Cricket is a sport that is considered to have a moderate injury risk. The high incidence of lumbar spine stress reactions and fractures (bone stress injuries) in fast bowlers that result in long recovery periods in part explains the high prevalence rates. There is not single factor that predisposes fast bowlers to pars injury, but a combination of factors related to localized bones stress, bone geometry and the magnitude and frequency of loading.
Localized bone stress
Fast bowling involves repetitive trunk extension, side flexion and rotation resulting in localized stress through the posterior elements of the vertebra, capable of creating a pars defect or bone stress injury.
High magnitude of bone stress
Ground reaction forces as high as eight times body weight have been reported at front-foot impact during delivery stride and are transmitted vertically through the lumbar spine to provide the high magnitude of load required to develop bone stress. Evidence has been brought out to support the theory that there are certain spin positions during the delivery stride that are responsible for the pars stress required for bone failure. Fast bowlers who deliver the ball from a greater height, have also been associated with an increased risk of lumbar bone stress injury. Ball release height is related to the degree of knee extension at front-foot impact, thus the higher the release height the straighter the knee. This results in stiffer knee segment and greater impact forces being transmitted to the lumbar spine.
Bone geometry (strength)
A bones ability to resist bending forces is dependant on its geometry and there fore its strength. The exact relationship between age and lumbar spine bone stress injury is unknown but it is commonly reported in younger athletes. It is likely that bone is more susceptible to failure before it has full matured making players younger than 25 more vulnerable to pars injury. In cricket it is an injury that is also commonly reported in younger fast bowlers. Studies have shown the relationship between the immature vertebral bone and bone stress injury.
Bone stress and ultimately fracture is thought to occur when insufficient time is given between loading sessions for the bone to adapt and repair. It has been identified that more than 300 deliveries in a single match or 180 deliveries in the second innings of a games increased the risk of injury in the following weeks after the match. It was found that it was the 21-28 day period after the match when the injury was most likely to occur.