Do you have a squat rack or platform to dead lift from?
Those two, and their variations, are by far the best exercises to develop the hips, butt and legs. forget the mtn biking.
different mechanics, different energy systems —
sure, for recreational purposes, by all means, have fun, but I can’t see how for a sprinter it would make any sense to regularly include mtn biking as part of your training, even for cross training/general prep.
PS - Chain yourself to the squat rack. squat. a lot. lather, rinse, repeat.
deep squats, deadlifts, snatch-grip deadlifts, roman deadlifts, reverse back extensions, back extensions to some extent, cable pull throughs. If you have a foot strap for the cable you can do hip extensions, same motion and a glute-isolator.