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Summary Literature EMGLecture 1 – Basics of EMG
Read: ABC of EMG (4-21 + 39) + Farina et al. (2004), up to spectral analysis (p. 1439)
ABC of EMG (p. 4-21 + 39)
Definition of EMG
“Electromyography (EMG) is an experimental technique concerned with the
development, recording and analysis of myoelectric signals. Myoelectric signals
are formed by physiological variations in the state of muscle fiber membranes.”
- Neurological EMG an artificial muscle response due to external
electrical stimulation is analyzed in static conditions
- Kinesiological EMG the study of the neuromuscular activation of
muscles within postural tasks, functional movements, work conditions and
treatment/training regimes.
Use and benefits of EMG
- Kinesiological EMG is established as an evaluation tool for applied
research, physiotherapy/rehabilitation, sports training and interactions of
the human body to industrial products and work conditions
o Medical research: orthopedic, surgery, functional neurology, gait &
posture analysis
o Rehabilitation: post surgery/accident, neurological rehabilitation,
physical therapy, active training therapy
o Ergonomics: analysis of demand, risk prevention, ergonomics
design, product certification
o Sports science: biomechanics, movement analysis, athlete’s
strength training, sports rehabilitation
Typical benefits of EMG
- EMG allows to directly “look” into the muscle
- It allows measurement of muscular performance
- Helps in decision making both before/after surgery
- Documents treatment and training regimes
- Helps patients to “find” and train their muscles
- Allows analysis to improve sports activities
Signal origin 1
The motor unit
- “the cell body and dendrites of a motor neuron, the
multiple branches of its axon, and the muscle fibers that
innervates it.
o All muscle fibers of a given motor unit act “as one” within the
innervation process
Excitability of muscle membranes
- An ionic equilibrium between the inner and outer spaces of a muscle cell
forms a resting potential at the muscle fiber membrane. The difference
in potential causes a negative intracellular charge compared to the
external surface, this is maintained by physiological processes (ion
pump).
- The activation of an alpha-motor anterior horn cell results in the
conduction of the excitation along the motor nerve. After release of
, transmitter substances at the motor endplates, an endplate potential is
formed at the muscle fiber innervated by this motor unit.
- The diffusion characteristics of the muscle fiber membrane are briefly
modified and Na+ ions flow in. this causes a membrane depolarization
which is immediately restored by backward exchange of ions within the
active ion pump mechanism, the repolarization
The generation of the EMG signal
The action potential
- The depolarization of the membrane causes an action
potential to quickly change from -80mV up to +30mV.
- The repolarization is followed by an after-hyperpolarization
period of the membrane.
- Starting from the motor endplates, the action potential spreads along the
muscle fiber in both directions and inside the muscle fiber through a
tubular system.
- This excitation leads to the release of
calcium ions in the intra-cellular space.
Linked chemical processes (electro-
mechanical coupling) finally produce a
shortening of the contractile elements of
the muscle cell.
- The EMG-signal is based upon action
potentials at the muscle fiber membrane
resulting from depolarization and
repolarization processes as described
above.
- The extent of this depolarization zone is described in the literature as
approximately 1-3mm2. After initial excitation this zone travels along the
muscle fiber at a velocity of 2-6m/s and passes the electro side
Signal propagation and detection
An electrical model for the motor action potential
- The depolarization – repolarization cycle forms a
depolarization wave or electrical dipole which
travels along the surface of a muscle fiber.
- Depending on the spatial distance between electrodes
1 and 2 the dipole forms a potential difference
between the electrodes.
- T1 the action potential is generated and travels
towards the electrode pair.
- T2 highest potential, an increasing potential difference is measured
between the electrodes
, - T4 if the dipole reaches an equal distance between the electrodes the
potential difference passes the zero line (T3) and becomes highest at T4,
which means the shortest distance to electrode 2.
- Because motor units consist of many muscle fibers, the electrode pair
“sees” the magnitude of all innervated fibers within this motor unit –
depending on their spatial distance and resolution. They sum up to a
triphasic motor unit action potential (MUAP) which differs in form and
size depending on the geometrical fiber orientation in ratio to the
electrode site.
