Direct measurement of inertial impact and propulsive force in a eukaryotic swimmer

Cilia and flagella are highly conserved biological motors that drive the motility of diverse eukaryotic cells. In the microscopic world, an organism's swimming kinematics are generally assumed to be a direct, instantaneous proxy for its underlying motor force output. However, as organisms scale in size—such as during the evolutionary transition to multicellularity—this fundamental assumption may break down. Using the multicellular alga as a model biophysical system, we perform direct, time-resolved force measurements during free-swimming collisions to deconvolve a swimmer's inertial impact force from its motor's propulsive force. We discover a ∼ 30 Hz propulsive pulse, which serves as the mechanical signature of collective ciliary action and hydrodynamic synchronization. Crucially, we demonstrate that while this high-frequency motor output drives a fluctuating velocity in the smaller , it is mechanically filtered by the inertia of the larger , resulting in a smooth swimming trajectory. Our work reveals that as organisms grow larger, their own mass acts as a mechanical low-pass filter. This fundamentally decouples high-frequency motor dynamics from whole-organism swimming kinematics, highlighting how physical constraints shape the mechanobiology of motility as organisms evolve toward complex multicellularity.