Insight Charts
Biomechanics Guide
TorqAI tracks four key biomechanical patterns in every stroke. Here's what each chart measures and how to read it.
The Four Insights
Kinetic Chain
Tracks how energy transfers sequentially from your hips through your shoulders to your elbow — the foundation of an efficient, powerful stroke.
Hips
Shoulder
Elbow (primary)
Peak markers (H/S/E)
Prep phase (blanked)
Formula:
Rating scale:
• Good: ≥ 4 frames delay (clear sequential transfer)
• OK: 2–3 frames delay (some separation)
• Needs work: 0–1 frames or reversed order (no chain)
$$v_{\text{segment}}(t) = \lVert \text{joint}(t) - \text{joint}(t\!-\!1) \rVert \times \text{fps}$$
Each segment (hip center, dominant shoulder, dominant elbow) velocity is smoothed with a 3-frame moving average. Chain delay = frames between hip peak and elbow peak.
Rating scale:
• Good: ≥ 4 frames delay (clear sequential transfer)
• OK: 2–3 frames delay (some separation)
• Needs work: 0–1 frames or reversed order (no chain)
Good
Hips peak first, then shoulders, then elbow — with clear sequential delay.
Needs work
All segments peak together — arm-only swing without ground-up power.
Racket Drop
Measures how far the racket head drops below your wrist before contact. A good drop creates the "lag" that generates racket-head speed and spin.
Racket position vs wrist
Zero line (wrist level)
Deepest drop point
Formula:
Rating scale (drop depth):
• Good: > 5 units below wrist (strong lag)
• OK: 2–5 units (minimal drop)
• Needs work: < 2 units (no drop, pushing)
$$\text{drop}(t) = \big(\text{wrist}_y - \text{racket\_center}_y\big) \times 100$$
Racket center detected per frame by YOLO. Positive = racket above wrist; negative = racket dropped below. The deepest negative value indicates maximum lag.
Rating scale (drop depth):
• Good: > 5 units below wrist (strong lag)
• OK: 2–5 units (minimal drop)
• Needs work: < 2 units (no drop, pushing)
Good
Deep drop below wrist — good lag and whip effect for power and spin.
Needs work
Little or no drop — pushing through the ball instead of swinging.
Contact Spacing
Measures how far your hitting arm extends from your body at contact. Proper spacing lets you swing freely and make clean contact in your power zone.
Extension ratio
Peak extension
Good zone (≥ 1.3x)
Prep phase (blanked)
Formula:
Rating scale (peak extension):
• Good: ≥ 1.3x hip width (comfortable spacing)
• OK: 0.9–1.3x (slightly cramped)
• Needs work: < 0.9x (ball too close to body)
$$\text{spacing}(t) = \frac{\lVert \text{wrist} - \text{hip\_center} \rVert}{\lVert \text{L\_hip} - \text{R\_hip} \rVert}$$
Wrist-to-hip-center distance normalized by hip width. This makes the metric body-size independent. First 40% of clip is blanked (preparation phase). Peak searched in 50–80% window (forward swing). Clamped at 3.5x max.
Rating scale (peak extension):
• Good: ≥ 1.3x hip width (comfortable spacing)
• OK: 0.9–1.3x (slightly cramped)
• Needs work: < 0.9x (ball too close to body)
Good
Peak extension ≥ 1.3x hip width — comfortable spacing with room to swing.
Needs work
Extension < 1.3x — cramped, ball too close to body.
Unit Turn
Measures shoulder-over-hip rotation during the backswing. A strong unit turn coils your torso like a spring, storing rotational energy for the forward swing.
Hip-shoulder separation
Peak rotation
Prep phase
Formula:
Rating scale (peak separation):
• Good: ≥ 30° (strong coil)
• OK: 15–29° (moderate turn)
• Needs work: < 15° (minimal rotation)
$$\theta_{\text{sep}}(t) = \big\lvert \angle(\text{L\_shoulder}, \text{R\_shoulder}) - \angle(\text{L\_hip}, \text{R\_hip}) \big\rvert$$
Angle of each line (shoulders and hips) computed with $\text{atan2}$ in the image plane. The difference shows how much the shoulders have rotated beyond the hips. Smoothed with a 3-frame moving average. Peak searched in first 60% of clip.
Rating scale (peak separation):
• Good: ≥ 30° (strong coil)
• OK: 15–29° (moderate turn)
• Needs work: < 15° (minimal rotation)
Good
30°+ separation — strong coil with stored energy for an explosive swing.
Needs work
< 15° separation — minimal rotation, losing rotational power.
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