Transcript The Use of Surface Electromyography in Biomechanics by Carlo De

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