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Approaching the finish line.
Castelli worked with pro triathletes racing through the demanding conditions at Kona to develop its T.O. ALII Run Top. The result is an airy, sleeveless top that doesn't quite go so far as to be called loose but is optimized to keep athletes cool and dry as they tackle the final 26.2 miles of the course that is arguably triathlon's proving ground.
Striking a balance between forgiving and loose is difficult, but Castelli executes the balance admirably in the T.O. ALII. A-Vent 70g polyester microfiber clings where you need it to while rapidly transferring moisture from the fabric to the skin and allowing air to move through the top. It's constructed with an open mesh knit, which is markedly different than most all-around tri tops.
We wouldn't recommend changing into the T.O. ALII top in T2 for any race much shorter than a 70.3, but after that point, comfort becomes a key concern, and the few seconds you spend swapping tops will translate to quantifiable gains in comfort and breathability later in the race. As an added plus, the top's UPF 15 rating means that your skin will be protected under the top whether you're exposed to the Hawaiian sun or running through powerful rays on your home course.