Answers to Common Questions Concerning Use of the F1000.

Q. Some literature recommends that the EMG ground lead (sometimes called the reference electrode) be placed equidistant between the two active leads. The F1000 uses a wrist strap ground. Explain.

A. The ground lead on the F1000 EMG system is not a reference. It merely provides a path needed by the amplifier and does not enter into the measurement. Therefore its position is not at all critical. The larger surface area of the wrist strap provides a much lower impedance ground reducing artifacts.

Q. What do you recommend in regard to skin preparation on the F1000?

A. The F1000 EMG amplifiers have a very high input impedance and common mode rejection ratio. Under most circumstances a simple alcohol wipe to remove skin oil is all that is necessary. We do not recommend excessive rubbing or abrasion of the skin.

Q. Please comment on checking electrode impedance with an ohm meter.

A. An ohm meter uses direct current to measure resistance rather than impedance. The direct current of the ohm meter polarizes the electrodes resulting in a false reading. Error due to this effect is not trivial and varies considerably with the particular ohm meter and the nature of the electrode/gel interface. Electrode impedance can only be reliably measured with a special impedance meter designed for the purpose.

Q. What effect does a high electrode impedance have on EMG data?

A. With early equipment, electrode impedance directly affected the accuracy of the measurement. Modern equipment with high input impedance amplifiers have virtually eliminated this problem.

High electrode impedance can make the measurement more susceptible to external interference. However, the effect on readings is dependent on a complex mix of factors including electrode impedance, proximity of interfering signals, filter bandwidth in use, and electrode placement.

Q. Comment on testing the F1000 EMG system using a dummy subject.

A. The dummy subject method involves replacing the subject with two 10,000 ohm resistors. A resistor is placed between the ground lead and each active lead of the EMG cable. The resistors are supposed to simulate the effect of a typical electrode impedance. Since the resistors do not produce biological signals (EMG), the reading on the equipment should represent system noise.

The dummy subject method is subject to a number of errors. The reading obtained is in fact the sum of system noise and environmental noise picked up by the unshielded portion of the EMG cables and dummy subject. With the low noise of modern amplifiers, environmental noise is likely to predominate. We do not recommend the dummy subject method as a valid test of the F1000 system noise. If attempted, the dummy subject resistors should be enclosed in a shielded enclosure with the shield grounded to the EMG ground lead.

Q. What are the time constants of the smoothing numbers in the EMG system?

A. The smoothing factors are arbitrary and have been chosen to provide a useful range. The time constant of the default smoothing factor of 20 is approximately 0.3 seconds. The time constant is directly proportional to the smoothing factor so a factor of 40 is approximately 0.6 seconds.

Q. How should the thermistor probe be attached to the finger? Should the cable be taped to the wrist to prevent stem effect errors?

A. The thermistor probe and cable design virtually eliminates stem effect errors. Air flow over the hand will affect the probe itself much more than the cable. We recommend routing the cable for comfort and protection from damage. Attaching the probe to the soft pad of one of the fingers will give the fastest response.

The following chart only covers the temperature and EMG system as required by the BCIA exam.

F1000 Characteristics

F1000 EMG Technical Specifications

Notes:

1. Actual input impedance is dependent upon a number of factors such as humidity, air pollutants etc.

2. Inputs shorted. 25-500 Hz bandwidth