RunningForm uses biomechanics reference ranges derived from peer-reviewed sports science research. This page documents the evidence behind each metric, our measurement methodology, and known limitations. We believe in transparency about what the science supports and where our analysis is approximate.
All biomechanics measurements are computed client-side from 2D video using MediaPipe Pose Landmarker. They are estimates, not lab-grade measurements. Reference ranges are pace-adjusted across three tiers: easy (> 6:00/km), tempo (4:30-6:00/km), and fast (< 4:30/km). Metrics tagged rear-view require an optional second clip filmed from behind the runner; without it, the analysis runs from the side view alone.
Unit: % of estimated body height (shoulder-to-ankle distance)
easy
< 5.5% good, < 8.0% moderate
tempo
< 6.0% good, < 8.5% moderate
fast
< 6.5% good, < 9.0% moderate
Computed as the total range (max minus min) of hip-to-ankle vertical distance across all analyzed frames, divided by the average shoulder-to-ankle distance. Using hip-minus-ankle cancels vertical camera panning. This total-range method yields higher values than per-stride averages reported by wearables like Garmin.
Lower vertical oscillation is beneficial for running economy, though few studies have manipulated VO as an independent variable.
5-10 cm vertical oscillation promoted proper running form and may mitigate injury risk.
Higher vertical oscillation moderately associated with higher energetic cost (r = 0.35).
Color zones: excellent < 6.8 cm, good 6.8-8.9 cm, fair 9.0-10.9 cm, poor 11.0-13.0 cm.
Unit: degrees forward from vertical
easy
2-8° good, 0-12° moderate
tempo
3-9° good, 1-13° moderate
fast
3-10° good, 1-15° moderate
Computed as the angle from vertical of the line connecting shoulder midpoint to hip midpoint, averaged across all frames with valid pose landmarks. Lean source (ankles vs. waist) is determined by comparing upper-body lean to lower-body lean.
Elite runners maintained ~3° lean across all speeds (12-20 km/h). Recreational runners increased from ~5° to ~7.5°.
Most economical group ran with ~5.9° trunk flexion; least economical at ~2.4°.
Running at ~8° lean increased metabolic cost by 8% vs. upright.
More upright trunk posture correlated with better performance across 97 endurance runners.
Increasing trunk flexion by ~7° reduced knee extension moment by ~7% but increased hip extensor demand.
Unit: % of estimated body height (shoulder-to-ankle distance)
easy
< 5% good, < 9% moderate
tempo
< 7% good, < 11% moderate
fast
< 9% good, < 13% moderate
Computed as the absolute horizontal distance between hip midpoint and the visible-side ankle at ground contact frames, divided by the shoulder-to-ankle body height estimate. Expressed as a percentage.
Overstriding results from foot position relative to COM, creating braking impulse proportional to foot-ahead distance.
Elite marathoners land ~0.30 m ahead of COM. Rearfoot vs. non-rearfoot difference was only 0.03-0.04 m — described as "too small to be meaningful."
Every +5 strides/min cadence increase reduced foot-ahead position by ~5.9%.
"There are no cutoffs at which this distance is determined to be abnormal."
Defines overstriding as horizontal distance between greater trochanter and lateral malleolus at foot contact.
Unit: steps per minute (spm)
easy
165–180 spm good, 155–190 spm moderate
tempo
170–185 spm good, 160–195 spm moderate
fast
175–195 spm good, 165–205 spm moderate
Estimated from stride times detected in the gait cycle analysis. A full gait cycle (same-foot to same-foot) covers 2 steps, so cadence = 120 / average_stride_time_seconds. Accuracy depends on detecting at least 2 clean gait cycles.
Increasing step rate 5–10% above preferred reduced energy absorption at the hip and knee, decreased braking impulse, and reduced center-of-mass vertical excursion — without changing speed. A low-barrier, evidence-backed intervention.
Cadence below 160 spm was associated with overstriding patterns across injured runners. Higher cadence correlated with reduced peak hip adduction.
Elite marathoners averaged 180–185 spm across both sexes at race pace.
Unit: degrees from full extension
easy
15–30° good, 10–35° moderate
tempo
15–30° good, 10–35° moderate
fast
15–30° good, 10–35° moderate
Computed as (180° − interior hip-knee-ankle angle) at each detected initial-contact frame, using MediaPipe's 3D worldLandmarks rather than image-space coordinates. Reported as the average across stance phases. Camera-independent: image-space angles bake in perspective and aspect ratio; worldLandmarks do not.
Knee flexion at initial contact was one of five 2D-video sagittal kinematic variables in a regression model predicting vertical average loading rate (R² = 0.51).
Trained runners typically present 20–30° knee flexion at initial contact; values below ~15° are associated with high loading rate and an overstride pattern.
Unit: degrees from full extension
easy
40–50° good, 35–55° moderate
tempo
40–50° good, 35–55° moderate
fast
40–50° good, 35–55° moderate
For each detected stance phase (initial contact → toe-off), we compute knee flexion at every visible frame and take the maximum, then average those peaks across stance phases. Uses worldLandmarks for camera independence.
"Normal peak knee flexion approaches approximately 45° at midstance." Lower values associated with patellofemoral pain; higher values with quad fatigue.
Prospective evidence linking reduced midstance knee flexion to PFP development.
Unit: degrees from vertical (absolute)
easy
0–8° good, 0–15° moderate
tempo
0–10° good, 0–17° moderate
fast
0–12° good, 0–18° moderate
Absolute angle of the shank (knee → ankle vector) from vertical in the sagittal plane at initial contact, using worldLandmarks. Direction-agnostic: we measure magnitude, not signed angle, so the metric is identical whether the runner moves left-to-right or right-to-left across the frame.
