Exposure misclassification: an "immortal time" bias in observational studies of training load and injury.

BACKGROUND: Several observational studies of the relationship between training load and injury have found increased risks of injury at low loads. These associations are expected because load is often assessed at the end of the injury follow-up period. As such, athletes who get injured earlier in the follow-up period will have systematically lower loads than athletes who get injured later in the follow-up period.

OBJECTIVE: In this commentary, we identify this problem as a type of exposure misclassification occurring from the misalignment of exposure measurement and start of follow-up. This methodological issue has previously been recognized in other areas of epidemiology as "immortal time bias." RESULTS AND

CONCLUSION: Similar to immortal time bias, exposure misclassification bias can be prevented by aligning the measurement of load with the start of follow-up for injury.