Vidlune
Measuring joint angles in a video: what it can and can't tell you
How to measure knee, hip and elbow angles on a paused video, how camera position affects the result, and how to use the numbers sensibly.
Coaches often talk about angles: a knee bent to about ninety degrees, an elbow at shoulder height, a straight line from shoulder to ankle. Measuring these on a paused video is easy, but the numbers are only useful if you understand where they come from.
How to measure an angle
Pause the video at the moment you want to check. Pick three points: the two ends and the joint in the middle. For a knee that is hip, knee and ankle; for an elbow, shoulder, elbow and wrist. The angle at the middle point is the joint angle as it appears on screen. Zooming in before you place the points makes them more precise.
In Vidlune you tap the three points and the angle appears in degrees. Lines and angles stay attached to the picture when you zoom or mirror, and they are saved with the video so you can come back to them.
Why camera position matters
A video is a flat picture of a three-dimensional body. An angle measured on screen is only close to the real angle when the joint moves in a plane facing the camera. A knee filmed exactly from the side shows its true bend; the same knee filmed from a diagonal can look straighter or more bent than it is.
To get useful numbers:
- Film the movement from the side for forward and backward bending, and from the front for sideways movement.
- Keep the camera at roughly the height of the joint you are measuring.
- Keep the camera still, and at the same distance and angle each time.
Compare yourself with yourself
Because of perspective, the most reliable use of angles is comparison: the same movement, filmed the same way, on different days or before and after a change. If your knee angle at the bottom of a squat goes from 110 to 95 degrees between two sessions filmed identically, that change is real even if neither number is exact.
Comparing your angles with a number from a book or with a video filmed differently is less reliable. Treat such numbers as rough guides.
Lines are often more useful than angles
Not every check needs a number. A straight line from shoulder through hip to ankle shows whether a plank or a swimming position is flat. A vertical line from the ball down to the court shows where a tennis toss lands. A line along the shin shows whether the knee travels past the toes. Drawing a line is quick and the result is easy to see.
Use the right frame
Angles change continuously during a movement, so the frame you choose matters. Decide the moment in advance, for example the lowest point of a squat, the moment of contact or the landing of a jump, and step frame by frame to find it. Placing a marker on that moment makes it easy to measure the same position on several attempts.
Share what you found
A screenshot with the lines, the angle and a short note is often clearer than a long explanation. Coaches can send one to a student, and students can send one to a coach with a question. Vidlune can save the paused frame with your drawings and a note as a single picture.
Measured carefully and compared fairly, angles turn "I think my knee is too straight" into something you can see change over time.