ADInstruments Bio Amp Owner's Manual page 19

There is a good introduction to data acquisition provided in the
documentation for LabChart and Scope. More information can be obtained
from your ADInstruments representative. It is highly recommended reading
for anyone recording biological signals using the front-end. Apart from the
general areas covered in that material, two things particularly affect the kind
of measurements made with the Bio Amp, and can cause 'artifacts' (spurious
readings) in the recorded waveform: electrode contact and motion effects.
Electrode Contact. Occasionally during measurement of a biological signal,
one of the lead wires connecting the source to the front-end may become
disconnected, or an electrode contact may become poor. If this should
happen, relatively high voltages (potentials) can be induced in the open wire,
due to electric fields caused by the power line or other sources close to the
front-end or to the subject. This induced potential results in a constant
amplitude disturbance of the recorded waveform at the power line frequency,
and loss of the desired signal. If the problem is a recurring one, one of the
leads may be faulty. Check connections and replace faulty leads, if necessary.
Make sure that skin is cleaned and lightly abraded before attaching electrodes
to it. Ensure that there is sufficient electrode cream to maintain a good
contact: if it dries out, the contact will be poor, and the recorded signal may
be degraded or lost.
Motion Effects. A common source of artifacts when recording biological
signals is motion of the subject or equipment. For example, muscular activity
generates its own electrical signals, which may be recorded along with an
ECG, depending on the location of the electrodes. If an electrode is not firmly
attached, impedance (and hence the recorded signal) may vary as the contact
area changes shape owing to movement. Movement of Bio Amp cables and
leads, particularly bending or rubbing together (triboelectric effects) may also
generate artifacts in a signal.
Subject respiration can also generate a signal: breathing can result in a slowly
changing baseline corresponding to inspiration and expiration. If the subject
is going to move during recording, then special care needs to be taken when
attaching the electrodes and securing the leads.
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Chapter 1 Overview