The Use of Surface Electromyography in Biomechanics by Carlo De Luca JAB Vol 13, p 135-163; 1997 To its detriment, EMG is too easy to use and consequently too easy to abuse. EMG provides easy access to physiological processes that cause the muscle to generate force and produce movement. EMG has many limitations that must be understood, considered, and eventually removed so that the discipline is more scientifically based.

EMG signal/force Relationship Pitfalls Is the EMG signal detected and recorded with maximum fidelity? What are the configuration, dimension, and electrical characteristics of the electrode unit? How Should the EMG signal be analyzed? How are initiation and cessation times of EMG signal measured? What are the preferred parameters for measuring the amplitude of the EMG signal? What are the preferred parameters for measuring the frequency spectrum? Where does the detected EMG signal originate? Is there any crosstalk? Where is the electrode placed on the surface of the muscle in relation to its anatomical structure? How much fatty tissue is there between the electrode and the muscle surface?

EMG signal/force Relationship Pitfalls Is the EMG signal sufficiently stationary for the intended analysis and interpretation? Does the muscle change length? Is the activation pattern of the motor units stable? That is, do some motor units alternate between the state of recruitment and derecruitment? Where does the measured force originate? What is the state of the synergistic and antagonistic muscles associated with the task? Are the motor control characteristics of the contraction stable for the intended interpretation? Is there any change in the relative force contribution among muscles during the contraction? Is the force generated homogenously throughout the muscle?

Electrode Structure and Placement Factors Electrode configuration describes: The area and shape of the electrode detection surfaces, which determine the number of active motor units detected by virtue of the number of muscle fibers in their vicinity, and the distance between the electrode detection surfaces, which determines the bandwidth of the differential electrode configuration. Location of the electrode with respect to the motor points in the muscle and the myotendinous junction, which influences the amplitude and frequency characteristics of the detected signal. Location of the electrode on the muscle surface with respect to the lateral edge of the muscle, which determines the amount of crosstalk. Orientation of the detection surfaces with respect to the muscle fibers, which affects the value of cond. vel., amplitude and frequency of signal.

Physiological, Anatomical, and Biochemical Factors The number of motor units at any particular time of the contraction, which contributes to the amplitude of the detected signal. Fiber type composition of the muscle, which determines the change in pH of the muscle during a contraction. Blood flow in the muscle, which determines the rate at which metabolites are removed during the contraction. Fiber diameter, which influences the amplitude and conduction velocity of the action potentials that constitute the signal. Depth and location of the active fibers within the muscle with respect to the electrode detection surfaces; this relationship determines the spatial filtering, and consequently the amplitude and frequency characteristics of the detected signal. The amount of tissue between the surface of the muscle and the electrode, which affects the spatial filtering of the signal.

Detection and Processing the EMG Signal Differential Electrode Configuration: Detection surfaces two parallel bars 1 cm apart Bandwidth of 20-500Hz with a rolloff of 12 dB/octave Common Mode Rejection Ratio > 80 dB Noise < 2 uV RMS (20-500 Hz) Input Impedance > 100 MegaOhms Locate the electrode on the midline of the muscle belly, between the myotendinous junction and the nearest innervation zone, with the electrodes aligned parallel to the muscle fibers. Use RMS or average rectified EMG to measure the amplitude.

Comparisons Among Subjects, Muscles and Contractions EMG/Force comparisons should be limited to isometric contractions with the joint constrained to limit the effects of other muscles In dynamic movements use contractions that have the least amount of shortening and the slowest velocity and interpret the results with caution. In repetitive dynamic contractions choose small sections of the motion to analyze. When normalizing the amplitude of the EMG signal, do so at less than 80% MVC. Above this level, the EMG signal and force (torque) are exceptionally unstable and do not provide a suitable reference. Measure MVC by choosing the greatest of three consecutive attempts.

Problems to be Resolved in EMG/Force Relations Develop a surface detection that follows the movement of the muscle fibers. Develop online EMG measurement Develop a method to estimate muscle force with +- 5% from surface EMG. How do muscle fibers transmit force throughout the muscle Does a muscle generate force homogeneously throughout its volume. Describe ansiotropy of muscle, fascia, fat and skin as related to EMG. Refine anatomically correct biomechanical models of the musculoskeletal system.

Issues for International Agreement Electrode configuration and dimensions. Electrode placement and orientation. Means for processing the EMG signal for amplitude and spectral analysis. Means for determining the delay between force and the EMG signal. Procedure for determining MVC Procedures for establishing repeatability of the EMG: among contractions when the experimental conditions are fixed among contractions when the electrodes are reapplied among muscles among subjects