What is Online First? What is a DOI and how is it used?
Online First publishing (i.e. publishing an electronic version of the article in advance of the print edition) considerably reduces the time it would have previously taken for critical discoveries in the research community to be communicated, often by many months.
Once a proof has been corrected and finalized, an article is ready for Online First publication and can be published online. This means that readers can access peer reviewed articles well before print publication. These articles are searchable on the journal homepage and on PubMed and they can be cited using their unique DOI (Digital Object Identifier). The formatting depends on the reference style of the journal, but in principle you can simply use the DOI instead of volume and page numbers, as in the following example:
Gamelin FX, Baquet G, Berthoin S, Thevenet D, Nourry C, Nottin S, Bosquet L (2009) Effect of high intensity intermittent training on heart rate variability in prepubescent children. Eur J Appl Physiol. doi: 10.1007/s00421-008-0955-8