The quote does not make sense.
I suspect that what it is intended to mean is that sitting around all day in an office (which is what 'working,' means for many people) is bad for a person's health..... it could be said that 'working in an office undermines a person's physical health.'
To undermine is a verb which means to reduce somebody's ability to do something or to reduce the effect of a key principle. So working in an office undermines a person's ability to live a healthy life.
For example, if you are a boss and you have somebody in your team who is openly disrespectful of you in front of other members of the team, then you might say "This person's behaviour undermines me," because the way they are behaving reduces your ability to be a boss (or could reduce it) as other members of the team may lose respect for you. In fact, this is the true meaning of the term "workplace undermining," which is why it makes zero sense in the above context.
Donald Trump's attempted bar (ban) on people claiming asylum in the United States of America can be said to "undermine both the letter and the spirit of the Geneva Convention," = this means that international law cannot have the effect that it is supposed to if one country opts out of the rules.