I think in some ways, padded shorts are MORE necessary on the trainer than the road. On the road, you stop, stand up, sprint, climb out of the saddle, etc. You change positions frequently. On the trainer, I'm finding I stay seated for long stretches. When doing intervals I now sometimes ride out of the saddle for 30 or so seconds during the 'recovery' segments.
Regarding hygiene, yeah - my bibs are always completely soaked in sweat after a session on the trainer. So I have at least 5 pairs of them and never wear a pair twice before laundering. them.
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