Backrests don't always work.
In a conventional chair, a backrest is a necessary counterbalance to keep your back from flattening. Even with a good back support, many people still slump. In a saddle seat your spine stays aligned naturally without a backrest. This occurs for two reasons: First, the open thigh-to-torso angle naturally aligns the spine, and second, the wide spread of the knees (called hip abduction) automatically holds your pelvis upright. Most saddle-sitter prefer no backrest.
Those who use a backrest on a saddle seat, either must sit in fixed postures for very long periods (e.g., micro-surgeons, microscope users, etc.), or are very weak with poor muscle control.
Your core muscles support you in a saddle seat. Not so in conventional seating.
When you saddle-sit your postural muscles are active, including your trunk, pelvis and leg muscles. In a conventional chair, your postural and leg muscles are dormant most of the time.
If your postural muscles are asleep when you sit, you will be dependent on arm support and back rests for support. If your postural muscles are active when you sit, you can be free of armrests and backrests.
Hip abduction stabilizes your pelvis upright.
The wide spread of your legs, called "hip abduction", stabilizes your pelvis in an upright orientation. This effortlessly supports your spine in perfect posture without additional support. In a conventional ergonomic chair your knees are closer together, which causes your pelvis to slump and roll backward.
The greater the hip abduction (leg spread) the more effortless good posture will be. The closer-together your knees, the greater your tendency to slump.
Sit with your knees together and see how far you can slump. Now sit with your knees apart and try to slump again. Feel the difference?
Your feet are ideally positioned beneath your body's center-of-mass in a saddle.
In a conventional chair, your feet are positioned forward of your body where they cannot provide support. With your feet underneath you providing support, you'll feel less need for armrests and you'll be able to move your arms more freely and with greater control. Plus, you'll be able to scoot around your work space more easily.
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