You may not associate the biceps with the primary muscles used for mountain biking, but this arm muscle stabilizes and steers the bike, which requires some work.
Rectus Abdominis Muscle
You probably know the rectus abdominis simply as the abdominal muscles, which are exercised during sit-ups. They are also heavily used during mountain biking, where they serve as pressure regulators during strenuous uphill rides.
The gluteus maximus is in the buttocks. You may feel this muscle in action when you stand on one leg with your arms extended for balance; the gluteus maximus makes it possible for your pelvis to remain in position during movement. As you are mountain biking, you use this muscle extensively when moving the thighs.
You can feel the semitendinosus in the back of the thigh. It is instrumental in bending the knees, and therefore becomes a major player during the pumping action of the pedaling movement. The semitendinosus works in conjunction with the gluteus maximus and the quads to accomplish this motion.
Farther down the leg to the calf, you can feel the gastrocnemius in the back of the lower leg. It reaches from the knee down to the ankle. You might notice this muscle during painful contractions that force you to stop your ride and massage the calves. The lower portion of the gastrocnemius is actually the soleus muscle.