Two strength-building exercises every mountain biker should do are split squats and single-leg hamstring curls. Here's why:
When you're in the saddle, you never get quite full leg range of motion when you're pushing down to the pedal. You'll get a similar range of motion and develop the same muscles used in riding with the split squat. Since your hips are in that flexed position so much, the split squat will also loosen up your hip flexors (on the front of your upper thigh), which will help alleviate any lower back pain.
The single-leg hamstring curl, using either a ValSlide or stability ball, helps optimize your upward pulling while cycling, which we know is one of the most powerful parts to efficient cycling mechanics. But what's really great is that this exercise trains your hamstrings and calves one leg at a time.
-- Craig Friedman, Director of Methodology, Athletes' Performance