Two people can both have a working knee and still bend it by very different amounts. To compare them you need a number, not a description. This page is about getting that number: what range of motion means, how a goniometer gives it to you, and how to avoid the sloppy technique that makes the reading useless.
📘 What you need to know
Range of motion is how far a joint can move. Joints move in different directions, sometimes called dimensions.
A joint that moves in several dimensions, like the hip, has a bigger range than a hinge joint.
A goniometer measures the angle a joint can move through, in degrees.
You take a reading at full extension and another at full flexion, then find the difference.
Computer analysis can track joint movement too, and there are phone apps that work as a goniometer.
You should have used a goniometer yourself at some point in the course.
What “range of motion” actually means
Range of motion is simply the amount of movement available at a joint. It depends on:
The type of joint. A hinge like the knee swings one way; a ball and socket like the hip turns in several directions.
The ligaments holding it, which stop the joint going too far.
The person. Age, training, injury and stiffness all change the answer.
Because different directions are involved, a full description of the hip needs several readings — flexion, extension, rotation, abduction and adduction — not one.
Range of motion is a number with a unit: degrees. If your answer to “what is the range of motion?” is “quite a lot”, you have not answered the question.
The goniometer
A goniometer is a protractor with two arms. One arm stays still, one moves, and the scale in the middle reads the angle between them.
The one part people rush is the centre. If it is not sitting over the joint, every reading you take afterwards is wrong.
🧩 How to take the measurement
Start with the joint fully extended and line the goniometer up against it: centre over the joint, one arm along each bone.
Read the angle while the joint is fully extended. Write it down before you move anything.
Bend the joint as far as it will comfortably go, then realign the goniometer in the new position.
Read the angle again.
Subtract the smaller reading from the larger one. That difference is the range of motion.
Range of motion
range of motion = angle at full flexion − angle at full extension
Notice that the straight leg does not read zero. Almost nobody’s does, which is exactly why you measure both ends instead of assuming one.
Getting a reading you can trust
This is a practical skill, so the examiner may ask how you would make your data reliable. Good answers talk about the technique, not just the equipment.
Repeat and take a mean. Three readings of the same joint, then average them. One reading tells you nothing about how consistent you were.
Use the same landmarks every time. Line the arms up with the same bony points, or your “same” measurement is not the same at all.
Keep the centre over the joint while the limb moves. It is easy to let it slide.
Same person doing the measuring. Two people place a goniometer slightly differently, which adds a systematic difference.
Do not force the joint. “As far as it comfortably goes” is the limit — pushing past it is both unsafe and unrepeatable.
Other methods.Computer analysis can track and measure joint movement from video, which is how sports scientists study running technique. There are also phone apps that use the phone’s sensors to work like a goniometer.
Worked examples
WORKED EXAMPLE
A student measures a knee. Fully extended it reads 5°; fully bent it reads 150°. Calculate the range of motion. [1]
Take the difference between the two readings150° − 5° = 145°Range of motion = 145°do not forget the degree sign — a number with no unit can lose the mark.
WORKED EXAMPLE
Three readings for full flexion of an elbow are 142°, 146° and 144°. Calculate the mean, and suggest why the three values differ. [3]
Step 1: add them up142 + 146 + 144 = 432Step 2: divide by the number of readings432 ÷ 3 = 144Mean = 144°Step 3: why they differthe goniometer is not placed in exactly the same spot each time, and the person may not bend the joint quite as far on every attempt.
WORKED EXAMPLE
Explain why the hip needs more than one range of motion measurement, but the knee does not. [2]
Compare the joint typesThe knee is a hinge joint, so it only flexes and extends — one direction of movement.Now the hipThe hip is a ball and socket joint, so it also rotates and moves sideways.The hip moves in several dimensions, so each one needs its own measurementuse the word “dimensions” — it is the syllabus wording.
💡 Exam tip
Always give angles in degrees with the symbol. Bare numbers can be marked wrong.
Range of motion is a difference, not a single reading. Show the subtraction.
If asked to improve a method, talk about repeats, means, fixed landmarks and the same operator.
Link range of motion back to joint type: hinge joints move in one dimension, ball and socket in several.
Name the tool correctly: goniometer. Spelling counts.
⚠ Common mix-up
Assuming a straight limb reads zero. Take the reading, do not guess it.
Reporting the bent-position angle as the range of motion. The range is the difference between two readings.
Measuring once and calling it data. Repeats and a mean, every time.
Letting the centre drift off the joint while the limb is moved.
Thinking a goniometer measures strength. It measures angle, and nothing else.
Confusing flexion and extension when writing up which reading was which.
Up next: Locomotion in Organisms — why animals bother moving around at all, and how bodies get reshaped by the way they travel.
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