Explainer: What is a stress fracture of the back and why does it happen?

A bone is a living tissue, which can be damaged when put under high physical stress. A build-up of inflammatory cells and swelling in the bone is called a bone stress injury or stress reaction and can be spotted in an MRI scan. But this can progress to a fracture when there is a break in the cortex - the thicker outer layer of the bone.

The fracture usually occurs in the 'pars' of the vertebrae in the lower back for fast bowlers. It is little bony bridge between the facet joints which take a lot of stress when you extend, laterally flex and vertically compress the spine. And it is normally on the side opposite the bowling arm: so if you are right-arm bowler it is generally the left side of the lower back where the stress fracture will happen.

A stress fracture can be an acute episode, but it is always in chronically loaded tissue. It is not like you twist your ankle and break your leg where the bone was previously fine. It happens when there is a sudden overload on a weakened area of bone that has not remodelled fast enough to absorb and transmit the physical forces involved.

Physical stress and the body's adaptation to it is a balance between fitness and fatigue. The fitter you are, the more stress resistant you are. Another simple way of looking at it is that bowlers absorb a lot of force on their front foot impact during bowling. Fitter bodies can cope with these forces more efficiently. But when your body is tired, your legs are tired, and your muscles or 'shock absorbers' are not doing their job, the bones and joints of the skeleton - essentially the chassis - takes more stress over time. So, if your suspension is broken, you are going to feel every bump in the road.

So, generally, yes, high workload is one of many reasons for bone stress injury. This includes both on- and off-field physical load, such as cricket activity and strength and conditioning training sessions. But research also tells us that there are certain patterns of workload associated with a higher generalised risk of injury. If there is a sudden exponential spike in workload above that usually experienced, the overload can be a factor in the development of injury even a few weeks afterward the event.

Chronically high workloads without rest presents an over-reaching scenario where the body's immune system can become compromised. Even lower workloads can indirectly contribute to a higher risk of injury due to the lack of a training effect (i.e. low specific bowling fitness) and poor preparation prior to bowling too much too soon. The scheduling of fixtures between T20 to ODI and Test formats has also been pointed to as a possible reason in raising injury risk.

It could also be a metabolic issue, such as low absorption/production of Vitamin D and Calcium, contributing to poor bone density. It could be any or more of a range of biomechanical factors. The bowler could have stiffness and reduced mobility, strength or control around his hips, ankles, upper back or shoulders, and he compensates through that part of the spine. He might be bowling a particular variation of delivery which increases the load in that area. It could also be related to nutritional factors, poor recovery, or even other aspects of his off-field training regime that need to be considered. Age is also a significant factor for bone stress injury in players under 25.

His action is certainly unique. As far as I know, this is the first occurrence of this type of injury with Bumrah. But he has bowled for many years now without suffering a stress fracture, so it is less likely to be technique related and more a function of his training and bowling workload.

If it is just a stress reaction, he could be fine in about four to six weeks with an active rehabilitation programme. But if it is further along the bone stress injury spectrum than this, then it could need three to six months. If the stress fracture is bilateral (a crack on both sides of the same vertebrae), then it might be more than year or longer.

The primary risk of any injury is a previous injury. The fact that he now has this, statistically raises the risk of recurrence than for someone else who has never had a stress fracture. But he will be put through a thorough rehabilitation programme at the National Cricket Academy in Bengaluru and be monitored carefully, and being a mature player will greatly minimise the risk of it coming back. The key is not to rush the player back and to address all of the factors mentioned above, including ensuring his bowling workloads at the end of his rehab being suitable for the format of the games he will be going into.