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Checking your tennis serve with slow motion and angles

How to break a serve video into its key positions, measure elbow and knee angles, and compare your timing with a reference serve.

3 min read

The serve happens too fast to feel. The whole motion takes around a second and the racket meets the ball for a few thousandths of a second. Slow motion lets you see the positions your body passes through, and measuring a few angles turns vague feelings into something you can check.

Film it properly

Film from the side, at about hip height, with the phone still. A view from behind the baseline is also useful for the toss and the direction of the swing. Use the highest frame rate your phone offers. At 30 fps you may get only one frame near contact; at 120 or 240 fps you get several.

Break the serve into positions

Step through the video frame by frame and mark these moments:

With markers on each position you can jump straight to any of them and compare one serve with another.

Measure the knee bend

Pause at the trophy position and measure the angle at the front knee, from hip to knee to ankle. Very straight legs mean little drive from the ground; a deep bend that is not used to push upwards does not help either. More useful than a single number is consistency: measure the same angle on several serves and see whether it changes.

Check the elbow

At the trophy position the hitting elbow is usually bent and roughly at shoulder height. At contact the arm is close to fully extended. Measure the elbow angle at both moments. An elbow that stays low or a bent arm at contact often goes together with less height on the ball.

Look at the toss

Pause at the moment the ball is highest and draw a line straight down from it. Where does it land relative to your front foot? A toss that drifts behind your head or far to the side forces the body to adjust in the middle of the swing. Comparing several serves shows whether the toss is consistent.

Compare timing with a reference

Put your serve next to a reference serve filmed from a similar angle. Sync them on the toss release and play both slowly. Watch when the knees bend, when the racket drops and when the hips rotate. Differences in timing are often more important than differences in a single position.

Measure how long the motion takes

Time a segment, for example from toss release to contact. Doing this on several serves shows whether your rhythm is steady. A rushed serve often has a shorter gap between the toss and the swing.

Practise with what you find

Choose one thing, such as a more consistent toss, and work on that only. Film again after the session and compare with the first video. Seeing the change is motivating, and seeing no change tells you to try a different drill.

Angles from a phone video are approximate: perspective and camera position affect them. They are best used to compare your own serves filmed the same way, not as exact measurements.

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