I

Week 3

What is Electromyography (EMG)?

  • EMG is the technique used to record changes in the electrical potential of a muscle when it is caused to contract by a nerve impulse.
  • In essence, EMG records the small electrical impulses that trigger muscle contraction.

What can EMG be used for?

  • Clinical EMG (diagnosis/monitoring)
    • Identify nerve or muscle dysfunction
    • Characterise neuromuscular conditions
    • Track rehabilitation progress
  • Kinesiological EMG (movement & sport)
    • Activation timing and coordination
    • Co-contraction and joint stability
    • Fatigue and asymmetry profiling

Example questions EMG can help answer

  • Were different muscles recruited to perform an activity when the movement pattern changed?
  • Did repeated task performance induce muscle fatigue?
  • Were subjects able to recruit their muscles to a greater extent after training?
  • Were different motor unit recruitment strategies adopted with learning?
  • What is the time between the initiation of a neural impulse and contraction (conduction velocity)?

Motor Unit - EMG

  • Smallest controllable muscular unit and fundamental unit of the neuromuscular system
  • Made up of a motor (α) neuron and the muscle fibres it innervates

Overview of Neuromuscular Transmission

  • Action potential (AP) travels along the α-motor neuron
  • Acetylcholine (Ach) released at the neuromuscular junction
  • Sarcolemma depolarises; AP spreads via T-tubules
  • Sarcoplasmic reticulum (SR) releases Ca^{2+} into cytosol
  • Cross-bridge cycling → force and movement
  • EMG reflects this electrical activity

Motor Unit Function (membrane dynamics & EMG sum)

  • Resting membrane potential: V_{ ext{rest}} \,\approx\, -70 \,\text{to} \, -90\, \text{mV}
  • Depolarisation: \text{Na}^{+} \text{ influx} → less negative
  • Overshoot: inside briefly positive
  • Repolarisation: K^{+} \text{ out} → back toward rest
  • After-hyperpolarisation: dips below rest
  • EMG is the sum of many MU action potentials (MUAPs)

Action Potentials and MUAPs

  • Action potential propagates along fibres; recordings capture the electrical changes during propagation
  • MUAP shape depends on multiple factors (see below)
  • There is a distinction between single-fibre APs and motor-unit action potentials (MUAPs)
  • The recorded EMG signal is influenced by electrode type, fibre type, conduction velocity, and recording setup

MUAPs vs APs and Direction of Propagation

  • A motor unit’s electrical activity is a composite of many fibre APs; the MUAP is the summed action potentials from all fibres within the recording volume
  • Direction and timing of AP propagation affect the recorded waveform

Raw EMG Data and Electrophysiology of Contraction

  • EMG data reflect multiple muscle fibres located within a recording area
  • Electrophysiology of muscle contraction is influenced by electrode properties and recording context

Electrophysiology of Muscle Contraction – Waveform Dependence

  • MUAP waveform shape depends on:
    • Type of electrodes
    • Recording location relative to muscle fibres
    • Electro-chemical properties of muscle and connective tissue
    • Recording equipment
    • Fibre type and conduction velocity
  • These factors influence data collection, processing, and interpretation

Motor Unit Recruitment and Tension

  • Two main ways to increase muscle tension: 1) Recruitment of additional motor units (typically early and up to submaximal force; varies across muscles)
    • More motor units = greater force
      2) Increased firing frequency of recruited motor units (more prominent at higher force levels)
    • Higher motor neuron stimulation → greater force
  • Consequences in EMG terms:
    • Firing rate increases lead to summation of twitches and eventual tetanic contraction
  • The size principle governs recruitment order:
    • Smallest motor units are recruited first, then progressively larger units as force demand rises

Types of Motor Units (Slow vs Fast Twitch)

  • Slow-twitch fibres (Type I)
    • Rich in mitochondria; highly capillarised
    • Twitches with slightly lower peak tension (normalised to diameter)
    • Long time to peak tension (≈ 60–120 ms)
  • Fast-twitch fibres (Type II)
    • Larger in size; fewer mitochondria; poorly capillarised
    • Larger peak tensions in shorter time (≈ 10–50 ms), not necessarily higher firing frequency
  • Subtypes of fast-twitch:
    • Fast-twitch fatigable (FF): type 2b fibres; fast, high-force, highly fatigable; innervated by thick, fast-conducting motor neurons
    • Fast-twitch fatigue-resistant (FR): type 2a fibres; intermediate diameter neurons; fatigue resistant
    • Slow (S): type 1 fibres; small motor neurons; low-tension, slow contracting, fatigue resistant

