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To examine the motility of actomyosin complexes in the presence of high concentrations of polymers, we investigated the effect of poly(ethylene glycol) on the sliding velocities of actin filaments and regulated thin filaments on myosin molecules in the presence of ATP. Increased concentrations and relative molecular masses of poly(ethylene glycol) decreased the sliding velocities of actin and regulated thin filaments. The decreased ratio of velocity in regulated thin filaments at − log[Ca2+] of 4 was higher than that of actin filaments. Furthermore, in the absence of Ca2+, regulated thin filaments were moderately motile in the presence of poly(ethylene glycol). The excluded volume change (ΔV), defined as the change in water volume surrounding actomyosin during the interactions, was estimated by determining the relationship between osmotic pressure exerted by poly(ethylene glycol) and the decreased ratio of the velocities in the presence and absence of poly(ethylene glycol). The ΔV increased up to 3.7 × 105 Å3 as the Mr range of poly(ethylene glycol) was increased up to 20 000. Moreover, the ΔV for regulated thin filaments was approximately two-fold higher than that of actin filaments. This finding suggests that differences in the conformation of filaments according to whether troponin–tropomyosin complexes lie on actin filaments alter the ΔV during interactions of actomyosin complexes and influence motility.Increased concentrations and molecular weights of PEG decreased the sliding velocity of both actin and regulated thin filaments on myosin molecules. The decreased ratio of velocity in regulated thin filaments at pCa 4 was higher than that of actin filaments. The conformation of the surface of filaments decorated with Tn-Tm affects the excluded volume for actomyosin interactions.