Op-amp instrumentation amplifier is initial input stage of EKG and EEG instruments

Op-amp instrumentation amplifier is initial input stage of EKG and EEG instruments due to it's high impedance, very high common-mode rejection ratio, and very high voltage gain. 
Read more:
http://www.cirvirlab.com/index.php/tutorials/162-op-amp-instrumentation-amplifier.html
and simulate online:
http://www.cirvirlab.com/simulation/op-amp-instrumentation-amplifier-online.php