There are two options for default tendon slack length for semimembranosus. The value in the model and the paper is slightly adjusted from the value predicted by the fiber length-joint angle relationship (this fact is included in the paper that presents this model).
The adjusted value, 0.378, gives appropriate passive behavior in high hip flexion-low knee angle cases, such as the experiments used to obtain maximum isometric force curves. Hence the adjustment.
In lower hip flexion cases, such as walking, this value results in very short fibers. The value predicted by the fiber length-joint angle relationship (measured in cadavers with low hip flexion) is more valid.
This contradiction is likely due to the limits of a lumped parameter model and has a large effect due to the high PCSA of semimembranosus.
If this muscle is a part of your analysis, please consider which tendon slack length is right for you. The lines below can be added to the OpenSim muscle semimem, comment out the value you don't want to use.
- Edith
<!--Resting length of the tendon, as predicted by fiber length-joint angle relationship. MAY RESULT IN OVER STRETCHED FIBERS AND HIGH PASSIVE FORCES IN HIGH HIP FLEXION CASES -->
<tendon_slack_length> 0.34800000 </tendon_slack_length>
<!--Resting length of the tendon, adjusted from value predicted by fiber length-joint angle relationship to give more reasonable passive forces in high hip flexion cases-->
<tendon_slack_length> 0.37800000 </tendon_slack_length>