Breaks in the Slides

  • Break slides indicate transitions between sections; content summarized above continues in subsequent sections

Electro-physiology of Muscle Contraction – Recording Considerations

  • EMG signal magnitude/content is affected by:
    • Motor unit recruitment/synchronisation
    • Fat overlaying muscle
    • Muscle temperature
    • Muscle cross-sectional area and length
  • It is difficult to keep all factors constant in research or applied work

Recording EMG – Signal Transfer to Skin and Tissue Effects

  • The EMG signal recorded is not the exact physiological signal due to tissue filtering
  • Signal must pass through body tissue; high-frequency content attenuated
  • Electrode–electrolyte interface reduces low-frequency content
  • Direct comparison of EMG between individuals is not valid without normalization

Normalisation of EMG

  • Essential to compare EMG across individuals, muscles, or sessions
  • Two normalisation approaches: 1) Normalize to a reference contraction (e.g., toMVIC) to express activity relative to max
    • %MVIC = (EMGtask / EMGMVIC) × 100%
      2) Normalize to another movement (e.g., compare CMJ vs SJ)
  • EMG signals are meaningless without a reference (normalisation)

Recording EMG – Standardisation vs Individual Variation

  • Extrinsic factors (controlled by the experimenter):
    • Electrode location relative to motor end plates
    • Electrode orientation relative to muscle fibres
    • Recording system characteristics
  • Intrinsic factors (not easily controlled):
    • Physiological factors: firing rates of units, conduction velocity
    • Anatomical factors: muscle fibre diameters, motor unit positions

Electrode Position and Configuration

  • Electrode position: mid-way between motor point and distal tendon (high reliability; reduces movement effects)
  • Electrode configuration:
    • Monopolar: amplified signal = gain(m + n) where n denotes noise
    • Bipolar (preferred): amplified signal = gain(m1 + m2)
  • Cross-talk and inter-electrode distance:
    • Cross-talk: signals from other muscles; bigger risk for small muscles or large inter-electrode distances
    • Inter-electrode distance effects (Bipolar):
    • Distance 1 cm: fibres contributing to signal ≈ 5–15
    • Distance 2 cm: fibres contributing ≈ 15–50
  • The effect of distance on AP size:
    • AP size diminishes with distance travelled; 0.1 cm movement can reduce AP amplitude by ~75%
    • The recording area affected by distance between fibre and electrode

Pick-up Area and Cross-talk Details

  • Bipolar electrodes capture a restricted subset of fibres depending on distance
  • Consequently, signals from nearby muscles can contaminate the recording if distances are large
  • For large muscles, 1 cm inter-electrode distance is generally appropriate; 2 cm may increase cross-talk risk for smaller muscles

EMG Electrode Types

  • Surface electrodes: detect averaged activity of superficial muscles
  • Indwelling electrodes (intramuscular): used for deep or fine movements; include needle and fine-wire electrodes
  • Surface electrode basics:
    • Usually silver/silver chloride disks (~1 cm diameter); smaller disks for smaller muscles
    • Pre-amplification often present in the electrode system
  • Electrode attachment/process to reduce impedance:
    • Abrasion
    • Dry shave
    • Alcohol rub
    • Gel or gel electrodes
    • Proper spacing between electrodes

Noise in EMG Recordings

  • Sources of noise:
    • Cross-talk from other muscles
    • Electromagnetic and electrostatic fields (power sources, equipment, materials)
    • Movement artefact: lead movement, electrode–skin movement, electrode–muscle movement
  • Minimising noise:
    • Proper electrode choice and placement
    • Use pre-amplified electrodes
    • Tape down leads and electrodes securely to maintain contact
    • Minimise movement of skin and fibre–electrode distance changes

EMG Amplifiers and Signal Quality

  • Common instrumentation example (BioPAC or similar):
    • Gain settings (e.g., 500–5000)
    • Low-pass and high-pass filters (e.g., 5 kHz LP, 100 Hz HP or others depending on system)
    • Shielded cables and grounding to reduce noise
  • Common-mode rejection (CMR):
    • With bipolar electrodes, signals that arrive simultaneously at both electrodes are treated as noise
    • CMR is achieved by amplifiers that reject common-mode signals
    • CMR is not perfect; some noise remains

