Lower back pain in mountain bikers
The culprit is mostly poor prolonged flexed postures on the bike.
What to do:
Find your neutral spine and build up the endurance of your postural muscles to maintain this position on your bike.
Make sure your bike has been set up correctly, and regularly check your saddle height and distance to the handle bars throughout your training and race events.
Avoid the hunched over position and over reaching on the bike
Try straighten up into extension on the long flats on the ride.
Pink dots on the handle bars/ watch/ Garmin to constantly remind you of your neutral spine position.