“Muscle confusion” is a term that is commonly used in bodybuilding culture. It is normally used as a justification to rotate your exercises and give your body something to adapt to.
For example, if you have been following the same program for a while and then you swap all the exercises, chances are you are going to feel significantly more sore, indicating you have done something new.
In this respect, it is common for people to chase soreness with the belief that soreness is indicative of muscle damage and therefore more soreness = more damage = more for your body to adapt to = more muscle growth.
Some people would argue against this because it is common for powerlifters and Olympic lifters to spend a lot of time focusing on a narrow number of lifts but many still appear to grow an appreciable amount of muscle.
Despite the fact that “muscle confusion” is a term has been used for years, it hasn’t ever been studied specifically, up until now.
A research paper split subjects into two groups, one had fixed exercise selection and the other had their exercises randomized by a computer-based app.
The workout length and number of exercises were the same between conditions but those in the muscle confusion group had exercises automatically assigned to them at random from a list of 80 choices.
In terms of muscle growth, both groups achieved similar increases.
In terms of strength, there was a trend for greater increases in the control group who kept their exercises similar. This isn’t hugely surprising because of course if you practice a bench press a lot you are likely to improve on the bench press more than if you do not.
That being said, it kind of highlights the need to repeat exercises you want to improve on rather than frequently rotating them in and out.
From the paper: “There may be a trade-off whereby too frequent rotation of exercises somewhat compromises muscle growth and strength; thus, those who wish to maximize these outcomes may wish to limit exercise variety”
Where the muscle confusion group did notice benefit however, was an increase in intrinsic motivation suggesting that some people may simply enjoy changing their workouts a lot.
As a compromise, the research paper authors suggested to keep a core staple of any complex exercises you want to improve on (like deadlifts or bench press) and rotate the simpler exercises that do not require as much motor learning (like machine exercises or isolation exercises such as eg extensions and bicep curls).
As always, it makes sense to pay attention to your own rate of progress. Use these as fundamental principles but experiment with your own workouts to find something that works well for you.
Reference: The effects of exercise variation in muscle thickness, maximal strength and motivation in resistance trained men