Concentration curls put no strain on the back. From a seated position, holding a dumbbell in one hand, lean forward resting the elbow on a padded surface. For instance, sit on a stool at a kitchen table or desk. Place a rolled up towel under the elbow. Lower and raise the dumbbell just like in a standard bicep curl. If you are tall, then this might work better just seated in a regular chair with the elbow resting on a table or desk.