Velocity times fraction motile = Velocity Index. The Velocity Index offers a more complete characterization of the motility behavior by multiplying velocity data by the fraction of moving filaments, which can be reduced at limiting myosin surface densities. (a) 2HEP WT velocity increases in proportion to the surface occupancy. (b) 2HEP WT fraction moving is reduced at low myosin surface occupancy where the numbers of active motors becoming limiting. (c) 2HEP WT Velocity Index = (velocity) × (fraction moving) using data in a and b. Data points represent mean values ± SD of three experiments from separate protein preparations. (d) Bihistogram of pooled alpha values for 2HEP WT and 15HEP WT across all surface occupancies (n = 3, see Statistics).