Warning: This is an applied rationality post, about rationality applied to a specific area of life, not a generalized rationality post.
Ergonomics is incredibly important. Sadly, so many of us in the techno-geek cluster ignore well-defined best practices of ergonomics and develop the infamous hunched back of late night computer toiling.
Seriously, ergonomics is basically a solved problem. The mathematics of anthropometry in relation to body mechanics and repetive stressors on the body are quite well understood.
I am here to offer you a basic, incredibly important, yet widely ignored lesson of rationality.
Spend money on ergonomics!
I really can't emphasize this enough. It's such low hanging fruit, yet I know way too many master aspiring rationalists with egregious ergonomic setups.
It is accepted wisdom on Less Wrong that optimizing your career is important, because you'll spend 80,000 hours working on your career. Strikingly, ergonomics presents an even larger time-based optimization opportunity. With straightforward monetary investment, you can dramatically improve the next hundreds of thousands of hours of your life. The effect size here is just enormous. Spend money on ergonomics, and you will be less fatigued, more energetic, more productive, and healthier into the later years of your life.
Chairs
If you must do your computing while sitting (and do consider alternative standing desks, treadmill desks, or a desk suited to computing while lying in bed), then a good chair is a stunningly good investment. If you make your living while sitting in a chair and computing, what is a $500 investment in your comfort and good health and productivity while sitting? A used Aeron from Craigslist costs around $500 and is the gold standard of ergonomic chair design.
At the low end of ergnomic chairs, the Ikea TORBJÖRN gets a hearty recommendation. It's only $39. Buy some extras for your visitors? That's what I did but then they all ended up in the rooms of my roommates. At the midrange, I have recommended the Ikea Verksam, but it appears to be discontinued. I think the current model Volmar is similar enough though I have not personally sat in it.
The important thing when getting your chair is to make sure it actually fits your body enough to let you sit in a proper ergonomic position. Note that the model in these OSHA images is committing an ergonomics no-no by using arm rests. Yes, I know they feel good to rest your arms on, but they're a crutch. Most all of the positions where you are resting your arms on your armrest are really bad for typing 8 hours a day. Just take the armrests off of your chair and start building up your arm strength. Similarly, avoid chairs with head rests.
Keyboard
Unsurprisingly at this point, I will declare that ergonomic keyboards are just better. They used to be a premium product, but now Microsoft's entry level ergonomic keyboard is only $25. Also, DVORAK is strictly better than QWERTY, ignoring the inconvenience of being forced to switch back and forth between keysets.
Sleep
Ironically, given that it is the default environment for computing, sitting is not very good for the body compared to standing or lying. This makes sense in an evolutionary biology sense -- the human body was definitely designed for working while sitting up, and sleeping while lying down. We can hack this a little by working while lying down, though many people have trouble focusing given the implied lack of focus of a lying down position.
So, a good mattress can be an investment in both your sleeping comfort and your working comfort. I think a good mattress is even more important than a good chair. You spent 1/4-1/3 of your life asleep! I can accomplish no useful work without a good night's sleep.
If you sleep with (or ever plan on sleeping with) a partner, get a queen size bed. A US full size bed is equal to 1.5 twin beds, which doesn't fit two full size adults. My parents sleep on a full size bed (along with a small dog!) and are plagued by insomnia, not enough space, and bouts of blanket stealing. Apparently, it was not uncommon among their generation to prefer the forced physical closeness of a smaller bed. This is ok sometimes, of course, but when we're talking every night, you'll sleep better when not forced to be crushed up against your partner.
A king size bed is even better, of course, if your room can fit it. I got a king size bed because my partner and I both like to compute while lying down in bed, and two people plus computers fit much better on a king size bed than a queen size bed.
I like memory foam mattresses. A minority of people really don't. My heuristic on this is that if you think you'll like a memory foam mattress, you will. One nice thing about memory foam is that it doesn't transmit vibrations from one side to the other. This means that you could probably sleep while someone else is jumping on the other side of the bed. That would not work on a conventional spring mattress. I've heard latex mattresses are even better but I'm too cheap to take my own advice to the full logical conclusion.
Feel free to skip the box spring, unless your bed requires one.
Driving
This is an area where my own ergonomics falls short. I'm 5' 11'' and I just can't quite fit in my Hyundai Elantra. No matter how I adjust the seat, I can't get in a perfectly ergonomic driving position. I refuse to buy another car until I can get one that drives itself, so for now, it seems like I am stuck with a somewhat unergonomic driving experience.
On hand positioning, note that the 10-2 advocated by some DMV and then driver's ed is basically wrong. Whatever slight advantage it might offer is offset by the risk that your arms are between the airbag and your body during a crash. 9-3 is a new conservative choice. I drive 8 and 4. The California DMV manual now supports this.
Fidget more often
One of the most important points of ergonomics is that injury comes from sustained stress. The body can handle a little bit of a stress for a short period of time without much in the way of problems. People often walk into a room and see me scrunched up in the most awkward seeming, obviously unergonomic and uncomfortable looking positions. Why do I do it? Well, it turns out that your body can tolerate almost any position at all for short periods of time. The important part is to notice when your body is experiencing too much stress and shift positions.
Take a step back from this article and note how your body feels, as you are situated. Do you notice any discomfort or stress in your neck, shoulders, back, or lower body? Try fidgeting into a more comfortable position. Next time you notice stress, fidget again. Repeat for the rest of your life.
The science of fidgeting is still surprisingly undeveloped, though more evidence is coming out in favor of it. Fidgeters are much less likely to be obese than non-fidgeters. Fidgeting also works as a technique to help with focus -- it's well documented for ADHD people, but fidgeting doesn't just help ADHD people focus.
Try barefoot shoes
Vibram Fivefingers are popular enough among aspiring rationalists that I frequently joke about the cult of the toe shoe. The evidence behind barefoot running as strictly superior to conventional running shoes at this point seems overwhelming. The evidence for barefoot walking as superior to shoe'd walking is less so, but it seems intuitive to me -- when you actually get tactile feedback from your feet painfully thudding against the ground, you're more likely to walk in such a way as to minimize stress on your body.
I really like Fivefingers, but got annoyed with random passerbys asking me about them everytime I leave my house. Also, they have a tendency to fall apart after heavy use and repeated washings.
The cult of the toe shoes seems to be moving onto Ninja Zemgears. They're also much, much cheaper than Fivefingers, so it's not as big of a deal when they inevitably fall apart. They are also much less intrusive as footwear than Vibrams. People notice them less, and when they do, they think you are wearing comfortable Japanese slippers (Tabi shoes) rather than monstrous toe forms.
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I've offered a lot of suggestions here for how to actually improve your life. If you do this sort of life-hacking, you will be able to actually notice that you are happier, less fatigued, more energetic, and more productive. Just try it. No one ever regrets improving their ergonomic well-being. You'll get to spend more of your day at your peak level of performance instead of in a tense, fatigued, or uncomfortable state.
I'm happy to answer specific questions or give product recommendations in the comments.
I have used a kinesis contoured for 4 years or so. It uses Cherry Brown switches. They're not especially clicky, but they have long travel and they have served me relatively well ergonomically over the last few years. The kinesis also has a "click" sound it plays on every key press (optional, but recommended).