Glossary · Biomechanics
Range of motion
Also called ROM, joint limits, joint range
How far a joint can move, in degrees, from one end of its travel to the other.
Each joint has a range it can move through before bone, ligament or muscle stops it. In a simulation these become joint limits: hard stops the physics won’t let the joint pass.
Typical adult ranges, as listed by the American Academy of Orthopaedic Surgeons (1965):
| joint and movement | range |
|---|---|
| knee flexion | 0–135° |
| hip flexion | 0–120° |
| elbow flexion | 0–150° |
| shoulder flexion | 0–180° |
| ankle dorsiflexion / plantar flexion | 20° / 50° |
Why limits matter in learning
Without realistic limits, a policy will happily use impossible poses if they help: knees bending backwards, a trunk folded in half. These look broken and couldn’t be performed by a person.
Limits also interact with the policy’s corrections. A large correction can push a joint into its stop, where it stays until something pulls it back (see action clipping). That’s one reason residual ranges in our survive mode are “limited by the joint ranges”.
Real people vary a lot. Fighters are often far more flexible in the hips than average, which matters for high kicks. Limits are one more place where the body model makes a choice about who it represents.
In this project
The muscular body’s 83 joints use ranges from the AAOS tables.