Composition of EMG signal
Superposition of MUAPs
- Interference pattern the motor unit action potentials of all active
motor units detectable under the electrode site are electrically
superposed and observed as a bipolar signal with symmetric distribution
of positive and negative amplitudes (mean value = 0)
Recruitment and firing frequency
- The two most important mechanisms influencing the magnitude and
density of the observed signal are the recruitment of MUAPs and their
firing frequency
- These are the main control strategies to adjust the contraction process
and modulate the force output of the involved muscle.
- The EMG signal directly reflects the recruitment and firing characteristics
of the detected motor units within the measured muscle.
Nature of the EMG signal
The “raw” EMG signal
- Raw EMG signal an unfiltered (exception amplifier bandpass) and
unprocessed signal detecting the superposed MUAPs
- Baseline is seen when muscle is relaxed, more or less noise-free
o Depends on many factors, especially the quality of the EMG
amplifier, the environment noise and the quality of the detection
condition
o Average baseline noise should not be higher that 3-5mV (1-2 is the
target)
- The healthy relaxed muscle shows no significant EMG activity due to lack
of depolarization and action potentials!
- By its nature, raw EMG spikes are of random shape, which means one
raw recording burst cannot be precisely reproduced in exact shape. This is
due to the fact that the actual set of recruited motor units constantly
changes within the matrix/diameter of available motor units
- By applying a smooth algorithm or selecting a proper amplitude
parameter, the non-reproducible contents of the signal is eliminated or
at least minimized.
- Raw sEMG can range between +/- 5000mV and typically the frequency
contents ranges between 6 and 500 Hz, showing most power between
~20 and 150Hz.
The influence of detection condition
Factors influencing the EMG signal
1. Tissue characteristics
, o The human body is a good electrical conductor, but unfortunately
the electrical conductivity varies with tissue type, thickness,
physiological changes and temperature
2. Physiological cross talk
o Neighboring muscles may produce a significant amount of EMG that
is detected by the local electrode site. This cross talk does not
exceed 10-15% of the overall signal contents or isn’t available at all.
o ECG spikes can interfere with the EMG recording, especially when
performed on the upper trunk/shoulder muscles
3. Changes in the geometry between muscle belly and electrode site
4. External noise
o The most demanding is the direct interference of power hum,
typically produced by incorrect grounding of other external devices
5. Electrode and amplifiers
o Internal amplifier noise should not exceed 5Vrms. Most of these
factors can be minimized or controlled by accurate preparation and
checking the given room/laboratory conditions.
EMG amplification
EMG – amplifiers
- EMG-amplifiers act as differential amplifiers and their main quality item is
the ability to reject or eliminate artifacts. The differential amplification
detects the potential differences between the electrodes and cancels
external interferences out.
- The term “common mode gain” refers to the input-output relationship of
common mode signals. The “Common Mode Rejection Ratio” (CMRR)
represents the relationship between differential and common mode gain
and is therefore a criteria for the quality of the chosen amplification
technique.
o The CMRR should be as high as possible because the elimination of
interfering signals plays a major role in quality. A value >95dB is
regarded as acceptable.
- State of the art concepts prefer the use of EMG pre-amplifiers. These
miniaturized amplifiers are typically built-in the cables or positioned on top
of the electrodes (active electrodes).
o Disadvantage of latter bulky electrode detection side with
increased risk of pressure artifacts and they typically do not allow
free selection of electrode types.
- The un-amplified EMG signal on the skin typically charges between a few
microvolts and 2-3 millivolt. The signal is generally amplified by a factor of
at least 500 to 1000. The input impedance of the amplifier should have a
value of at least 10x the given impedance of the electrode.
- The frequency range of an EMG amplifier (bandpass settings) should
start from 10Hz high pass and go up to 500Hz low pass.
- Any notch filtering (to cancel e.g. power hum) needs to be avoided
because it destroys too much signal information.
- Both cable and telemetry systems are available and applied concepts
range from handheld 1 of 2 channel biofeedback units up to 32 channel
systems for complex and multi-parametric setups.
Computation of the EMG signal
A/D resolution
- Before a signal can be displayed and analyzed in the computer, it has to be
converted form an analog voltage to a digital signal (A/D conversion)
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