What this stroke shows
The reference is a forehand by one player in the 3DTennisDS motion-capture set. The player is left-handed; the picture can mirror the stroke to right-handed. The legs and hips turn first, the chest opens, and the arm and racket are pulled through last, so the racket head meets the ball in front of the body at about 25 m/s (55 mph) and the ball leaves at about 40 m/s.
Change the body
On these cards the racket is only 5–30% slower, yet most balls no longer land in. A timing change costs the face and the ball as well as speed.
- Pushing with the arm, 60 ms before the hips and chest: about 15% slower, and the ball does not clear the net.
- Legs half a beat late: about a fifth slower at 60 ms; each 10 ms of delay costs about 3%.
- Chest stays closed: a good 10% slower, into the net.
- Hips turn half: about 27% slower, and the ball flies long. If the chest still turned as far, the loss would be only 1–4%: hip rotation matters through the chest it carries.
- Wrist released early (40 ms): about 18% slower, into the net.
- Wrist not laid back: barely slower (2–5%), but into the net. Laying the wrist back sets the racket face more than the speed.
Change the racket at contact
- Face 5° more open: the ball clears the net by 1 m and lands 12 m long, with two thirds less topspin. Face 5° more closed: into the net.
- Swinging 5° more upward: about 40% more topspin, just long. Swinging 5° flatter: 40% less topspin and a faster ball, into the net.
- Contact 3 cm towards the tip: only about 1% slower. At 8 cm the ball is 8% slower.
- A slice at contact gives about 2,300 rpm of backspin and a low ball at 26 m/s instead of 40. This forehand cannot be bent into a real slice, though: a slice is a different swing from the start.
Change the grip or the contact point
These cards keep the racket speed and face and show the effort instead. An eastern grip raises the wrist’s load from 53% to over 300% of its measured strength (above about 200% the model overstates it, so read it as clearly harder). A western grip raises it to 107%, and meeting the ball 20 cm further forward to 160%.
Limits
One player, one stroke and a typical incoming ball rather than the one this player received. Where a card says the landing is hard to call, the model cannot say. Use the numbers to compare strokes with each other.