AD620 Instrumentation Amp EMG circuit
The extra circuitry in green is to ensure that the optimum common mode voltage (for making accurate measurements) is "applied" to the body to which measurements are being made. This is done by using the "leg electrode". It's sometimes called the driven right leg circuit.

See this wikipedia page for more details: -


  A Driven Right Leg Circuit or DRL circuit is an electric circuit that
  is often added to biological signal amplifiers to reduce Common-mode
  interference. Biological signal amplifiers such as ECG
  (Electrocardiogram) EEG (Electroencephalogram) or EMG circuits measure
  very small electrical signals emitted by the body, often as small as
  several micro-volts (millionths of a volt). Unfortunately, the
  patient's body can also act as an antenna which picks up
  electromagnetic interference, especially 50/60 Hz noise from
  electrical power lines. This interference can obscure the biological
  signals, making them very hard to measure. Right Leg Driver circuitry
  is used to eliminate interference noise by actively cancelling the
  interference.