Electromyography

There are somw factors, which need to be metioned when talking about EMG signal. Following factors may affects the signal and may do the signal interpretaion more difficult.

1) Signal-to-noise ratio (SNR) is the best generally  accepted measure of the quality of the EMG signal. It indicates the ratio of the wanted EMG signal verus unwanted signal (baseline noise, line noise, movement artefacts).

The amplitude of the wanted EMG signal depends on the contraction strength, but also on the location of the sensor on the muslce of interest, its orientaion with respect to muslce fibres, and the sensor characteristics, such as the size of the electrodes and the inter-electorde distance.

It means that one of the factors is the

2) Position of electrodes

The location of the sensor on the muscle is the single most important factor for obtaining good SNR. This advice was first provided by Basmajian and De Luca in 1985 in their book Muscles Alive and has been confirmed by numerous studies since. The muscle, fat and skin tissues present an internal impedance to the propagation of electric currents and act as a low-pass filter on the detected EMG signal. The filtering characteristics of these tissues are a function of the distance between the active muscle fibres and the detection surfaces of the electrodes. Thus, the location of the sensor on the muscle renders dramatically different surface EMG signal characteristics. For instance, locating the sensor in the proximity of the tendon origin or the innervation zones yields lower amplitude signals. The fibres in the middle of the muscle belly generally have a greater diameter than those at the edges of the muscle or near the origin of the tendons. Because the amplitude of action potential from the muscle fibres is proportional to the diameter of the fibre, the amplitude of the EMG signal will be greater in the middle of the muscle.

For example of differneces of electrode position see picture Electrode spacing

There are more factors which affects EMG siganls: As interelectrode distance, Skin-electrode interface, Motion artefact, Physiological Noise, Power Line and Electrical Noise, Crosstalk...

For more infromation about these factors I will reffer you to Comprehensive Textbook of Clinical Biomechanics by J. Richards (pp218-222)