Glossary · Biomechanics
Hyperextension
Also called hyperextend, hyperextended, ligament, ligaments, joint lock
A joint straightened past its normal range. Ligaments and bone stop it; pushed further, they tear or break.
Every joint has a range of motion. Near its end the muscles can no longer hold the joint, and what stops it is passive: ligaments, the tough bands that tie bone to bone, and in the end the bones themselves. Straightening a joint past that end is hyperextension. A little stretches the ligaments; more sprains or tears them, and at the extreme the joint dislocates or a bone breaks.
In combat sports this is the point of a joint lock. An armbar in Brazilian Jiu-Jitsu levers the elbow straight against the hips until it would hyperextend, and the opponent taps before the ligaments go. In striking, a kick or punch that is blocked or misses can hyperextend a knee or an elbow too.
In this project
In Muscles instead of motors every joint’s range is a hard limit in the simulation: the body can rest on it with any force and nothing is ever damaged. In stage 2 the policy found exactly that, a rear leg held straight against its hip and knee limits with the muscles idle, and a cost for resting near the limits was added to stop it. Ligaments that strain and can be injured are not modelled yet.