I would say that a programme of heavy squats, plyometrics, weighted jumping, leg curls, super setting your calves, and jogging would be the best for the lower body and fitness of a trials rider. For the upper body, I would do the bench press, working from low to high weight and back. And do shoulder presses, and also bicep curls, and forearm exercises. Then I would do weighted sit ups and oblique crunchs, aswell as weighted back rises, and I would also do serious amounts of lat machine work, and work on your traps. What is heavy is defined by you. Also a sprinting programming such as 20 fifty metre sprints in a row, with no more than a minute rest between sprints, always going as fast as possible, will be great for speed and endurance. As you can see, trials requires a lot of work, because it is a discipline and sport that uses the entire body.