Now, we add C1 which decouples the 1st and 2nd stage amplifier. It filters out any DC from from by the instrumentation amplifier. Otherwise the gain of 100 of the OP-amp would amplify the DC further and would drive it into saturation. The cutoff frequency is at f=1/(2 pi RC)=0.2Hz but note that it's a first order highpass so that the transition is very wide.
Note that the value of 1uF is OK for EMG (muscle activity) but for ECG (heart) and EEG (brain wave) a capacitor of 4.7uF is recommended (because it's just 1st order).
next step: Ground connections