MTB underwear by the best brands
In cross-country, steep climbs and fast descents alternate at short intervals. The base layer must therefore cover both: heavy sweating uphill and cooling down downhill.
The layer that handles temperature changes
If the base layer reliably transports sweat outwards, the skin stays relatively dry even under high load, and the following descent does not become a shivering experience. In XC, a thin, close-fitting short sleeve or sleeveless model has become standard.
A second aspect is added off-road: under protectors and backpack straps, bare skin chafes.
When choosing between merino and synthetic fibres, the length of the ride decides: synthetic fibres dry faster on short, intense rides; merino stays warm when damp on long tours and stays odour-neutral for longer.
Look for flat or bonded seams in the shoulder area. That is exactly where backpack straps rest.
