I have had a spinal fusion for the past 10 years. can bending down too much or twisting cause the rod to break/fracture the spine?

Usually not: If the rod breaks, it is more likely sign that the fusion did not fully heal. Very early rod systems had some design issues that led to breakage even with good fusion achieved. The other possible options: the rod broke as you were fusing or the rod diameter was too small for your size & extent of the fusion levels. It could have been recontoured too many times at surgery & stress riser made.

Not Usually: Today's rods are made of titanium. They go through a vigorous mechanical testing to determine their fatigue factor. Although breakage may occur it is highly unusual with today's equipment. I would recommend that you keep your core muscles strong, perform flexibility exercises, and follow proper biomechanical principles in lifting. Go out and enjoy your indoors and outdoors activities.

Unlikely after 10yrs: The rods are there to stabilize the fused spine until the bony fusion takes. As soon as the fusion takes, there is very little stress on the rods with bending motion. Stress is transferred and concentrated in the level above and/or below the fused segments. If your fusion didn't take, then there will be repeated torque/mechanical loading cycles on the implants, which can lead to fatigue fracture.