KINE 4930 Laboratory Experiences in Kinesiology II: EMG Detailed Notes

Electromyography (EMG) Overview

  • Definition: EMG is the recording of electrical signals related to muscle contraction.
    • Origin of words:
    • "Electro" = motor unit action potential (MUAP)
    • "Myo" = muscle
    • "Graphy" = resulting waveform
  • Significance:
    • Important for studying human movement.
    • EMG can provide insights into:
    1. Active muscles during movement.
    2. Timing of muscle activation and cessation.
    3. Magnitude of muscle electrical response during tasks.
    4. Signs of muscle fatigue.
  • Action Potential Propagation:
    • Action potentials (APs) generated in muscle fibers induce muscle contractions.
    • APs spread from the neuromuscular junction (NMJ) along the sarcolemma and down transverse tubules.
    • Electromyogram consists of summated electrical activities from muscle fibers innervated by motor units.

Motor Units and Action Potentials

  • Motor Units (MUs):
    • Consists of a motor neuron and all muscle fibers it innervates.
    • Innervation Ratio: Number of muscle fibers innervated by one motor neuron; varies widely.
  • Motor Unit Action Potential (MUAP):
    • Sum of electrical activities from all active muscle fibers in a motor unit.
    • Amplitude of MUAP influenced by the number and size of muscle fibers.

Electromyogram Signals

  • Recording Techniques:
    • Surface EMG:
    • Uses electrodes placed on the skin surface over muscles.
    • Advantages:
      • Non-invasive, easy to apply.
      • Good for large muscle activity.
    • Disadvantages:
      • Prone to noise from skin and cross-talk from adjacent muscles.
    • Intramuscular (Indwelling) EMG:
    • Electrodes inserted directly into the muscle.
    • Advantages:
      • Reduced cross-talk; better signal quality.
    • Disadvantages:
      • Invasive, pain potential, requires sterilization.

Electromechanical Delay (EMD)

  • Definition: The time (20-100 ms) between the onset of the AP signal and muscle tension development.
  • Importance: Required for biochemical processes that lead to contraction.

EMG Display and Data Acquisition

  • Sampling Rate: Must adhere to the Nyquist Sampling Theorem (at least twice the highest signal frequency).
  • EMG Frequency Spectrum:
    • Typically varies between 5-2000 Hz depending on the electrode type; most activity below 1000 Hz.
    • Surface EMG signals are often in the range of 20-500 Hz.

Noise Reduction and Filtering

  • Common Mode Rejection Ratio (CMRR): Affects signal quality by reducing common electrical noise from the EMG signal.
  • Digital Filtering: Used to clean up signals:
    • Low-pass filters: Allow low frequencies while discarding high frequencies.
    • High-pass filters: Allow high frequencies while discarding low frequencies.
    • Band-pass filters: Allow a specific range; attenuate outside frequencies.
    • Notch filters: Suppress specific known frequency interferences (e.g., 60 Hz hum).

Processing the EMG Signal

  • Steps for Processing:
    1. Rectification: Taking the absolute value to ensure all values are positive.
    2. Linear Envelope: Producing a smooth representation of muscle activity over time.
  • Integrated EMG (iEMG): The total activity over a specified time period; distinct from linear envelope as it reflects cumulative activity.

Factors Affecting EMG Signals

  • Blood Flow: Alters signal due to low-pass filtering effects, particularly at greater tissue depths.
  • Muscle Length & Depth: Affect action potential propagation speed and amplitude, complicating EMG readings.
  • Crosstalk: Concurrent signals from nearby muscles can contaminate results.

Applications of EMG

  • EMG provides insights for:
    • Muscle performance measurement.
    • Ergonomic studies.
    • Rehabilitation and surgical decision-making.
    • Sports performance analysis.

Muscle Fatigue

  • Fatigue indicated by:
    • Increased amplitude during sustained contractions.
    • Decreased conduction velocity and shifting frequency content to lower values as fatigue progresses.

Relationship Between EMG and Muscle Force

  • General trend:
    • Increase in contraction intensity typically leads to higher EMG amplitude and frequency.
    • Variations exist, especially in eccentric movements where EMG can be lower despite force exerted.

EMG Lab Stations Overview

  • Station 1: Skin preparation and electrode placement.
  • Station 2: Equipment familiarization with EMG technology and concepts.
  • Station 3: Acquiring EMG signals through proper electrode attachment and initial testing.
  • Station 4: Signal normalization to establish baseline values for contractions.
  • Station 5: Processing EMG data, focusing on rectification and analysis.
  • Station 6: Investigating the relationship between EMG and muscle force through a force-EMG device.
  • Station 7: Analysis of frequency domain and fatigue effects on EMG signals.