Why the Forehand Is Hard to Self-Diagnose
In a typical match you hit more forehands than any other stroke. That volume is exactly why the forehand resists improvement: you have repeated the same pattern thousands of times, and the pattern feels normal from the inside. What feels like a full shoulder turn may be 15° short. What feels like clean extension at contact may be a bent, crowded arm.
The other problem is speed. A forehand from preparation to follow-through takes well under a second. Contact itself occupies a single frame of video. You cannot see your own contact position while hitting, and neither can a coach standing ten feet away — not with the precision needed to say whether your elbow was at 150° or 170°.
So most forehand advice stays at the level of outcome: the ball sailed long, the ball found the net, you were late. Useful, but it describes the symptom rather than the cause. Measurement closes that gap.
The Six Phases of a Forehand
Tennis HUD segments every forehand automatically — you do not mark anything by hand. Each phase is located from the pose data itself, so the boundaries reflect what your body actually did rather than a fixed time slice.
Ready
Your split-step and neutral stance before the ball is read. Establishes the baseline the rest of the swing is measured against.
Preparation
Unit turn and shoulder rotation away from the net. This is where most power is won or lost, long before the racket moves forward.
Backswing
The racket travels back and down. Depth and shape here set up the low-to-high path through contact.
Forward Swing
The kinetic chain unwinds — legs, hips, trunk, shoulder, arm. Sequencing matters more than raw effort.
Contact
The single most important frame in the stroke. Arm extension and contact position are measured precisely here.
Follow Through
Deceleration and finish. A truncated follow-through usually signals a problem earlier in the chain.
What Gets Measured
Pose estimation tracks 33 body landmarks on every frame. From those landmarks Tennis HUD derives the measurements that map onto things a coach would actually tell you to change:
Shoulder rotation
Peak rotation between preparation and contact, in degrees. Under ~25° typically means you are arming the ball instead of turning into it.
Knee bend
Average bend on your hitting-side leg through the preparation phase. Straight legs cap how much upward drive you can generate.
Arm extension at contact
Elbow angle on the exact contact frame. Cramped contact — a heavily bent elbow — costs both pace and consistency.
Swing path
The trajectory angle of the racket through the hitting zone. A negative value means you are swinging high-to-low, which kills topspin.
Separation (X-factor)
The gap between shoulder and hip rotation, which stores elastic energy. Only reported when you film from behind — the angle is not recoverable from a side view.
Filming a Forehand That Analyses Well
A phone is enough. Resolution matters far less than angle and framing, and Tennis HUD detects both your handedness and the camera view automatically — you do not need to tell it which hand you play with.
Film from the side if you care most about swing path, contact position, and arm extension — a side view shows the low-to-high shape of the stroke clearly. Film from behind if you want rotation and separation, since shoulder-to-hip separation simply is not recoverable from a side angle.
Keep your whole body in frame through the entire stroke, including your feet. Landmarks that leave the frame cannot be measured, and a clipped follow-through weakens the phase detection for everything before it.
From Numbers to Something Actionable
Raw angles are only useful if you know which ones matter. Every analysis produces a score for each area of the stroke plus written feedback that explains what the numbers mean and what to work on next — in plain language rather than joint-angle notation.
Because each analysis is stored, the more useful comparison is against yourself. Upload a forehand this month and again after six weeks of work, and the change in shoulder rotation or contact extension is a number rather than an impression.
The same pipeline analyses your backhand and your serve. If you want to see the output before uploading anything, the sample analysis walks through a full annotated breakdown.