Mike Gittleson was the Director of Strength & Conditioning at the University of Michigan for 30 years and was a part of 15 Football Championships in that time. He explains, that muscles don't actually contract.
Do muscles contract? A simple test show you they don’t. Jump in a pool and retain in your respiratory system as much air as you need not to sink. You should be barely floating. If you are floating easily, well, that is another issue.
Now squeeze those muscles, notice I did not say contract. If your muscles really contracted you would become a little smaller and your mass being unchanged, you would become denser.
Being denser you would begin to sink. But you don’t sink because your muscles didn't really contract. Muscles pull their ends together and get thicker and change shape, but their volume remains the same.
Think. If you push aginst a wall the muscles are actually pulling but they certainly aren't getting any shorter! Muscles can get longer and lengthen or muscles can get shorter and shorten, but they don't contract by scientific definition.
Depending on the interaction between the force developed by the muscle and the load on the muscle, the muscle will either shorten, remain at a fixed length (isometric), or be lengthened.
In the late 30s, Hubbard and Stetson set out to fix the incorrect usage of "contraction". They decided to change the verbage in the literature to reflect what was occuring scientifically.
The three words they chose for the muscles' condition were termed "miometric," "isometric," and "pliometric," by coupling the Greek prefixes "mio" (shorter), "iso" (same), and "plio" (longer) to the noun "metric", defined as "pertaining to measures or measurement."
This made things crazier all of a sudden we had the term "isometric contraction", an oxymoron!
In 1963, Fenn tried once again to let everyone know about the contraction 'snafu'. Fenn gained ground for years. Then, at the National Strength Coaches convention in the late 70s, somehow the term pliometric became "plyometrics" for conditioning. Oh no!
European track drills were introduced with high-powered jumps that involved repeated, rapid, and forceful shortening and lengthening actions. The coaches were calling them "plyos", which now means many things to many people except a scientist. Pliometric (lengthening) became a new term (lengthen shorten).
Dr. John Faulkner, PHD, who runs the University's Molecular & Intergrated Physiology lab has been a great friend over the years. He got mad. In 2003, once again tried to straighten everyone out. He published in the Journal of Applied Physiology the proper terminology for muscles.
Don't get angy with me Doc for changeing things again, but the bottom line is this: when you get in the weight room you must 'shorten em', 'same em' and 'longer em' to GET STRONG. And when you run be careful when you 'Plyo em'