Why are pacers' backs under constant threat?

In his piece for The Guardian, Greg Chappell weighs in on what makes fast bowlers susceptible to back injuries, and how selectors needed to have a 'bank of fast bowlers of varying ages' equipped to handle the workload

The latest wisdom is that, to prevent injury, a bowler must increase their chronic load (number of balls bowled) in training to the level at which they will be bowling in games. A 50% spike over that chronic load in games can be managed, but much over that will put a developing body under strain and it is likely to break.

Those that develop their pace from the ground up by using the big muscles from the legs upwards are likely to have fewer problems than those who use considerable upper body rotation to generate exceptional pace. Most of those who have stress injuries are in the second category