that happens because your lower back is weak and you use your lower back a lot when riding trials. your back just needs some training, it helps against the pain and you will get extra inches in gaps, taps, pedal ups and other moves where you use your lower back a lot http://www.nismat.org/orthocor/programs/lo...backex.html#Ex1 you can start with these ones, those helped me and now im doing deadlifts and a few more excersises for my back with heavy weights