A jump-diffusion process is proposed to describe the displacements 
performed by single myosin heads along actin filaments during the 
rising phases. The process consists of the superposition of a Wiener 
and a jump process, with jumps originated by sequences of 
Poisson-distributed energy-supplying pulses.
In a previous paper \cite{DCMR2002}, the amplitude of the jumps 
was described by a mixture of two Gaussian distributions. 
To embody the effects of ATP hydrolysis \cite{BR2002}, we now refine 
such a model by assuming that the jumps' amplitude is described 
by a mixture of three Gaussian distributions. 
This model has been inspired by the experimental data of T.\ Yanagida 
and his co-workers concerning observations at single molecule 
processes level as described in \cite{KTIY1999} and in the references 
therein.