An analog EMG signal can be sampled in time by taking instantaneous voltage values at a particular rate or sampling frequency (samples/s or Hz). Related terms: Aliasing, A/D conversion.When converting a signal from analog to digital form, the signal must be sampled at a rate which is greater than twice the highest frequency component in the signal bandwidth (Nyquist’s theorem) in order to retain all the information contained within the signal. In practice, the sampling rate should be 2.5 – 3 times higher than the highest frequency component contained within the EMG signal.

The relative power of wanted (EMG) to unwanted signal components (noise, interference etc.) that are contained in the overall signal. It measures the quality of the signal. SNR is typically measured on logarithmic scale and expressed in decibels (dB).

An array or grid of electrodes can be used to sample the distribution of the EMG signal in space, e.g., across the skin surface (using a surface electrode array) or within the muscle (with multi-channel intramuscular EMG). Related terms: Filter, aliasing, sampling in time.

A signal exhibits stationarity when the statistical properties of the processes that generate the signal do not change over time. A signal exhibits quasi-stationarity when it can be assumed that the processes generating the signal are stationary over short time intervals (time windows or epochs used for estimating the signal properties such as ARV, RMS, power spectrum, MNF, MDF, etc.), but can vary when considered over longer time intervals or across different epochs. A signal exhibits nonstationarity when the statistical properties of the processes that generate the signal, and consequently those of the signal itself, change over time. Isometric constant force contractions produce surface EMG signals that are quasi-stationary within time epochs of 0.5–1.0 s.

Muscle that supports the work of the agonist and facilitates the completion of the task goal. Synergist muscles include muscles that contribute to the joint moment of the agonist and/or stabilize the origin of the agonist muscle to enable efficient movement during the task. This “functional” or “biomechanical” definition should not be confused with “muscle synergies” (also called modules or motor primitives). Muscle synergies are coordinated activations of a group of muscles.