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The rails on a seat allow you to set it towards the front or rear, which is a complex and important optimisation. The position of the seat with respect to the handlebars and pedals make the main difference between the steep 74 degree seat tube of a racing bike and the 71 degree tube of a touring bike.

Moving your saddle back puts you in a lower posture, which is more aerodynamically efficient, allows you to use all of your leg muscles, and is better for your back and breathing in the exact same way as dropped handlebars. The best way to achieve this may be to lower your headset and get one which extends farther, however, as sliding the seat back can mess up your leg geometry. The farther forward your are, the more total power output you have available -- hence the steep seat-tube angle on racing or sprinting bikes -- and farther back allows you to "ankle" more effectively and is conducive to long-haul output.

One way to find the correct fore/aft position is when the knee is directly over the pedal spindle with the crank at the 3 o'clock position. Have a friend dangle a plumb line from just below your kneecap when at this position, and set you saddle forwards or backwards accordingly for the plumb to line up with the pedal spindle. This is generally the optimum position for exerting muscle power on the spindles.

Older riders generally prefer seats towards the back of the usual 1 3/4" to 2 1/2" range from the nose of the saddle to a vertical line through the crankset.
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