6'1" and 150 lbs is not anywhere close to "way too skinny". It's well within the normal range of BMI. Sure you don't look like Schwarzenegger. But that's very different from being way too skinny.
I disagree. Here's why. I used to work out quite a bit and, with the help of P90X, I got up to 170-175 pounds of lean weight. I maintained it for a while, but adverse life events, a huge workload, and a crappy diet made me lost most of the weight (and a lot of the muscle mass) that I put on before.
Since my weight tumbled to 145 lbs, I noticed a huge difference in my physical well being, especially in sedentary positions. My posture worsened, my back constantly ached, and I started to suffer from RSI. I've been using the same chair all the while, a Herman Miller Mirra.
So, from my experience, weight and muscle mass was a primary determinant of my sedentary well being. Believe me, I was putting a lot less stress on my spine when I was doing 48 pullups in a workout, vs 8 now. I'm slowly working my way back up, and the back pain from sitting has lessened.
