The big idea: The range of motion of a joint is the angle through which it can move. It is measured in degrees, in each direction the joint allows.
The points to remember
- Use a goniometer or a photo or video analysed on a computer or app (a protractor on an image).
- Line up the centre on the joint and an arm along each bone.
- Move the joint to its limit, then read the angle; straight counts as 0°.
- Repeat each reading, and measure the opposite direction too.
| Direction | Hip (ball and socket) | Knee (hinge) |
|---|---|---|
| Bending forwards (flexion) | about 120° | about 140° |
| Bending backwards (extension) | about 20° | about 0-5° |
| Out to the side | about 45° | almost none |
| Twisting (rotation) | about 45° each way | very little |
Remember it as: Centre on the joint, arms on the bones, straight = 0.
Real example: hip v knee. The hip moves in every direction, so its range is measured in several planes; the knee mostly bends one way, like a hinge.
Trap: A protractor alone cannot measure a living joint well: name a goniometer or computer analysis of images.
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The big idea: Joint-angle practicals are judged like any experiment: name the variable, read the graph, compare methods, and weigh up the design.
The points to remember
- The dependent variable is the joint angle; degrees is only its unit.
- On a box plot, the line is the median, the box the interquartile range; circles are outliers.
- Two methods agree when their medians and ranges are similar (box plots show medians, not means).
- A correlation coefficient near +1 (such as r = 0.8) is a strong positive correlation.
- Hypermobile joints bend backwards past straight, well beyond the normal range.
| Evaluate the design | Examples |
|---|---|
| Strengths | Large sample; several joints; variables controlled (same finger, healthy people); two methods compared |
| Limitations | Finger length or flexibility not taken into account; joint position judged by eye; one hand only |
Remember it as: Variable, graph, agreement, design.
Real example: diagnosis. Doctors judge hypermobility from how far joints, such as the little finger, bend backwards beyond the normal range.
Trap: When asked to evaluate the experiment, comment on its design (sample, controls, methods), not on what the results show.
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How it is asked: This is a practical: expect a data question on joint angles (variable, box plot or scatter graph, comparing methods, evaluating), and 'describe how to measure the range of motion'.
Measurements of the elbow show a smaller angle when the arm is straight than when it is bent. Explain this result.
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