Frame-by-frame analysis is the review of video by stepping forward or backward one frame at a time. It allows positions and events that pass too quickly for the eye at normal speed to be located, compared, and measured.
Each frame represents a sample of the movement. The interval between frames equals one divided by the frame rate: about 33 milliseconds at 30 frames per second, about 8 milliseconds at 120, and about 4 milliseconds at 240. Anything that happens between two frames is not recorded directly.
Common Uses
- Finding events such as foot contact, toe-off, ball release, or impact
- Measuring joint angles at a defined position
- Estimating durations by counting frames between two events
- Checking the order in which body segments move
- Comparing the same phase across repetitions or sessions
Timing From Frames
A duration can be estimated by dividing the number of frames between two events by the frame rate. For example, 9 frames at 120 frames per second is 0.075 seconds. The result can only be as precise as the frame interval, so a short event such as sprint ground contact is captured in just a few frames at 30 frames per second but many more at 240.
Practical Tips
- Use the highest frame rate that the available light supports for fast movements
- Define events consistently, for example the first frame in which the foot visibly contacts the ground
- Keep the device stable so changes between frames come from the athlete, not the camera
- Check that footage was recorded at a real high frame rate rather than slowed down from a lower one
Common Pitfalls
Motion blur can make the exact frame of an event ambiguous, and compressed or edited video may drop or duplicate frames. Some slow-motion effects generate interpolated frames that look smooth but are estimates rather than recorded samples, so they should not be used for timing.
KineVision includes frame-by-frame scrubbing alongside slow-motion playback, which helps locate the frames that define a movement before drawing or comparing.