  Molecular motors and cytoskeletal filaments work collectively most of the
time under opposing forces. This opposing force may be due to cargo carried by
motors or resistance coming from the cell membrane pressing against the
cytoskeletal filaments. Some recent studies have shown that the collective
maximum force (stall force) generated by multiple cytoskeletal filaments or
molecular motors may not always be just a simple sum of the stall forces of the
individual filaments or motors. To understand this excess or deficit in the
collective force, we study a broad class of models of both cytoskeletal
filaments and molecular motors. We argue that the stall force generated by a
group of filaments or motors is additive, that is, the stall force of $N$
number of filaments (motors) is $N$ times the stall force of one filament
(motor), when the system is in equilibrium at stall. Conversely, we show that
this additive property typically does not hold true when the system is not at
equilibrium at stall. We thus present a novel and unified understanding of the
existing models exhibiting such non-addivity, and generalise our arguments by
developing new models that demonstrate this phenomena. We also propose a
quantity similar to thermodynamic efficiency to easily predict this deviation
from stall-force additivity for filament and motor collectives.
