension of biceps whilst holding a weight.)
  • Isometric - muscle under tension but no change in muscle length. eg carrying an object at arm's length - the biceps have not changed length and therefore have an isometric contraction.
  • Isotonic - a name applicable to both concentric and eccentric movements in a muscle.

Muscle Actions

Antagonistic - eg biceps / triceps - opposing sets of muscles. Agonist - bicep (primary muscle), antagonist - tricep

Synergist - a muscle that steadies a movement. When making a fist, the finger muscles are the agonists and the wrist extensors are the synergists.

Fixator - a fixator holds the origin of the agonist stable. Eg, abducting arm - deltoid is abductor, fixators are other flexors keeping the scapula in position eg rhomboids.

Muscle Antagonistic Pairs
Stronger Muscle Weaker Muscle
Bicep Tricep
Quadriceps Hamstrings
Lattisimus Dorsi Deltoids
Gastrocnemius Tibialis Anterior
Erector Spinae Rectus Abdominis
Adductors Abductors
Pectoralis Trapezius
Iliopsoas Gluteus Maximus
Obliques R / L


Notes on above:

  • Chest muscles consist of pectoralis major and pectoralis minor. The major is higher up the body than the minor.
  • Gluteus minimus and gluteus medius are part of the abductor group.
  • The muscle over the shoulders is called the deltoid and has posterior, mid and anterior parts.
  • The stomach muscles include the internal obliques, external obliques, rectus abdominis and the deep muscle the transverse abdominis.
  • Rhomboids consist of major and minor parts and are contracted when you squeeze your shoulder blades together.
  • The erector spinae are on both sides of the spine for its full length.
  • The multifidus muscles consist of lots of little muscles that are situated between the vertebrae.
  • The quadriceps consist of four muscles, the main one is called Rectus Femoris.

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