Do not capitalize all letters of the title of a journal when abbreviated, in other words it should appear as Med Sci Sports Exerc, and not as MED SCI SPORTS EXERC. There are rare exceptions to this, such as JAMA. Find your source in the NLM Catalog and use the abbreviation that they utilize. If any of your journal articles come from a journal which is not available in the NLM Catalog, you will need to create your own abbreviations. According to 220.127.116.11.3 of the CSE manual, "Abbreviate 'significant words'...and omit other words such as articles, conjunctions, and prepositions."