Whereas two different feedback thermometers should register the same room temperature, we can't assume the same for two electromyographs' measurements of SEMG voltage.
SEMG amplitude is a relative measurement of skeletal muscle electrical activity because it depends upon a surprising number of parameters, some controllable and others intrinsic to your client or electromyography.
The relativity of SEMG voltages characterizes all biofeedback modalities. Temperatures are no more absolute than SEMG values. When you attach a temperature probe to a client, factors like cold exposure before entering the clinic, stabilization period, sensor placement, task, and your relationship with the client can greatly affect temperature readings. All psychophysiological measurements are affected by hardware, environmental, procedural, and client factors. Experienced clinicians and researchers manipulate controllable factors and standardize those not controllable to increase measurement validity.
To compare SEMG values across session pre-baselines, standardize the electrodes, placement, skin-electrode impedance, baseline conditions, location in the room, bandpass and notch filter settings, and rectification and integration methods.
Consider Your Client's Adiposity When Interpreting SEMG Values
Adipose tissue filters the SEMG signal and reduces its amplitude. Consider your client's subcutaneous fat when interpreting SEMG measurements from different sites on the same individual or the same location between individuals (Shaffer & Neblett, 2010).
A reading of 5 μV obtained from one electromyograph could easily be 8 μV on another due to different bandpasses and integration methods. In contrast, temperature measurements are absolute because two feedback thermometers should register the same room temperature.
Summary
EMG voltages are relative measurements that are affected by hardware, environmental, procedural, and client factors. Take factors like adiposity into account when interpreting EMG values. Low values could mean greater subcutaneous fat instead of relaxation. Always compare apples with apples (e.g., standing versus standing EMG).
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References Shaffer, F., & Neblett, R. (2010, Summer). Practical anatomy and physiology: The skeletal muscle system. Biofeedback, 38(2) 47-51.