Exercise program design is a complex topic with a lot of variables to consider. Within just a single weeks training a coach must look at volume, frequency, rest intervals and exercise selection and weigh those variables against the recovery capacity of the individual doing the program.
The exercise selection is the most obvious of those variables. It is the variable that most clients and athletes look at first when given a new program. People want to know what exercises they are going to do. Many people like going to a trainer because it breaks up the monotony of training on their own. Trainers are often aware of new exercises that clients have never seen before and can make workouts different every time, which people love. Variety in training is important for maintaining interest and avoiding plateaus but some trainers take this concept too far, either in an attempt to appease their clients or because of a lack of understanding of basic principles of adaptation.
If you are doing a completely new workout every single time you train and new exercises every week you will never reach your maximal potential in improving any of your fitness qualities. This philosophy of constantly rotating exercises often takes the name "muscle confusion" Like many ridiculous fitness trends muscle confusion has a small grain of truth to it. Your body can become adapted to certain types of training but it has far more to do with volume and intensity than it does exercise selection. Assuming you have adequate muscle balance and mobility, as well as adequate sleep and calories, you can do almost any exercise several times per week all year round with great results. What you can't do is the same rep scheme year round, especially if it's very high intensity.
Regardless of how you design your program your muscles will never become confused. They also won't get happy or sad. They will not become upset if you don't do their favorite exercises. Muscles are just contractile proteins. They can get better or worse at contracting at certain intensities and in conjunction with other muscles. These qualities are what lead to improvements.
One of the bedrock principles in exercise science is the SAID principle (specific adaptation to imposed demand). Your body will adapt to chronic stressors and therefore get better at whatever you make it do consistently. But if you change your routine every week you aren't really doing anything with consistency and significant improvements will never be made.
You may be saying to yourself, "I have a friend that did P90X and lost x lbs". You can replace P90X with any number of other gimmicky high intensity training systems for this example (Insanity, every group exercise class I've ever seen, etc.). Your friend was likely a novice to high intensity training. Training new exercisers is great because they will adapt to anything. I am trying to promote a style of training with more sustainability.
As I mentioned before muscles can adapt to training intensities and plateau so some variation is required. So how can we design programs to allow for consistent adaptation without wasting our time with a gimmicky routine?
Typically when I write programming I have overall goals for each month for the next three to four months. The specific program (exercises, sets, reps) is usually planned one month at a time. This is by no means the only way to do it but has always worked well for me and many other coaches I know.
A client may be on a MWF weight training schedule with cardiovascular/ energy systems work/ sport competition on weekends or other days. For this client every Monday for the month will have the same exercises. Same with Wednesdays and Fridays. What changes is the rep scheme, and the weight, or maybe the rest interval.
For example, a client looking to improve strength may receive a program that looks like this:
Back Squat 5x3@80/4x3@85/6x3@85/3x3@90
Bench Press firstname.lastname@example.orgemail@example.comfirstname.lastname@example.org/3x3@90
This program format allows some form of progression every week. When volume decreases the weight goes up. The following week the volume goes back up and weight stays the same. With this system there is always some form of improvement. The consistency of the exercise selection is what allows that improvement to occur. At the end of the month you can either keep the same exercise and adjust other variables or change to a new exercise depending on the goals and experience of the athlete.
So next time a trainer tells you that the program you're doing is designed around "muscle confusion" or keeping the body guessing or any other BS tagline like that kindly ask for your money back, because they have failed at understanding the basic principles of adaptation and overload that allow for progress in the gym.