Let me explain what I mean with this. In an effective muscle-building regimen you need to go to the gym with a plan and preferably a notebook that has the specific weights and reps you did that last time you were there performing exactly those same exercises.
This is what I do most of the time when I go to the gym but there are other days of the week where I go to the gym with no plan other than to keep my muscles remembering that I am going to use them every day.
src
First off, even though I do it, I advise against going to the gym without a plan unless you are already going to the gym WITH a plan 3 times a week. On my "no plan days" I just do a little bit of this, a little bit of that, and I don't really try to do any sort of heavy weights either. This could be considered some sort of a HIIT (high intensity interval training) workout because the idea is that I just use whatever machine or weights are not being used by someone else when I am in there. I don't even pay any attention to what body part it is for. This only becomes embarrassing on the glute machine that is used mostly by women and I don't really, even to this day, have much of an idea as to how i am meant to use it properly.
src
I always feel like a jackass when I am on the above machine but then I just remember what I always say about ego-lifters in the gym: Nobody cares what you are doing and nobody is watching you or judging you. They also aren't admiring you if you are jacked and shouting... so stop doing that.
To get back to the point of all of this I want to say that although I don't have any clinical proof that my "no plan day" is beneficial in any sort of efficiency type of way, I will say that i don't need to have gone to sport science school to know that any workout is better than not working out.
On these days this workout is more about keeping my heart rate up and just activating as many muscles as possible in say, 40 minutes or so. Mostly I don't even try to work particular groups to exhaustion and this is more of a just keeping moving and staying in the gym mindset type of thing.
I find that it actually ends up being one of my favorite gym days of the week because I am not sticking to a very specific set of exercises nor am I working singular muscle groups to hypertrophic exhaustion, which is generally considered the best way to gain muscle mass.
So if you are going to have a "no plan workout" day in your mix I say that you should go for it, but like I mentioned before, it is absolutely essential that on most days that you actually DO have a plan if you want to get results.
However, a day with no plan activates muscles and most importantly, probably keeps your heart rate up longer than on days where you focus on a specific muscle group because of the zero breaks. Weight lifters tend to forget how important cardio is for overall health and for this reason, I feel as though a winging it day can actually benefit people who focus primarily on weight training.
I have no professional qualifications to back up my words but I do have results from my own life where I lost over 50 lbs and kept it off after I decided to make gradual and accomplishable changes to my life. I believe that if I can do it, so can almost anyone else