Academy / Kinetics
Inverse dynamics in one page
From ground reaction force to a joint moment — the free-body diagram students actually need. · 8 min read
The idea
Forward dynamics asks: given muscle forces, how does the body move? Inverse dynamics asks the lab question: given the motion and the external forces, what net joint moment must have been present?
Start at the ground. A force plate gives GRF and centre of pressure. The foot has mass, acceleration, and a known anatomy. Newton–Euler gives the ankle moment. Walk up the chain: ankle → knee → hip. Each step uses the distal moment as an input.
What Kineos estimates
Without a plate, GRF is reconstructed from COM acceleration (GRF ≈ m(g + a_com) during stance, 0 in flight). That is a single resultant, not a two-foot split, and it ignores horizontal shear. Treat peak GRF in BW as an order-of-magnitude teaching signal.
Joint moments are not displayed as gold-standard kinetics. Where a value is modelled, the report says so. That is a design rule, not a disclaimer in six-point type.