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During fatiguing contractions muscle fiber conduction velocity (CV) decreases progressively. The exact cause of this is not yet fully known, although recent studies suggest that changes in extracellular potassium concentration play an important role. A model was developed to examine the effect of accumulation of extracellular potassium ions on the muscle fiber action potential and its CV. As the extracellular potassium concentration was increased, the action potential progressively broadened, reduced in peak-peak amplitude and its CV decreased. However, when the inward rectifier channels were blocked, the changing shape of the action potential and the reduction of its CV with increasing extracellular potassium were dramatically reduced. The simulation results support the hypothesis that the reduction in muscle fiber CV observed during sustained fatiguing contractions may be due in part, to increased accumulation of extracellular potassium ions and suggest that the inward rectifier currents play an important role on the relationships observed.