Exercising is the only way to build and maintain physical fitness. While the popular "no pain, no gain" slogan is overstated, it's true that working the muscles vigorously enough to make them stronger requires breaking down weaker muscle fibers so that your body can replace them with hardier ones. An inevitable part of this load-compensation cycle is soreness in stressed muscles, and since your butt muscles are among the largest in your body, and are used in a variety of different exercises, they can be a real pain in the you-know-where.
The primary muscles of the butt are, in descending order of size, the gluteus maximus, the gluteus medius and the gluteus minimus. Layered roughly atop one another, their job is to extend the hip -- that is, move the rear of your leg closer to your back, as when drawing back to kick something -- and laterally rotate the hip joint, i.e., move your leg in a way so that your toe points outward. The gluteus maximus is the largest and strongest muscle in the body.
Cycling is a great no-impact form of aerobic exercise, but you'd never know it from the way your butt can hurt after a long ride. Sitting for a number of hours on a bicycle seat leads to reduced blood flow in the glutes because of mechanical pressure, and the reduced ventilation combined with sweating can compound the problem through chafing. The website BicyclingLife.com includes various tips on avoiding the pain of "saddle seat," including angling the saddle differently or adjusting the seat upward or downward.
Weight Training and Plyometrics
Many people focus on working their gluteal muscles for the purpose of developing a firmer, more shapely backside, and as a consequence they incur a great deal of muscle soreness, especially if they are not used to doing these specific exercises. Common exercises to target the butt include jump squats, lunges, calf raises, squats, and various exercises done without the use of weights, including stair-climbing, running up hills, and squat thrusts.
Very common among distance runners, piriformis syndrome affects a small butt muscle beneath the glutes called the piriformis, which helps stabilize the pelvis during exercise. The repetitive motion of running in concert with the small size of the piriformis results in tightening, inflammation, and pain between the center of the buttock and the hip bone, usually on one side. Treatment includes rest, massage, and the use of anti-inflammatory medications as directed by a health professional.