I need help to findout the mechanism

Dear friends, I have a question regarding to below chest compression system.


[IMG]http://www.engineersedge.com/engineering-forum/attachment.php?attachmentid=1698&stc=1[/IMG]


What is the mechanism of compression? it is used a hydraulic system or a step motor (considering the volume of device)?
As you know for CPR it is needed to compress the chest about 100 times/min for 2-2.5 in and we need significant force (about 500 N).
Please respond, because I need to know for one of my projects for using similar system.

Probably an electric motor, gear, eccentrically located pin that drive a linearly channeled rod or similar.

Thank you for your reply.
probably you are right, but can you help me which type of an electric motor (step motor) can produce this amount of torque and rotate 100 rpm?!