Ground Reaction Force
Also known as: GRF, ground force
Ground Reaction Force (GRF) is the force the ground exerts back against an athlete's feet — the foundational energy input for the kinematic chain in every swing sport.
By Newton's third law, when an athlete pushes into the ground, the ground pushes back. Skilled athletes learn to leverage this: golfers shift weight into the lead foot during the downswing, generating GRF that feeds into hip rotation; batters stride and push off the back foot; tennis players use leg drive to power the serve. GRF cannot be measured from video alone — it requires force plates — but its effects (weight transfer, vertical ground force timing, lead-knee extension) are observable from pose estimation and labeled as estimates when SwingVantage infers them.
Example
A golfer whose weight stays evenly distributed through impact never fully loads the ground, limiting the GRF available to drive rotation and reducing club speed.
Related terms
- Kinematic ChainThe kinematic chain is the sequence of body segments — from ground to tip — through which force and speed are transferred in a swing, with each segment's energy amplifying the next.
- Movement SequencingMovement sequencing is the timed order in which body segments accelerate and decelerate through a swing — correct sequencing multiplies speed; incorrect sequencing bleeds it.
- BalanceBalance in a swing context is the ability to maintain a stable base and controlled center of mass through the full motion — from setup through the end of the follow-through.
- Stability vs MobilityStability vs mobility is the biomechanical principle that joints alternate between roles — some must be stable anchors, others must be mobile movers — and faults often come from reversing these roles.
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