I hope this can help some of you as much as it helped me.
As much as I don't like giving out unsolicited advice, I feel that there are some commonly recurring health myths that need to be nipped in the bud. The current rhetoric surronding spinal health is pretty detrimental in society unfortunately, and something that can be improved upon.
Firstly, I actually agree with
@Dozer about avoiding all these positions, in terms of performing them as
static stretches. The issue with static stretching can be a whole other debate for another time, however if anything is going to be detrimental about those positions, it would be for an entirely different reason than the poorly written explaination on that sheet.
Flexion isn't bad for discs. Positions of flexion aren't inherently unhealthy. In fact, appropriate loading in flexion is beneficial for not only disc health but also muscle strength and size.
Here is one such example, relevant to our predominant hobby, where it was found that high volume road cyclists (who spend a heap of time in flexion) have beneficial changes compared to non-cyclists. There are heaps of other articles which back up this concept, from similar case studies in different flexion-based sports, biomechanical force testing, and fluid dynamics modelling of disc compression via vertebrae through spinal range of motion.
Yes, following an acute disc injury it can be wise to avoid aggravating flexion-based activites for a few weeks during recovery. However, avoiding positions of flexion in the long term is not only incredibly detrimental, but also a pretty inconvenient way to live. It's a great way to turn an mild-moderate acute injury into a long-term chronic issue via catastrophisation and atrophy via activity avoidance.
TL;DR - ride your bike, do normal things, don't be afraid to bend your back, but probably beneficial to avoid static stretching in extreme ranges of flexion (or static stretching at all really)
Again, as a disclaimer this is generally speaking, there will always be outlying cases and extreme scenarios, however for the vast, vast majorty of cases these are valid points.