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Two different relations between motor unit electrical and mechanical properties were used to simulate isometric force and surface EMG signals of the first dorsal interosseous (FDI) muscle in man: 1) motor unit action potential (MUAP) amplitude was linearly proportional to the square root of twitch force; 2) MUAP amplitude was linearly proportional to twitch force. The EMG amplitude-force relations that emerged from simulations indicated that the experimentally observed linear EMG amplitude-force relation was most likely due to the linear electrical and mechanical relation at motor unit level. When MUAP amplitude was proportional to the square root of twitch force, the amplitude of the simulated surface EMG increased slower than force, which was unlike that observed experimentally. The simulation results also suggested that estimating the number of MUAP's in the surface EMG may be a more physiologically …