Force per cross-sectional area from molecules to muscles: a general property of biological motors

We propose to formally extend the notion of specific tension, i.e. force per cross-sectional area-classically used for muscles, to quantify forces in molecular motors exerting various biological functions. In doing so, we review and compare the maximum tensions exerted by about 265 biological motors operated by about 150 species of different taxonomic groups. The motors considered range from single molecules and motile appendages of microorganisms to whole muscles of large animals. We show that specific tensions exerted by molecular and nonmolecular motors follow similar statistical distributions, with in particular, similar medians and (logarithmic) means. Over the 1019 mass (M) range of the cell or body from which the motors are extracted, their specific tensions vary as Ma with a not significantly different from zero. The typical specific tension found in most motors is about 200 kPa, which generalizes to individual molecular motors and microorganisms a classical property of macroscopic muscles. We propose a basic order-ofmagnitude interpretation of this result.