EMG of reinnervated motor units: a simulation study

Using computer simulation techniques, reinnervation of motor units (MUs) was studied by increasing the number of muscle fibers in the MU without changing the MU territory. The fiber density (FD) measured by single fiber EMG electrodes, the amplitude, area and number of turns of concentric needle (CN) EMG motor unit action potentials (MUAPs) and the amplitude of macro EMG MUAPs were most affected by partial reinnervation changes. The values of these features increased during simulated advanced reinnervation, as did the number of CNEMG MUAPs that had increased numbers of phases or turns and the mean CNEMG MUAP duration. The increase in macro EMG MUAP amplitude, FD and CNEMG MUAP area were proportional to the increase in the number of muscle fibers in the MU. When loss of muscle fibers due to so-called MU fractionation was simulated, values of all EMG features fell, but were still increased compared to normal. Two patterns of change in SFEMG and macro EMG values were identified that may distinguish between recordings made from reinnervated low force threshold MUs and those from higher force threshold MUs.