11.1 Essential ideas

11.1.2 Movement

The roles of the musculoskeletal system are movement, support and protection.

Bones and exoskeletons provide anchorage for muscles and act as levers

  • A lever is a simple machine that amplifies the input force about an axis point (the fulcrum) in order to overcome resistance. The output of a muscular contraction is movement in the direction opposite to the resistance.
  • There are three classes of lever, as shown in the table below:

Diagram

Description

Example joint

Movement 
produced

Figure 11.1.2aFigure 11.1.2a – 1st class

Fulcrum lies between the force and the resistance

Atlanto-occipital (skull to spine)

Tilts head

Figure 11.1.2bFigure 11.1.2b – 2nd class

Resistance is between the fulcrum and the effort

Ankle

Lifts heel

Figure 11.1.2cFigure 11.1.2c – 3rd class

Effort is between the fulcrum and the resistance

Elbow

Raises hand towards shoulder

Synovial joints allow certain movements but not others

  • The location where two bones meet is called a joint, or articulation. Some joints, like the bones fused in the skull, are immobile.
  • At synovial joints, bones move with respect to one another. This allows muscles to produce motion.
  • The arrangement of bones, cartilage and ligaments in the joint determines the range of motion.

Some types of joint are described in the following table:

Arrangement

Example

Comparative
range of motion

Possible movements

Hinge

Knee, fingers (phalanges)

Small

• Flexion and extension only

Saddle

Wrist, thumb base

Medium

• Flexion/extension

• Adduction/abduction

• Circumduction

Ball and socket

Shoulder, hip

Large

• Flexion/extension

• Adduction/abduction

• Internal/external rotation

• Circumduction

Figure 11.1.2d – Anatomical movements in the knee, shoulder and hipFigure 11.1.2d – Anatomical movements in the knee, shoulder and hip

Skill: Annotate a diagram of the human elbow

The elbow is an example of a synovial joint that allows movement in two directional planes: flexion/extension and supination/pronation.

  • You should be able to annotate a diagram of the elbow joint.
  • Print a copy of Figure 11.1.2e. Click on the image for help with your annotations.

Figure 11.1.2e – Elbow joint Figure 11.1.2e – Elbow joint 

Application: Antagonistic pairs of muscles in insects

  • Movement of the body requires muscles to work in antagonistic pairs.
  • In the elbow joint shown in Figure 11.1.2e, the biceps and triceps are attached to bones, and work to move the arm in opposite directions. They are antagonistic pairs.
  • The antagonistic muscles of arthropods (and some molluscs) are attached to an exoskeleton.

Figure 11.1.2f – Antagonistic pairs in the leg of a grasshopperFigure 11.1.2f – Antagonistic pairs in the leg of a grasshopper

  • Powerful flexor and extensor muscles in the tarsus allow the jumping force to be produced over a large angle. Grasshoppers can cover distances 20 times their body length in a single jump. 

Muscle fibres are multinucleated cells made of myofibrils

  • A single plasma membrane, called sarcolemma, surrounds each muscle fibre.
  • Each muscle fibre holds many myofibrils, each made of many contractile sarcomeres.
  • Each myofibril is surrounded by a specialised type of endoplasmic reticulum called sarcoplasmic reticulum.

Figure 11.1.2g – Levels of structure in muscle fibresFigure 11.1.2g – Levels of structure in muscle fibres

Skill: Draw a labelled diagram of the sarcomere structure

  • You should be able to draw a labelled diagram of a sarcomere.
  • Use Figures 11.1.2g and 11.1.2h to help you. Include the following:
    Z-lines, thin actin filaments, thick myosin filaments (with heads), light and dark bands.

Figure 11.1.2h – MicrographFigure 11.1.2h – Micrograph

Figure 11.2.1i – ExoskeletonFigure 11.2.1i – Exoskeleton

Key questions

  • Describe how exoskeletons and bones provide support and protection while allowing movement.
  • Annotate a diagram of the human elbow.
  • Explain why movement requires antagonistic muscles.
  • Draw a labelled diagram of a sarcomere.

Concept help

  • Fibrocartilaginous joints are found in between vertebrae – these do not have a joint capsule containing synovial fluid, and therefore, do not allow as much movement as synovial joints.

Try it

  • Identify the types of motion that are possible in the neck and ankle.

Figure 11.1.2j – PronationFigure 11.1.2j – Pronation
Supination and pronation of the elbow joint turn the palm of hand.

Language help

  • Skeletal muscle is so named because it attaches to bones. It may also be called striated muscle because of its appearance as light and dark bands under the microscope.
  • Smooth muscle is found around organs and is not attached to bone.
  • Cardiac muscle surrounds the heart.

Nature of Science

Development follows improvement in apparatus: fluorescent calcium ions have been used to study the cyclic interactions in muscular contraction.