Background: Resisted swimming, including tethered and semi-tethered swimming, has been used for almost 100 years for performance assessment and for over 50 years as a training strategy. Aim: To investigate the acute effects of resisted swimming on kinematics, muscle activity, energetics, and active drag estimation. Methods: Three-dimensional whole-body kinematics, surface EMG from six major propulsive muscles and energetic variables were analysed across three protocols during front crawl under external resistance conditions. During energetic testing, parachute drag was quantified at paired metabolic power (60-80% Vmax), under active swimming conditions (Coloretti, 2025, JSCR), and active drag was estimated using a residual-thrust method derived from resisted swimming (Cortesi, 2024, JSSM). Drag efficiency was calculated as the fraction of metabolic power converted into power to overcome drag (Zamparo, 2020, EJAP). Results: At sprint pace, external resistance did not significantly affect free swimming kinematics in terms of movement amplitude and timing. Only shoulder roll decreased (*6.0° at 10% and *9.1° at 80% resistance), whereas stroke rate remained unchanged. Likewise, EMG amplitude of the six propulsive muscles was unaffected by resistance. At paired aerobic metabolic power, the ~45% increase in energy cost with a 900 cm² parachute was proportional to the reduction in swimming velocity and was explained by the additional parachute drag (~40-70 N at 0.65-0.85 m·s*¹). The apparent reduction in drag efficiency during parachute swimming was mainly due to lower speed, as parachute swimming per se did not affect drag efficiency. Conclusions: Resisted swimming does not appear to modify the fraction of force used to propel the body. The absence of meaningful changes in stroke kinematics, muscle activation, and drag efficiency supports resisted swimming as a valid approach for monitoring active drag during short-duration efforts, rather than as a training strategy to increase force application
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