A single fiber action potential (SFAP) can be modelled as the convolution of a biolectrical source and a filter impulse response. In the Dimitrov-Dimitrova (D-D) convolutional model, the first temporal derivative of the intracellular action potential (IAP) is used as the source, and Tspl is a time parameter related to the duration of the IAP waveform. This paper is centred on the relation between Tspl and the main spike duration (MSD), defined as the time interval between the first and third phases of the SFAP. We show that Tspl essentially determines the MSD parameter. As experimental data, we used fibrillation potentials (FPs) of two different muscles to study the D-D model. We found that Tspl should have a certain statistical variability in order to explain the variability in the MSD of our FPs. In addition, we present a method to estimate the Tspl values corresponding to a given SFAP from its measured MSD.