...
| Input Target Mass (TM) | Preserve Mass Distribution (PMD) | Scaled Model Mass |
|---|---|---|
| TM > 0 | PMD is checked | Each body mass is scaled by the product of its own scale factors, then each body is scaled again by the ratio of the target mass to the sum of the scaled masses. The scaled model mass will equal the input target mass. |
| TM > 0 | PMD is unchecked | The mass of each body is proportionally scaled by the ratio of the target mass to original mass. The scaled model mass will equal the input target mass. |
| TM <= 0 | PMD is checked | None of the body masses change. The scaled model mass will not have changed. |
| TM <= 0 | PMD is unchecked | Each body mass is scaled by the product of its own scale factors. The scaled model mass will equal the sum of all the newly scaled body masses. |
Step 4 - Scaling Muscles and Other Model Components that Depend on Length
Components in a model that depend on distances or lengths, like ligaments (as of version 3.3) and muscle actuators, are next updated. For example, a muscle’s new optimal_fiber_length and tendon_slack_length are computed during the scaling process. A scale factor is computed as the ratio of the length before scaling to the length after scaling and the result is used to scale the component's length-dependent properties. The process is complicated by the fact that parameters like a muscle's length are configuration dependent, thus OpenSim tries to maintain the model configuration across scaling.
...