Link Segment Inverse Dynamics

Forces act along a line (push or pull); moments (torques) cause rotation about an axis. In a knee during jumping, vertical forces compress the joint; internal moments rotate the tibia relative to femur. Both are important: high compression forces and high rotation moments both indicate high loading.

Accuracy is typically ±10-15% for moment magnitude, depending on marker placement error, mass estimation, and soft tissue artifact. Relative comparisons (left vs. right, technique A vs. B) are more reliable than absolute values. Always report uncertainty ranges.

2D inverse dynamics is possible but less accurate, limiting analysis to movement in single plane. 3D motion capture (6+ cameras) is preferred but more expensive. For gross assessment, calibrated 2D video can suffice; for detailed research, 3D is necessary.

Heavier individuals typically experience higher absolute joint moments due to larger inertial forces. Relative comparisons should normalize by body mass or body weight. Expressing moments as multiples of body weight (Nm/kg or %BW) enables cross-subject comparison.

Not always. Strong athletes can produce high joint moments safely. High moments relative to individual capacity (determined by strength testing) are more concerning than absolute magnitude. Context (movement velocity, intended demand) matters; same moment may be normal during sprinting but excessive during recovery.