Ground Reaction Force (GRF) is the force exerted by the ground on a body in contact with it, equal in magnitude and opposite in direction to the force applied by the body to the ground, according to Newton's third law of motion.
GRF has three components:
- Vertical Force: Perpendicular to the ground surface (typically the largest component)
- Anterior-Posterior Force: Forward-backward direction
- Medial-Lateral Force: Side-to-side direction
Key characteristics and applications:
- During standing still, GRF equals body weight
- During movement, GRF can be several times body weight (e.g., 2-3x during running, 5-8x during landing from a jump)
- GRF patterns provide insight into movement strategy, impact loading, and injury risk
Common GRF measurements in different activities:
- Walking: Characteristic double-hump pattern in vertical GRF
- Running: Single peak in vertical GRF during stance phase, typically 2-3x body weight
- Jumping/Landing: High impact peaks that can exceed 5x body weight
- Cutting Maneuvers: Large lateral forces combined with vertical forces
GRF is typically measured using force plates embedded in the ground or pressure sensors in footwear. While video analysis alone cannot directly measure GRF, it can be combined with inverse dynamics calculations to estimate forces when body segment kinematics and anthropometric data are known.
Understanding GRF patterns helps:
- Identify injury risk factors (especially for stress fractures, ACL injuries)
- Optimize performance (maximizing force application in desired directions)
- Evaluate rehabilitation progress (return to normal loading patterns)
- Design better footwear and surfaces