You can flex them but it takes alot. The BB usually gives up first but either way, it takes a lot. I raced BMX alot and up until my 30s , where you do alot of standing starts and then literally torque the whole bike under you. I've twisted crank arms, BBs , pedal axles , bar stems ... pretty much you name it. But thats the price you pay if you ride the balance between strength and maximum light weight.