Background: Tethered swimming is a method for quantifying swimmers` performance including intra-stroke changes in propulsive force. Tethered swimming assumes that the swimmer remains stationary, and the center of mass (CoM) acceleration contribution is negligible. Thus, the force measured by the sensor (Fs) equals the propulsive force (Fp). Aim: To test this assumption by examining the relationship between sensor force, sagittal body motion, and propulsive forces. Methods: Five male swimmers performed submaximal effort tethered trials using only their arms. A force sensor (120 Hz) measured cable tension, while an underwater camera (60 Hz) recorded sagittal motion. Our markerless system SwiMe recorded joint location and the hip location was used as a proxy for CoM. Propulsive force was modeled dynamically as Fp = ma + Fd + Fs, accounting for force applied by the cable on the swimmer (Fs), inertial forces (ma) and active drag (Fd) Fd= ½ * *Cd *A* v²(Cd and A are based on values form the literature). Results: Fp average max was approximately 125N, Fd was approximately 1N. Swimmer`s average forward-backward range of motion of 11 cm, resulting in peak accelerations of 0.84(±0.21) m/s2. Thus, the inertial effect of body mass can contribute approximately 90 N, indicating it cannot be neglected. The sensor force (Fs) showed a weak Pearson correlation with CoM acceleration (R2=0.1), whereas propulsive force (Fp) demonstrated a strong correlation (R2 = 0.87). Additionally, the maximum propulsive force occurs when the hand is at its deepest (start of the push phase). These results align with the hand forces vs trajectory as predicted using CFD and SWUM simulations. Conclusions: In only arm tethered swimming the assumption that sensor force is equivalent to propulsive force is flawed due to inertial effects caused by intra-cycle acceleration. Thus, accurate intra-cycle evaluation of propulsion requires accounting for body mass acceleration.
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