The influence of changes in the intracellular action potential (IAP) spatial profile on motor unit potentials (MUPs), number of turns per second (NTs), and mean turn amplitude were simulated and analyzed. We show why measurement of NTs was “the best indicator of neurogenic affection” and why the lower diagnostic yield of turns/amplitude analysis in myopathy could be due to changes in IAP shape caused by elevated free calcium concentration. The results explain the complications observed when interference electromyographic signals obtained during high levels of isometric contractions were analyzed. We show that, in contrast to earlier assumptions, the effect of increased IAP spike duration on NTs was stronger than that of a decrease in muscle fiber propagation velocity (MFPV). The decrease in the NTs could occur without a drop-out of MUs and/or a decrease in their firing rates, and without a change in MFPV and synchronous firing.