Use the chair as less as possible - ideally under an hour per day. Sitting on a chair (office, car seat, dining chair, sofa etc) any more than that is likely to be as damaging as say smoking.
At the molecular level, your body was designed to be active and on the move all day long. When you stop moving for extended periods of time, it's like telling your body it's time to shut down and prepare for death.
• Heart: When you sit, blood flows slower and muscles burn less fat, which makes it easier for fatty acids to clog your heart. Research published in the Journal of the American College of Cardiology, for instance, showed that women who sit for 10 or more hours a day may have a significantly greater risk of developing heart disease than those who sit for five hours or less.5
• Pancreas: Your body's ability to respond to insulin is affected by just one day of excess sitting, which leads your pancreas to produce increased amounts of insulin, and this may lead to diabetes.
• Brain damage: Your brain function slows when your body is sedentary for too long. Your brain will get less fresh blood and oxygen, which are needed to trigger the release of brain- and mood-enhancing chemicals.
• Strained Neck and Shoulders: It's common to hold your neck and head forward while working at a computer or cradling a phone to your ear. This can lead to strains to your cervical vertebrae along with permanent imbalances, which can lead to neck strain, sore shoulders and back.
• Back Problems: Sitting puts more pressure on your spine than standing, and the toll on your back health is even worse if you're sitting hunched in front of a computer. It's estimated that 40 percent of people with back pain have spent long hours at their computer each day.
The disks in your back are meant to expand and contract as you move, which allows them to absorb blood and nutrients. When you sit, the disks are compressed and can lose flexibility over time. Sitting excessively can also increase your risk of herniated disks.
Personally, after I reduced my normal 12 to 14 hours of daily sitting to under one hour, the back pain I'd struggled with for decades disappeared.
• Standing requires you to tense your abdominal muscles, which go unused when you sit, ultimately leading to weak abdominals.
• Hip Problems: Your hips also suffer from prolonged sitting, becoming tight and limited in range of motion because they are rarely extended. In the elderly, decreased hip mobility is a leading cause of falls.