Academy / Kinematics
Joint angle conventions
How Kineos reports flexion, extension and the ISB sign convention — and why 0° is never ‘straight’ in every joint. · 6 min read
What the number means
A joint angle is the relative orientation of two segments. Kineos uses a clinical flexion-positive convention for the hip, knee and ankle: 0° at the knee is full extension (segments colinear), 90° is a right angle. The trunk angle is inclination of the thoracolumbar segment from vertical, not lumbar lordosis.
This is close to how a goniometer is taught in clinic, and different from some ISB cardan sequences used in 3D labs. When you export CSV, the column headers name the convention so a methods section can cite it.
Finite differences and smoothing
Angular velocity is a first difference of angle with respect to time. Raw differences amplify landmark jitter, so the Capture Lab applies a light sliding window before derivatives. Peak velocity is therefore a modelled estimate — labelled as such — not a hardware gyro.
If you collect with a camera, frame rate sets the Nyquist limit. 30 Hz will miss the fastest part of a drop-jump contact. Prefer 60 Hz or the kinematic engine at 60–120 Hz for teaching traces.