Processing EMG – Time vs Frequency Domain

  • Time-domain processing:
    • Linear envelope / RMS (moving average)
    • Ensemble average
    • Normalisation
    • Time of Activation
  • Frequency-domain processing:
    • Fourier analysis
    • Median frequency

EMG Signal Processing Examples

  • RMS example: raw EMG can be processed to obtain RMS values over time
  • Linear envelope (IEMG):
    • Full-wave rectified signal filtered with a low-pass filter (typically 2nd order; 3–6 Hz)
    • This captures the overall trend of muscle activation while suppressing high-frequency content
  • Ensemble average:
    • Time-averaged waveform used for cyclic or repetitive movements (e.g., gait)
    • Example: averaging six strides as a percentage of gait cycle

Normalisation and Comparison Across Trials

  • Reminder: EMG must be normalised to compare across sessions or subjects
  • Common normalisation targets include MVIC or a reference task

Interpreting EMG – Kinesiological EMG Usage

  • Kinesiological EMG provides information on timing and relative intensity of muscle activation
  • Factors affecting interpretation:
    • Magnitude of tension
    • Rate of tension buildup
    • Fatigue state
    • Reflex activity and joint angle

EMG – Force Relationship

  • EMG vs. Force relationship illustrated in hamstrings: Normalised RMS EMG vs Force (% MVIC)
  • Relationship is not strictly linear, especially during dynamic acts
  • EMG cannot be used alone to precisely measure muscle force; the relationship is complex

Why EMG and Force are Nonlinear in Practice

  • Reasons for non-linearity:
    • Difficulties in measuring true EMG (cross-talk)
    • Temporal disconnect between EMG and force due to electromechanical delay (EMD)
    • Movement of the muscle relative to electrodes
    • Muscle length-specific activation and mechanical constraints
    • Force influenced by reflexes and contraction history (e.g., isometric vs concentric)

Electromechanical Delay (EMD)

  • EMD is the time between EMG onset and force onset (as defined in literature)
  • Commonly used definition: the time interval from EMG onset to the corresponding force onset; sometimes defined as between direct nerve or muscle stimulation and force
  • EMD causes:
    • Conduction velocity across muscle fibers
    • Ca^{2+} release from sarcoplasmic reticulum
    • Cross-bridge formation and cycling
    • Tendon compliance (transfer of force to the skeleton)

Practical Questions for EMG Testing

  • Muscle(s) to be tested
  • Recording/testing setup and type of electrodes
  • Preparation and placement
  • Normalisation test (e.g., MVC)
  • Test processing (RMS/linear envelope, moving average)
  • Frequency analysis (e.g., median frequency)
  • Interpreting/analyzing EMG: maximums, timing, frequency content

Summary Takeaways

  • EMG measures electrical activity related to muscle contraction, reflecting motor unit recruitment and firing behavior
  • Both recruitment and firing rate contribute to muscle force; the size principle governs recruitment order
  • EMG signals are influenced by numerous extrinsic and intrinsic factors; normalization is essential for meaningful comparisons
  • Recording quality hinges on electrode choice, placement, inter-electrode distance, and noise management
  • Data processing spans time-domain (RMS, envelope, activation timing) and frequency-domain (Fourier, median frequency) analyses
  • Interpreting EMG in relation to force requires consideration of electromechanical delay, muscle length, fatigue, and history of contraction
  • Fatigue manifests as slower conduction velocity, altered AP morphology, reduced amplitude, longer duration, a shift to lower frequencies, and selective dropout of fast-twitch units

Notation and Equations Used in EMG Context

  • Resting potential: V_{ ext{rest}} \approx -70 \text{ to } -90~\text{mV}
  • Normalisation to MVIC:
    • ext{%MVIC} = \frac{EMG{task}}{EMG{MVIC}} \times 100\%
  • EMD (definition):
    • EMD = t{ ext{force on}} - t{ ext{EMG onset}} > 0
  • Low-pass filtering for linear envelope (example):
    • Filter order ~2nd; cutoff ~f_c \approx 3-6\,\text{Hz}