The effect of subcutaneous fat on myoelectric signal amplitude and cross-talk.

The effect of subcutaneous fat on myoelectric signal amplitude and cross-talk was studied using finite element (FE) models of electromyogram (EMG) signal propagation. A FE model of the upper arm consisted of skin, fat, muscle and bone tissues in concentric layers. Single muscle fibre action potentials were simulated for muscle fibres at a variety of depths and combined to simulate surface EMG interference patterns. As fat layers of 3, 9 and 18 mm were added to the model, the RMS (root mean square) amplitude of the surface EMG signal directly above the centre of the active muscle decreased by 31.3, 80.2 and 90.0%, respectively. Similarly, surface EMG cross-talk above the region of inactive muscle increased as the fat layer thickness increased. The surface EMG RMS amplitude fell below 5% of its value above the centre of the muscle at 14 degrees, 17 degrees, 34 degrees and 47 degrees from the edge of the active muscle with fat layers of 0, 3, 9 and 18 mm, respectively. An additional model was developed with the subcutaneous fat layer thinned from 9 mm to 3 mm in a small, focal region under a pair of recording electrodes. Reducing the fat layer in this manner caused the surface EMG amplitude at the electrodes to increase by 241% and decreased the EMG cross-talk by 68%; this was near the values for the 3 mm uniform fat layer. This demonstrates that fat reduction surgery can increase surface EMG signal amplitude and signal independence for improved prosthesis control.