Initial Contact — Knee Flexion + Stride Angle

Why two readings, one drawing

The shank line (knee → ankle) carries the information for both measurements:

• Knee flexion is the rotation-invariant angle between the thigh and shank — how much the leg is "loaded" at touch-down.
• Stride angle is the inclination of the shank alone vs vertical — how far the foot lands ahead of (or behind) the body's centre of mass.

Asking the analyst to mark the thigh and shank once gives both readings at the same frame with the same landmarks. The standalone Stride Angle tool is deprecated because this composite replaces it.

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A. Knee Flexion at IC

What it measures. The angle between the thigh vector (hip → knee) and the shank vector (knee → ankle), reported as the interior joint angle (180° = a straight leg; a smaller number means a more flexed knee). Rotation-invariant — the value is the same regardless of which lateral side the camera sat on, so no auto-mirror applies.

Target range: 160° – 170° included (≈10°–20° of bend).

What good looks like. A small bend at the knee. The leg looks "soft", not locked. The quadriceps + patellar tendon are pre-loaded to absorb the impact rather than transmitting it straight up the kinetic chain.

Common faults

• Stiff knee at IC (> 177°) — running on a locked leg. The strike is invariably heel-first and the impact pulse runs straight up the tibia and femur. Common in runners who haven't built eccentric quad strength.
• Excessive flex (< 160°) — collapsing onto the leg as it lands. Often paired with overstride (see below) and a high vertical oscillation.

Coaching cues

• "Soft landings." Conscious cue to bend the knee on contact.
• Strength work: single-leg squats, step-downs build the eccentric quad capacity that gates this angle.

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B. Stride Angle at IC

What it measures. The tilt of the shank (knee → ankle) off vertical. The arc anchors at the foot with a dashed vertical reference through the ankle so the analyst can see exactly what the angle is measured against. Axis-relative — the auto-mirror system flips the sign when the camera sat on the opposite-side lateral so the convention stays standard.

Target range: 4° – 8° forward of vertical (foot lands slightly behind the knee).

What good looks like. The foot lands close under the body's centre of mass — a small forward shank tilt of 4°–8° from vertical. The knee sits just ahead of the foot at touch-down, which lines the leg up to roll over the foot smoothly into midstance.

Common faults

• Excessive forward shank tilt (> 10°) combined with the foot reaching out ahead of the body — the analyst will see the foot land ahead of the knee. This is overstriding: long reach, heel-first landing, big braking force on every step. Often paired with low cadence (< 170 spm).
• Backward shank tilt (foot lands behind a vertical line through the knee) — uncommon but seen in runners who consciously try to "land under the hip" and over-correct.
• Asymmetry between L and R — frequently traces back to a hip flexor / glute imbalance.

Coaching cues

• "Land under the hip" — fixes both knee-stiffness and overstride simultaneously by changing where in the gait cycle the foot makes contact.
• Increase cadence by ~5%. Shorter steps naturally bend the knee more and reduce shank reach.
• Watch the arms — over-reaching arms (across the midline) almost always pair with over-reaching feet.

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How the two relate

The same physical configuration produces both readings, but each captures a different concern:

Reading	What it tells you
Knee flexion low + Stride angle low	Stiff legged, landing under the body. Impact absorption is poor; check vertical oscillation.
Knee flexion low + Stride angle high	Overstride with a locked leg. Worst combination — long braking force on a stiff lever.
Knee flexion high + Stride angle low	Soft, well-flexed landing under the body. The "look" the cues above are aiming for.
Knee flexion high + Stride angle high	The leg reaches and collapses. Often a sign of hip flexor weakness or hip-drop on the contralateral side.

Related measurements

• Midstance Knee Flexion — same geometry, measured at peak load. Reads the absorption phase of the same spring action.
• Foot Strike Angle — heel / midfoot / forefoot at the same frame.
• Hip Extension at Push-Off — the next phase in the same step.