Figure 1.

Two states, long and short, of the motile element. The probability of state L is $P$ and that of state S is $1-P$. The energy level of state L is higher than that of S by $-aF-qV$ when the membrane potential is $V$ and the force applied to the motile element is $F$. Notice that $a>0$ and $q<0$ because depolarization shrinks the cell.