Muscle confusion is currently a big buzzword in the fitness field. Program “X” is based on the principle of muscle confusion to ensure maximum growth. Trainer “Y” says, “You gotta confuse your muscles to get them to grow!” But what exactly is muscle confusion?
To start, muscles do three things: they contract and shorten (a concentric contraction), they contract and lengthen (an eccentric contraction) or they contract and stay the same length (an isometric contraction). Also, muscles respond to stress. Muscles will not get bigger/stronger if the stress placed on them is not more than they can currently handle (the term for this is overload). The overload must periodically be increased to maintain an overload (this is called a progressive overload).
(A quick word about stress on a muscle-it doesn’t matter what the stressor is, be it a machine, free weight, or a big bag of potatoes, as long as the stress is a little more than what the muscle is used to, growth will occur)
Anyway, back to confusion. Proponents of muscle confusion will tell you that you need to do something different each and every time you workout to shock the body and force it to grow. That is a good way to get sore after each workout, but not totally necessary.
It comes back to the progressive overload. If you use more weight or do more repetitions or just work harder on your next workout, your muscles will be “confused”. To me muscle confusion is a marketing word for variability. Pick out some exercises, work hard, rest and do it again.