I worked in an office doing spreadsheets and had to really use my brain for other tasks as well and yes that office work which I though was gonna be easy work sitting on my bum in a nice warm or cool comfy office was actually more physically draining to me than physical work that I used to do.
I eventually got out of that office job and took on a landscaping job where I'm outside and I'm actually less tire and stressed out than I was when I was working the office job.
Office Jobs may seem easy but they're not really easy jobs.