Vertical shank at initial contact is the clinical sagittal-plane overstride marker; deviation from vertical increases braking impulse and tibial bending moment.
Lower-leg angle at initial contact was a predictor in the 5-variable GRF/loading-rate regression (R² = 0.51 for loading rate).
Unit: degrees behind vertical (absolute)
easy
10–20° good, 5–25° moderate
tempo
10–22° good, 5–27° moderate
fast
12–25° good, 7–30° moderate
Absolute angle of the thigh (hip → knee vector) from vertical in the sagittal plane, measured at each detected toe-off frame using worldLandmarks. Direction-agnostic. Averaged across detected stance phases.
Trained runners typically extend the hip 10–20° behind vertical at toe-off; <5° suggests restricted hip extension, often secondary to hip-flexor tightness.
Unit: flight time / contact time (dimensionless)
easy
0.30–0.70 good, 0.20–0.90 moderate
tempo
0.45–0.85 good, 0.30–1.05 moderate
fast
0.60–1.05 good, 0.45–1.30 moderate
Derived from contact-time and stride-time measurements: for a full gait cycle, flight time = stride/2 − contact, and the ratio is flight ÷ contact. This is the body-mass-free form of Morin's spring-mass model — when scale calibration is unavailable, t_flight/t_contact tracks the same direction of change as absolute leg stiffness in kN/m.
Sine-wave approximation of vertical stiffness from contact and flight times — the foundational method we derive from.
Vertical stiffness r=−0.31 and leg stiffness r=−0.28 with energy cost — strongest individually validated correlates. Duty factor was non-significant (r=−0.06).
Low duty-factor runners exhibit stiffer legs; high duty-factor runners use longer ground contact with a softer leg. Equally efficient at endurance speeds.
Unit: degrees (interior shoulder–elbow–wrist angle)
easy
80–110° good, 70–120° moderate
tempo
80–105° good, 70–115° moderate
fast
80–105° good, 70–115° moderate
Mean interior elbow angle on the visible (near) arm across all frames where shoulder, elbow and wrist are visible. Computed from worldLandmarks for camera independence.
Maintained arm swing and arm posture were among the technique variables correlated with running economy.
Recommends ~90° elbow flexion as the clinical reference posture for the running arm.
Unit: % of estimated body height
easy
25–65% good, 18–80% moderate
tempo
30–70% good, 22–85% moderate
fast
35–75% good, 25–90% moderate
Front-to-back excursion of the near-arm wrist across the capture window, expressed as a percentage of estimated body height. Uses image-space X relative to the shoulder midline (which cancels camera pan) since worldLandmarks are hip-centred and oscillate around zero by construction.
Active arm swing reduces metabolic cost of running ~3–5% relative to running with arms restricted.
Maintained arm swing benefits running economy across recreational and trained runners.
Unit: coefficient of variation, % (SD ÷ mean × 100)
all
< 8% good, < 15% moderate, > 15% significant
For each kinematic metric with per-stride samples (trunk lean, knee flexion at IC, peak knee flexion, tibial inclination, hip extension, elbow angle), we compute the coefficient of variation across detected strides and attach it to the metric. Requires ≥3 valid samples.
Cited for the framing principle: spatiotemporal between-stride variation is not, on its own, a validated injury predictor in recreational runners.
Unit: degrees
easy
0–4° good, 0–7° moderate
tempo
0–4° good, 0–7° moderate
fast
0–5° good, 0–8° moderate
Pelvic tilt of the swing side relative to the stance side at midstance, measured from the rear-view clip when one is provided. Averaged across detected stance phases.
Contralateral pelvic drop was the kinematic variable that best discriminated injured runners. Injured runners averaged ~7–10°; healthy ~4–5°.
Unit: degrees
easy
0–8° good, 0–12° moderate
tempo
0–8° good, 0–12° moderate
fast
0–10° good, 0–14° moderate
Peak femur tilt toward the body midline during stance, measured from the rear-view clip. Averaged across detected stance phases.
Peak hip adduction was elevated in injured runners across PFP, ITBS and MTSS cohorts.
Elevated peak hip adduction prospectively predicted PFP and ITBS development.
Unit: % of pelvic width (hip-to-hip distance)
easy
8–25% good, 4–32% moderate
tempo
8–25% good, 4–32% moderate
fast
6–25% good, 3–32% moderate
Lateral distance between contralateral midstance foot positions, expressed as a percentage of pelvic width. Computed from the rear-view clip.
Narrow step width (toward crossover gait) increased peak iliotibial band strain — a mechanical link to ITBS development.
Many running coaches suggest shortening or lengthening stride length as a coaching cue. RunningForm deliberately does not prescribe a target stride length, and the science is clear on why.
Cavanagh & Williams (1982) showed that trained runners self-select a stride length that is at or very near their individual metabolic optimum — the point where oxygen uptake is minimized for their speed. Deviating from preferred stride length in either direction (too short or too long) increased VO₂ by an average of 2.6–3.4 ml/kg/min. The body finds its own optimum through training adaptation.
What we do flag is overstriding — when the foot lands significantly ahead of the centre of mass, creating a braking impulse regardless of stride length. The fix is increasing cadence slightly or shortening the reach of the foot at contact, not prescribing a specific stride length target.
All subjects showed a U-shaped relationship between stride length and VO₂ with an individual optimum. Trained runners self-selected a stride length at or near their metabolic optimum.
This analysis is AI-generated and intended for educational purposes only. It is not a substitute for advice from a qualified running coach or physiotherapist. Biomechanics metrics are estimates from 2D side-view video, not lab-grade measurements.