Direct Measurement of Inertial Impact and Propulsive Force in a Eukaryotic Swimmer

The transduction of force into motion for microswimmers at intermediate Reynolds numbers ($Re\sim 1$), where inertia becomes relevant, is a fundamental problem in active matter. Using the multicellular alga $\mathit{\text{Volvox}}$ as a model physical system, we perform the first direct measurements that deconvolve a swimmer’s inertial impact force from its motor’s propulsive force. We discover a $\sim$30 Hz propulsive pulse, the mechanical signature of collective ciliary action. This high-frequency motor output drives a fluctuating velocity in the low-$Re$ $\mathit{\text{V.}}\mathit{\text{carteri}}$, but is mechanically filtered by the inertia of the larger $\mathit{\text{V.}}\mathit{\text{ferrisii}}$, resulting in a smooth swimming trajectory. Our work demonstrates that for swimmers beyond the Stokes regime, kinematics are not a direct proxy for the underlying motor dynamics, a foundational assumption in the study of microscopic motility.