Muscles accommodate to a specific type of stress ("habituate" or "plateau") when you continually apply the same stress to your muscles over time, so you must constantly vary exercises, sets, reps and weight to avoid accommodation – Fred Hatfield describing Joe Weider’s “Muscle Confusion Principle”
Look at what the author is saying: “Muscles accommodate to a specific type of stress when you continually apply the same stress to your muscles over time…” So let’s say you’ve been doing 3 sets of 5 on bench. Wouldn’t a new stress be 4 sets of 5? Or maybe you could increase the weight and do 5 sets of 3? That’d be a new stress, too. Looky there, you’ve just altered your sets, reps, and weight with that last idea – a whole new stress to accommodate.
But why then do I see people lying on the ground doing cable curls and squatting concentration curls between their knees? Does squatting somehow help bicep contraction?! BECAUSE ALL THEY’VE PAID ATTENTION TO WAS THE “constantly vary exercises” BIT. They don’t even know where the term “muscle confusion” originated, but they see it all over the place with Muscle Rags, P90X, and Crossfit. Blasphemy I say! I don’t even care for Weider’s thoughts on the Iron Game, but at least get your facts straight! These gym-rats do this crap because they think it’s what MUST be done, and they’re running out of ideas on how to confuse a muscle, as if a muscle could be confused!
So, why does varying exercises help (when it’s not done in the above fashion)? Well, let’s say you’re bench numbers are slowing down, and you think it might be because your triceps are weak. You might try close-grip benching for a while before coming back to regular benching. That didn’t involve tricep pushdowns or skull crushers (though they can sometimes be handy), and it was a simple change; nothing drastic! The only other reason for varying exercises is out of boredom, which is another big reason I think most gym-rats do the crazy things they do. I can’t argue there, except that progress is harder to track and harder to come by.