3 EMG Amplifier Specifications Amplifiers should be described by the following:If single, differential, double differentialInput impedanceCommon Mode Rejection Ratio (CMRR)Signal to noise ratioActual gain usedFrequency range of amplifier
4 Noraxon Telemyo EMG Amplifier Input Noise < 1 μV RMSDifferential Input Impedance 16 Meg OhmsCommon Mode Rejection Ratio CMRR 85 dBFrequency Response 16 – 500 HzThroughput Circuit Gain 1000
5 Electrodes Reports on surface EMG should include: Electrode material (Ag/AgCl)Electrode geometry (disc, bars, rectangular)Size (diameter, radius, width x length)Use of gel or paste, alcohol applied to cleanse skin, skin abrasion, shaving of hairInterelectrode distanceElectrode location, orientation over muscle with respect to tendons, motor points and fiber directionIntramuscular wire electrodes should be described by:Wire material (stainless steel)Single or multi strandInsulation materialMethod of insertion (hypodermic needle)Depth of insertionSingle or bipolar wireLocation of insertion in the muscleInterelectrode distanceType of ground electrode used, location
6 EMG AmplitudeRoot Mean SquareAverage Rectified Value
7 EMG Linear EnvelopeFull wave rectify the EMGLow pass filter with a time constant?
8 EMG FilteringSince the power density spectra of the EMG contains most of its power in the frequency range of Hz at the extremes, surface EMG should not be filtered above 10 Hz as a low cutoff and below 350 Hz as a high cutoff.Intramuscular signals should be filtered in the Hz range.
9 EMG Frequency Spectrum The Median Frequency is defined as the frequency that divides the power spectrum in two regions having the same power or area under the Amplitude – Frequency CurveExplain of Power Spectrum should include:Time epoch used for each calculationType of window used prior to FFT (Hamming, Hanning)Number of zero padding pointsEquation used to calculate Median or Mean FrequencyMuscle length or fixed joint angle
10 EMG - ForceRecruitment of a motor unit. A quanta of force in contributed to the muscle contraction; however, the contribution to the EMG signal amplitude depends on the proximity of the detection surfaces of the electrode to the nearest fibers of the recruited motor unit.A newly recruited MU will increase its firing rate as the force demand increases.The force increase rapidly as a function of the increasing firing rate, whereas the contribution to the amplitude of the EMG signal increases less rapidly
11 EMG NormalizationSubjects should be adequately trained to elicit an MVC.Provide subjects with immediate feedback of obtained force/torque.Give the following information relative to normalization:How the subjects were trained to obtain MVCJoint angle or muscle lengthRate of force developmentVelocity of shortening or lengtheningChange in muscle lengthRange of joint angle in non – isometric contractionsLoad applied in non – isometric contractionsWhen normalizing the amplitude of the EMG signal, do so at values less than 80% MVC. Above this level the EMG signal and the force (torque) are exceptional unstable and do not provide a suitable reference point (De Luca, JAB 13, p. 154)
12 EMG CrosstalkMake every effort to determine that EMG crosstalk from muscles near the muscle of interest did not contaminate the recorded signalSelect an appropriate electrode size, interelectrode distance and location when working on an area where many narrow muscles are tightly gatheredCare also should be employed when recording surface EMG from areas with subcutaneous adipose tissue as it is known that adipose tissue enhances crosstalk.