o Sub-acute (e.g. duration 6–12 weeks).
o Chronic (e.g. duration more than 12 weeks).
* Although this classification is necessary for scientific studies, it may not be useful in clinical practice

o The limits for the groups have not been standardized. For example:

+ Sub acute is sometimes combined with the acute category (i.e. acute is sometimes classed as a duration of less than 12 weeks).

+ Sub acute is sometimes combined with the chronic category (i.e. chronic is sometimes classed as a duration of more than 6 weeks).

o Classification as acute or chronic is a substantial simplification of reality, and can be misleading because:

+ Low back pain treatments tend to follow a course that fluctuates over days, weeks, and months. It is thus not always possible to distinguish between a new acute event and an exacerbation of a chronic process.

+ Chronic pain and disability often seem to become dissociated from the original physical problem.