Swimming Art & Science
Summary
Under water, most of a video measurement's error comes from refraction at water, glass and air, and from how the camera was calibrated for it. Frames, wands, chessboards and land calibrations compared; why two action cameras measured better under water than in air; the two errors of the shutter; what a fixed pair of cameras solves; and why millimetres matter for forces. The second of five posts on video analysis in swimming.
Topics
Video analysisBiomechanicsAuthor V.Karpov
Summary
Video at the pool is mostly replay. The same camera is a measuring instrument: a stopwatch, a speedometer and a protractor in one housing, with tested accuracy and known limits. What it measures from the stands, what measuring the body's movements takes, and what no camera measures: the first of five posts on video analysis in swimming.
Topics
Video analysisBiomechanicsAuthor V.Karpov
Summary
Passive drag is a force you can hang a weight against. Active drag is a quantity defined by subtraction. The first of five posts on how swimming resistance is measured.
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Active dragBiomechanicsAuthor V.Karpov
Summary
The two oldest approaches to active drag sit at opposite ends of the scale: breathing measurements say resistance is 1.6 to 3 times passive drag, fixed pads say it is about equal. Both have reasons.
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Active dragBiomechanicsAuthor V.Karpov
Summary
The small perturbation method and assisted and resisted towing share one equation and one assumption; the naval-architecture variant shares only the assumption. How the equation works, what the assumption costs, and why the size of the perturbation is a dilemma rather than a setting.
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Active dragBiomechanicsAuthor V.Karpov
Summary
Tether the swimmer and change the flow, or film the swimmer and write down the force balance instant by instant. The flume, pool-tether and HRM methods, and the question of which force any of them is reporting.
Topics
Active dragBiomechanicsAuthor V.Karpov
Summary
Direct comparisons on the same swimmers and whether the methods rank them alike, a map of published values, seven reasons for the gap, one reliability table, and a short list of what to ask of any number called active drag.
Topics
Active dragBiomechanicsAuthor V.Karpov
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