EMG Onset Detection Protocol

Before data collection, patients’ skin over the vastus lateralis was shaved, rubbed with abrasive skin prep, and cleaned with alcohol to improve the electrode–skin contact and minimize skin impedance. Bipolar, disposable, pre-gelled Ag/AgCl surface electrodes with 20 mm distance between electrode centers were placed on the belly of the vastus lateralis. The exact placement of the electrodes followed the recommendations by Surface Electromyography for the Non-Invasive Assessment of Muscles (SENIAM 2008 ). The reference electrode was placed over the proximal end of the fibula of the same leg. Signals were analog filtered at 10–500 Hz (with first order filter at lower cutoff frequency and sixth order filter at higher cutoff frequency), amplified 2000× and sampled at 1 kHz using a TeleMyo 900 telemetric hardware system (Noraxon USA, Inc., Scottsdale, AZ, baseline noise < 1 uV RMS, Common Mode Rejection min. 85 dB through 10–500 Hz operating range).
EMG signals were recorded as each of 17 subjects performed one near-maximal voluntary isometric contractions, starting with 1 s of rest interval to establish baseline. The raw signals were visually inspected and the pre-contraction portions of the baseline as well as the steady portions of the EMG burst were identified. The baseline and the EMG burst from each recorded signal was then used to construct 17 reference EMG signals by adjoining the baseline and the burst portion at a known onset time t₀ (see Fig. 1). Length of the EMG baseline, EMG burst, and position of the true onset t₀ varied for all reference signals. The known, true onset times, t₀, were used as a reference to quantify the accuracy of estimated onset times t₁ identified by three onset detection methods.Fig. 1

Construction of the reference signal. From the raw signal (top panel), a portion of the baseline and a portion of the EMG burst was selected (middle panel) and re-joined at the known onset time t₀ (bottom panel). To demonstrate the importance of signal conditioning, the baseline in this example contains fluctuations in the signal amplitude. These fluctuations were not associated with the muscle contraction

The precise EMG onset was not known in the experimental signals recorded from old adults during gait. In these trials, we determined the onset time, t₀, by visual detection because computerized techniques should detect EMG onset close to the onset time selected by individuals with EMG expertise (Staude et al. 2001 (link)).
SNR of the reference signals was calculated to test the influence of signal quality on onset detection accuracy. The SNR of the signals was defined as: where A is RMS amplitude. All data analysis was performed in MATLAB (MathWorks, Natick, MA).