Identification of Motor Unit Discharge Patterns from High-Density Surface EMG during High Contraction Levels

One major limitation in motor unit (MU) studies is the difficulty in assessing the properties of high-threshold units, which are recruited at high force levels. The aim of this study was to validate the decomposition results obtained from high-density surface EMG by the rate of agreement with the decomposition of concurrently recorded intramuscular EMG during high muscle contraction forces. Surface EMG signals were recorded with a grid of 9x10 electrodes from the tibialis anterior muscle of four healthy men (age, range 24-35) during isometric contractions ranging between 50% and 70% of the maximal voluntary contraction. Bipolar intramuscular EMG signals were recorded with three pairs of wire electrodes. Surface and intramuscular EMG were independently decomposed into contributions of individual MUs. For jointly identified MUs, the rate of agreement between both decomposition techniques was calculated as a percentage of jointly identified MU discharges normalized by the number of all MU discharges. On average, 26 +/- 8 MUs per contraction were identified from the three channels of intramuscular EMG, but only 6 +/- 3 MUs with highly regular discharge pattern and clearly distinguishable action potentials were kept for further analysis. At the same time, surface EMG decomposition allowed to identify 16 +/- 6 M Us per contraction. Due to the strict selection of the MUs from the intramuscular EMG recordings, the number of MUs identified by both techniques was relatively low (1 +/- 1 MU per contraction). For these M Us, decomposition of surface EMG demonstrated a good match in identified discharges with decomposition of intramuscular EMG (average rate of agreement, 94 +/- 3 %). These results demonstrate that MU behaviour can be non-invasively investigated reliably during isometric contractions at relatively high forces.