Question(s) about modelling slack in the muscle-tendon unit

[Bas Van Hooren]

Hi,

Recent research found that slack can be present in both the fascicles and tendon/aponeurosis of the muscle-tendon unit and that the length at which slack is taken up can differ between both. I’m very interested in modelling the effects of slack on muscle fibre length changes during locomotion. For example, if slack is modelled, an increase in the distance between the attachment points of the muscle may not necessary represent active lengthening of the CE, but the uptake of slack. However, I’m not sure about the best way to model the effects of slack.

I’m currently thinking about two possibilities:
1. Simulating the electromechanical delay by providing the model with a time delay to account for slack (and tendon compliance). However, what should I use as value for tendon slack length? Should this be set to zero, since slack is already taken into consideration in the mechanical delay?
2. Providing the model with information about the muscle-tendon unit length at which the fascicles and tendon/aponeuroses fall slack. Does the model assume that the fascicles and tendon/aponeuroses fall slack at the same length when a value is provided for tendon slack length? Or does the model assume there is never slack in the fascicles?

I’m also wondering whether there has been research so far that has used real experimental data on slack length, rather than estimates?

Furthermore, some models need a minimum amount of activation. Does this affect slack? If the muscle is active, it cannot be slack since there will be some tension.

Finally, is the activation time constant the same as the electrochemical delay?

Bas