Why were swimming records frozen after the swimsuit ban?

Because a generation of suits produced such dramatic improvements that the records set in them stood for years afterwards, distorting the sport's record books.

What the suits were. From around 2008, manufacturers introduced full-body suits made from polyurethane and similar non-textile materials, most famously the Speedo LZR Racer and subsequent full-polyurethane suits from other makers.

Why they were so effective:

Buoyancy. The materials trapped air and were themselves less dense than water, lifting the swimmer higher. A higher body position reduces drag substantially, since the body presents a smaller wetted surface and better alignment.

Compression. The suits held the body in a streamlined shape and reduced muscle oscillation.

Reduced surface drag from an engineered surface.

Improved body alignment, particularly reducing leg sink.

The scale of the effect. In 2008 and 2009 the record books were rewritten wholesale. At the 2009 World Championships in Rome, an extraordinary 43 world records were set in a single meet. Swimmers who had never been near a record were breaking them, and the phenomenon was sufficiently absurd that commentators described the suits as technological doping.

The ban. FINA — now World Aquatics — introduced rules effective from 2010 requiring suits to be made of textile materials only, and limiting coverage: men from waist to knee, women from shoulder to knee, with no zips or fastening systems.

The consequence. Several 2008–09 records became effectively unreachable. Some stood for a decade or more, meaning the best swimmers of the following generation could be outstanding and never hold a world record in their event. Paul Biedermann's 200m freestyle record, set in a polyurethane suit against Michael Phelps in textile, is the standard example and stood for years.

Records were not annulled, which was debated — the governing body chose to let them stand, on the reasoning that they were legal when set.

Most have since fallen, which took genuine generational improvement.