Transcript Lecture
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 – Amplitude Frequency domain – Frequency Examples… Time Domain Changes in EMG amplitude over time EMGrms – Root-mean-square (rms) value N 1 2 xi N i 1 Same as standard deviation – Time-averaged integrated EMG values Integrated = AUC AUC ÷ time 1 Average Rectified EMG N N x i 1 i 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 Displacement Sensor Accelerometer Laser Beam Bipolar EMG Electrodes Force Transducer Orizio, C., Gobbo, M., Diemont, B., Esposito, F., Veicsteinas, A. Eur J Appl Physiol. 2003.