EMG Analysis and Interpretation Extracting information from the signals

Interference EMG Superimposed muscle action potentials Positive deflections Negative deflections Isoelectric line Algebraic sum of muscle action potentials Amplitude cancellation I.e., interference

Two Signal Domains Time domain Frequency domain Examples… Amplitude

Time Domain Changes in EMG amplitude over time Root-mean-square (rms) value Same as standard deviation Time-averaged integrated EMG values Integrated = AUC AUC ÷ time

Normalization Drawbacks of surface (interference) EMG is comparing raw values Between subjects Between muscles Repeated over time Solution = normalize EMG amplitude as a percentage of the maximal voluntary contraction (MVC). Result = comparing the patterns of response under different conditions or tasks, not the absolute value. Example…HRmax

Frequency Domain Determining the frequency content of an EMG signal epoch Epoch = any given signal portion/duration Measured in Hertz (Hz) Cycles (turns) per second Every signal can be represented by a set of sine and cosine terms = Fourier series. The fundamental, its harmonics, and the amplitude of each harmonic Fourier Transformation Determine the Power Density Spectrum (PDS) Mean Power Frequency (MPF) Mean (Hz) of the PDS Median Power Frequency (MDF) Frequency (Hz) that bisects the PDS in half

EMG Signal Conditioning Units Volts (V), millivolts (mV), and microvolts (μV) Zero Mean Subtracting the mean value Digital filtering Reducing unwanted signal components Low-frequency noise (movement artifact, etc) High-frequency noise (random white noise, etc) More about this after Fourier analysis…

Quantified Values EMG Amplitude RMS Represents: Algebraic sum of muscle action potentials traveling within the recording area Number of motor units recruited Firing rate of activated motor units Linear or curvalinear relation with force production

Quantified Values EMG Median Power Frequency (MDF) Derived from Fourier Transformation Indication of what frequencies dominate the EMG signal (power of the signal) Related to: The shape of the action potentials Most significant contributor… The relative timing of the action potentials discharged by different motor units The discharge rate of motor units (firing rate) Least significant contributor… Mostly used to track fatigue

Isometric muscle action at 30% MVC Orizio, C., Gobbo, M., Diemont, B., Esposito, F., Veicsteinas, A. Eur J Appl Physiol. 2003. Displacement Sensor Laser Beam Accelerometer Bipolar EMG Electrodes Force Transducer Isometric muscle action at 30% MVC