Okay , i assume you're already aqcuiring this signal okay. So I'd go to the library browser , under simulink\sinks you will find the "to workspace block". Wire your signal to this block . Change block settings to output a vector. The block stores this vector in the workspace called simout (you can change the name if you want) . Now after you have finished collecting data you can go to the Matlab workspace and say :
mylast50 =simout((length(simout)-50): length(simout))
thats how i'd roll ... let me know if it works ..
Go MIZZOU mechanical !!!