Glossary · Physics simulation
Tendon path
Also called spatial tendon, muscle path, via point, wrapping surface, muscle wrapping, line of action
The route a simulated muscle takes over the bones: anchored at its origin and insertion, through via points, and wrapped round bones, so its length and lever change as the joints move.
A real muscle doesn’t pull in a straight line from bone to bone. Musculoskeletal models describe it as a path: an origin on one bone, an insertion on another, via points it must pass through on the way, and wrapping surfaces (cylinders, spheres) it slides over where it curls round a bone or a joint, like the quadriceps over the knee.
The simulator works out the path’s length at every pose . Two things follow from it:
- the muscle’s length, which sets its force (force–length–velocity);
- its moment arm on each joint it crosses, .
That is the difference from a joint muscle, which acts straight on one joint with a fixed lever. Paths give muscles across two joints, levers that change with the angle, and the real body’s weak and strong positions.
In this project
In stage 2 of Muscles instead of motors the leg muscles of MyoSuite were moved onto the fighter’s legs: every point mapped bone by bone and scaled to his segment lengths (about 7 % longer legs than MyoSuite’s). MyoLeg26 has 26 paths with via points; the full MyoLeg has 80, with 15 wrapping objects a leg. MyoLeg26’s knee uses moving points that slide with the angle; ours replaced them with a 4.2 cm cylinder the quadriceps wrap over.