i hurt my back so bad 6 years ago on a little bike like that, i was hauling ass down the road that our lake cottage is on riding a bike thhat size cuz i was teaching my brother how to ride a bike, and i tried to clear a speed bumb and i completely forgot about teh friction brake (where it stops if you push the pedals in reverse) and so in standing up to hop, i pushed back on the pedals. long story short, i had horrific back spasms for days and couldn't leave my bed except for using the restroom, the pain was so unbearable that it made me throw up.