Now the use of computer in our day to day life is increasing day by day. Now the number of jobs without direct contact with computer is very less. Most of the jobs require you to sit in front of computer for a long time. You have to take care of your posture if you are using computer for more than 2 hour a day,(you have to make your position more ergonomic ).
The picture depicts how to sit in front of computer ideally. But I agree that it is very difficult to sit like this for a very long time. (But you have to considering the future impacts an improper posture can cause)
About suitable chair
You have to look for a chair with adjustable height and adjustable back support. This is because, it is very difficult to get a non adjustable chair matching your requirements. So it is always better to get one chair with adjustable height.
How to sit in chair
Once you have purchased an adjustable chair, next thing that you have to take care is how to sit on it. First of all make the height adjustments so that your foot touches ground and your knees are at 90 degrees, keeping your hip to the back as possible. Also you have to adjust your adjustable back support, so that your lower and upper back gets support. If necessary you can use inflatable cushions. Now keep chair at an arm length from monitor. After completing these steps you are done with the chair.
Positioning of monitor
Positioning of the monitor and height of top of monitor and even the tilting angle of monitor are important. As if offers to keep your heads in optimum angle while sitting, else either you have to look up continuously or look down continuously .Also if you learn to type without looking in keyboard, you can keep your head in one direction itself, otherwise you will have to look intermittently to keyboard and monitor while typing. Also get an ergonomic keyboard, which will help you to keep your hands strain free while typing.
Also looking continuously to monitor for a long time will cause eye problems, first of which is drying of eyes and irritation in eyes. To overcome this, after every 30 minutes you must take a 5 minutes break and for 30 seconds you have to look to some distant object or keep your eyes closed. Also it is better to get up from your chair and walk little bit in this 5 minutes. If you want you can set a reminder in your phone or system to remind you to take break in every 30 minutes.
Working in front of system for long hours is not avoidable for most of the jobs. The only thing we can do is to keep good ergonomics so that we will not get affected by this badly in future.
