Z Polarization in $e^+e^-to tbar t Z$ for testing the top quark mass structure and the presence of final interactions

We show that the measurement of the $Z$ polarization in the $e^+e^-to tbar t Z$ process would allow an interesting determination of the role of the top quark mass. This can be used for testing the possibility of top compositeness or of the occurence of final state interactions related to the mass generation in particular the interaction with dark matter.