Introduction of Surface EMG (SEMG) Purpose of Surface EMG signal To observe the activation of motion neuron The Motor Unit The combination of a single motor neuron and all the muscle fibers it innervates is called a motor unit. Small muscle such as extraocular muscles have about 5-6 muscle fibres per motor unit for fine control Large muscles of the lower limb such as gluteus maximus and gastrocnemius have about 2000 muscle fibres per motor unit, allowing only relatively coarse control. Small Motor Unit Muscle Fiber Motor Unit Action Potential Spinal Cord In response to an action potential from the neuron, a muscle fiber depolarizes as the signal propagates along its surface. Firing Rate Recruitment The combination of the muscle fiber action potentials from all the muscle fibers of a single motor unit is the motor unit action potential (MUAP) The repetitive firing of a motor unit creates a train of impulses known as the motor unit action potential train (MUAPT) Model of EMG generation Figure. Schematic representation of the model for the generation of EMG signal MUAP
Properties of Electrodes Instrumentation Electrode configuration 1. Needle electrode 2. Wire electrode 3. Surface electrode Multi-electrode SEMG Multi-channel SEMG Properties of Electrodes Electrode configuration Bipolar: with differential amplifier Mono-polar: Placement direction: Spatial canceling effect An inter-electrode surface spacing of 1.1 cm is recommended Type of Electrodes Mono-polar Bi-polar
6 Placement direction: Electrodes Spacing and Position Recommendation: Place electrode half-way between the distal end of the fibers and the innervation zone Place electrode parallel to fibers Fallacies: Electrodes should be positioned over the motor points Filter Electronic Considerations EMG can be mathematically decomposed to sine waves of different frequencies Low-pass, high-pass, & band pass filter, notch filter LPF: attenuate fast changing of components of a signal HPF: attenuate slowly changing of components of a signal BPF: a specific spectral range of a signal is preserved Notch filter: power line interference (50 or 60 Hz) is removed
Filtering Filtering Signal processing - in time domain A. De-trend DC offset due to electronic problems is likely to happen during measurement. Subtraction of the raw EMG mean is often necessary for further EMG processing (especially important for spectral analysis, RMS, and LE) Because the raw signal is biphasic, its mean value is zero. B. Rectification "flips" the signal's negative content across the zero axis, making the whole signal positive. Absolute value of EMG raw signal Raw EMG Rectified EMG C. Root Mean Square (RMS) 1 RMS{ m( t)} T t T= duration of a segment D. Smoothened EMG t T 2 m ( t) dt 1/ 2 Moving average of rectified EMG using a window function Window length determines the smoothness of EMG
RMS_longer window RMS_shorter window Windowed EMG RMS Window length?? ms E. Integration Calculating the area under the linear envelope, a quantity analogous to electrical work or energy. F. Linear envelope (LE) To represent muscle phasic activity, particularly in gait and other dynamic activity Raw EMG + phase-invariant band-pass filter (40-400Hz) + rectification + phase-invariant law-pass filter (cut-off frequency: 6-10 Hz) eliminate unwanted movement artifact (< 10 Hz) Raw EMG G. Normalization Normalized EMG is grossly associated with changes in muscle force Typically at 100% MVC in a fixed length for the same muscle Comparison across muscles is especially needed!! Integrated EMG G. Turn-amplitude Analysis Turn: occurring at a peak of the EMG signal where the signal changes it direction, provided that the signal amplitude changes by more than 100 uv compared to the previous and subsequent turns. Amplitude: peak-to-peak amplitude of the MUAP or Interference pattern (IP) Traditionally used to classify between neurological and muscular problems Signal processing - in frequency domain
EMG Processing in the Frequency Domain A. Concept of the Fast Fourier Transform (FFT) periodic waveform can be expressed with sinusoid functions of different frequency components Perform FFT after de-trend Filtering Xa1 Xa3 Xa5 Xa EMG Processing in the Frequency Domain B. Removal of noises High pass filtering: rejects at least three established sources of artifact: offsets due to the recording apparatus, motion artifacts and electrocardiographic (ECG) artifacts. Typical filter cut-off frequencies are between 10 and 40 Hz (optional). Notch filtering: Specific frequency component is removed, such as 60Hz. Low pass filtering: 1. Cut-off frequency: 400 Hz (500Hz) for surface EMG 2. Cut-off frequency: 10K Hz for needle EMG EMG Processing in the Frequency Domain C. Power Spectrum of the EMG Surface v.s. invasive (wire or needle) Model of MUAP and EMG Power spectrum of the surface EMG (SEMG) Spectral features of the SEMG Mean frequency Median frequency Mode frequency Dominant frequency Ratio of high frequency band to low frequency band (H/L ratio)
Frequency Domain Spectrum Impulse Trains (t-t i ) Time Domain Spectrum = Motor Unit Action Potential h(t) = SEMG Spectrum FOURIER TRANSFORM Motor Unit Action Potential Trains u i (t) EMG Processing in the Frequency Domain D. Application of SEMG in Clinical Aspects 1. Atypical muscle activation in patients Stroke SCI, PD. etc. Post-polio syndrome & ALS Low back pain 2. Understanding control of motor units 3. Fatigue assessment/ Post exercise effect 4. Functional electrical stimulation 5. Helping diagnosis