A mountain bike's saddle cannot be overlooked. The tires provide contact between bike and trail; the saddle is the key point of contact between bike and rider. It supports some of the rider's weight and also provides lateral control. The wrong kind of seat can cause discomfort over long rides and reduce the rider's ability to effectively control the bike.
To find the right seat, you first have to consider two important parts of the seat: the pads and the nose. The pads are the rearmost portion of the seat; this is where most of your weight will rest when you're sitting back on the seat. Specifically, the two bony protrusions of your pelvis known as the ischial tuberosity (or sitting bones) bear your weight when you're in a seated position. Make sure these bones are actually resting on the pads. If your pelvis is too wide for the seat, the sitting bones will extend beyond the edges of the pads. Soft tissues will end up bearing your weight, which isn't very comfortable. Women tend to have wider pelvises than men, which is why companies offer saddles specially designed for female riders.
The nose can be a source of discomfort if the seat is not adjusted properly. However, it serves an important function, allowing the rider's thighs to exert lateral control over the bike. There are alternative saddle designs that omit the nose, but serious riders wouldn't consider them. If you focus on long cross-country rides, then comfort may be a more important factor than control.
The best way to find the right saddle and adjust it properly is to take it on a trail (wearing your usual biking clothes). A spin around the bike store parking lot won't tell you much about a saddle. Luckily, some bike shops offer test saddles that can taken on trails for real testing. There's definitely trial and error involved, but eventually you'll find the saddle perfectly suited to your riding style and body shape.
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