KINESIOLOGICAL ELECTROMYOGRAPHY - Biomechanics and Motor Control of Human Movement - page 258

below). The most common distortion is overdriving of the amplifier system
such that the larger signals appear to be clipped off. Every amplifier has a
dynamic range and should be such that the largest EMG signal expected will
not exceed that range. Noise can be introduced from sources other than the
muscle, and can be biological in origin or man-made. An electrocardiogram
(ECG) signal picked up by EMG electrodes on the thoracic muscles can be
considered unwanted biological noise. Man-made noise usually comes from
power lines (hum) or from machinery, or is generated within the compo-
nents of the amplifier. Artifacts generally refer to false signals generated by
the electrodes themselves or the cabling system. Anyone familiar with EMG
recording will recall the lower-frequency baseline jumps, called movement
artifacts , which result from touching of the electrodes and movement of the
cables.
The major considerations to be made when specifying the EMG ampli-
fier are:
1. Gain and dynamic range
2. Input impedance
3. Frequency response
4. Common-mode rejection
10.2.1 Amplifier Gain
Surface EMGs have a maximum amplitude of 5 mV peak to peak, as recorded
during a MVC. Indwelling electrodes can have a larger amplitude, up to 10
mV. A single m.u.a.p. has an amplitude of about 100 μ V. The noise level
of the amplifier is the amplitude of the higher-frequency random signal seen
when the electrodes are shorted together, and should not exceed 50 μ V,
preferably 20 μ V. The gain of the amplifier is defined as the ratio of the
output voltage to the input voltage. For a 2-mV input and a gain of 1000, the
output will be 2 V. The exact gain chosen for any given situation will depend
on what is to be done with the output signal. The EMG can be recorded on
a pen recorder or magnetic tape, viewed on an oscilloscope, or fed straight
into a computer. In each case, the amplified EMG should not exceed the
range of input signals expected by this recording equipment. Fortunately,
most of these recording systems have internal amplifiers that can be adjusted
to accommodate a wide range of input signals. In general, a good bioamplifier
should have a range of gains selectable from 100 to 10,000. Independent of
the amplifier gain, the amplitude of the signal should be reported as it appears
at the electrodes, in millivolts.
10.2.2 Input Impedance
The input impedance (resistance) of a biological amplifier must be sufficiently
high so as not to attenuate the EMG signal as it is connected to the input