The electrocardiogram (ECG) is the recording of the heart's electrical activity, obtained by measuring the biopotentials generated during depolarization and repolarization of the cardiac muscles. Measured using surface electrodes placed at specific points on the body.

The interior of the cell membrane is considered to be negative with respect to the outside during resting conditions. When an electric impulse is generated in the heart, the interior part becomes positive with respect to the exterior. This change of polarity is called depolarization. After depolarization the cell comes back to its original state. This phenomenon is called repolarization. The ECG records the electrical signal of the heart as the muscle depolarizes (contract) and repolarizes.

Signal Characteristics:

• Amplitude: Approximately 0.1 – 4 mV
  
• Frequency content: From DC up to ~100 Hz
  

Clinical significance:

• Used to assess heart rate and rhythm
  
• Helps detect cardiac abnormalities such as arrhythmias and
  
• Provides a valuable timing reference in cardiovascular measurements
  

ECG Waveform:
The diagram below shows the principal components of a standard ECG signal:

Figure 1: The Electrocardiogram Waveform.

Key Components:

• P wave: Atrial depolarization
• QRS complex: Ventricular depolarization
• T wave: Ventricular repolarization

Various intervals, such as the PR interval and QT interval, correspond to different phases of the cardiac cycle. Each feature can be analyzed to assess heart function and detect abnormalities.

2. Electroencephalogram (EEG)

The electroencephalogram (EEG) is the recording of the brain's electrical activity, obtained by measuring the biopotentials generated by neuronal activity at the surface of the scalp or directly within brain tissue. Recorded using surface electrodes placed on the scalp, or in some cases using needle electrodes inserted into brain tissue.

Origin:

• Generated by the electrical activity of neurons in the brain, resulting mainly from postsynaptic potentials of large groups of cortical cells.
  

Signal Characteristics:

• Amplitude: Approximately 10 – 100 μV (peak)
  
• Frequency content: From DC up to ~100 Hz
  

Clinical Significance:

• Used to study characteristic brain waveforms (e.g., alpha, beta, delta, theta rhythms)
  
• Helps in detection and diagnosis of neurological disorders, such as epilepsy
  
• Useful for spectral analysis of brain activity and for measuring evoked potentials in response to stimuli
  
• Plays a key role in monitoring brain states like sleep, consciousness, and anesthesia
  

Typical EEG Waveforms and Frequency Bands:
EEG signals differ based on electrode placement and brain activity. To interpret them, they are commonly divided into distinct frequency bands, each named after a Greek letter, and each associated with specific mental states.

Figure 2: Human BrainWaves (EEG)

Frequency Bands:

• Delta (below 3 Hz): Prominent in deep sleep
  
• Theta (3–8 Hz): Associated with light sleep and drowsiness
  
• Alpha (8–13 Hz): Seen when relaxed with eyes closed, representing synchronized brain activity
  
• Beta (13–30 Hz): Linked to alertness and active thinking, showing desynchronized patterns
  
• Gamma (30–100 Hz): Related to higher cognitive functions such as perception, memory processing, and consciousness
  
  

3. Electromyogram (EMG)

The electromyogram (EMG) is the recording of the electrical activity produced by skeletal muscles during contraction and relaxation. These signals arise from the depolarization and repolarization of muscle fibers, reflecting the summed action potentials of motor units.

Methods of Measurement:

• Surface Electrodes: Placed on the skin above the muscle group; capture activity from multiple fibers
  
• Needle Electrodes: Inserted directly into the muscle tissue for localized recording, useful when nearby large muscles interfere
  

Signal Characteristics:

• Amplitude: Approximately 50 μV to 1 mV
  
• Frequency content: From about 10 Hz up to 3 kHz
  

Clinical Significance:

• Indicates muscle contraction intensity and timing
  
• Used to study muscle fatigue and neuromuscular function
  
• Aids in diagnosing neuromuscular disorders such as myasthenia gravis, neuropathies, or muscular dystrophy
  
• Important in rehabilitation monitoring and prosthetic control applications
  

Typical EMG Waveform:
The EMG signal is a complex, noise-like pattern representing the instantaneous sum of action potentials from multiple muscle fibers. When the muscle is at rest (baseline), the signal is minimal. During muscle contraction, the waveform amplitude increases, producing an interference pattern that obliterates the baseline. The energy and shape of the EMG waveform depend on the level of muscle activity and electrode placement.

Figure 3: Typical ElectroMyoGram (EMG) waveform.