Isometrics is mostly for strength, where you are pushing or pulling against a unmovable object. Usually allowing a 3 to 6 second effort, if I remember right. It is a static hold in one position. Usually a three point position; start, middle, finish. Has, and is, used in lifting programs, usually at the end of a specific workout. Though it's popular with some, most never include in in a regular lifting program. Big thing in the late '60's and '70's for the York Olympic lifting gym. Latter found out most of those guy's were into D-bol, etc. Physical rehab centers use isometrics (and forms of) for restrengthing the body.
Static holds are also good for strength increases. Just holding a extra heavy weight on the shoulders, in the top DL position or bench just before/complete lockouts tend to increase the positive contraction of those lifts. The body has to get use to handling extra heavy weight, even if it's just a hold, to get stronger. Also very good for people who want to increase their chinning strength, by just holding a certain position for a short perior of time. Good Luck.
