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Extract Envelope from EMG Signal

Source: R/preprocess_envelope.R
extract_envelope.Rd

Extracts the amplitude envelope from rectified EMG signals using various methods.

Usage

extract_envelope(
  data,
  sampling_rate,
  method = "lowpass",
  cutoff = 6,
  window_ms = 50,
  filter_order = 4,
  zero_phase = TRUE
)

Arguments

data

Rectified EMG signal. Can be a numeric vector (single channel), a matrix (T x M, time x channels), or a list of matrices. Should typically be rectified before envelope extraction.

sampling_rate

Sampling frequency in Hz.

method

Envelope extraction method:

  • "lowpass": Low-pass filter (default, most common)

  • "rms": Root Mean Square with sliding window

  • "moving_average": Simple moving average

cutoff

Cutoff frequency for low-pass filter in Hz. Only used when method = "lowpass". Default is 6 Hz.

window_ms

Window size in milliseconds for RMS or moving average. Only used when method is "rms" or "moving_average". Default is 50 ms.

filter_order

Filter order for low-pass method. Default is 4.

zero_phase

Logical. If TRUE (default), applies zero-phase filtering for the low-pass method.

Value

EMG envelope signal, same structure as input.

Details

The envelope represents the amplitude modulation of the EMG signal, showing muscle activation patterns over time. Common approaches:

  • Low-pass filter: Applies a Butterworth low-pass filter to the rectified signal. Cutoff frequency typically 4-10 Hz.

  • RMS: Computes root mean square in a sliding window. Good for preserving amplitude information.

  • Moving average: Simple smoothing with a sliding window. Fast but may introduce artifacts at window edges.

Examples

if (FALSE) { # \dontrun{
# Generate and process EMG
fs <- 1000
raw_emg <- rnorm(1000)
rect_emg <- rectify(raw_emg)

# Low-pass envelope (most common)
env_lp <- extract_envelope(rect_emg, sampling_rate = fs, method = "lowpass")

# RMS envelope
env_rms <- extract_envelope(rect_emg, sampling_rate = fs,
                            method = "rms", window_ms = 100)
} # }