Look at the seat of the chair. Look for the side contour on the seats, as well as the waterfall front, shown on the Humanscale Freedom Chair to the right. These two features allow for optimal blood flow through your legs, and optimal blood flow reduces yawning and tiredness(because you yawn and get tired from a lack of oxygen, which can come from bad circulation drinking water helps this too).
Some chair companies offer the option of a saddle seat, or a tractor seat. These, shown to the lower right, are highly specialized solutions for people that need to remain in the same position, and generally geared towards men. We don't recommend them, unless there are specific seating needs required that necessitate a tractor seat, as for a standard person looking for basic ergonomic support, they would be uncomfortable. We do have great manufacturers that have tractor seat options, though. Call us for more information!
Final Word: The contoured seat pan, shown upper right, is definitely an ergonomic benefit. Luckily, it is also a standard feature in most true ergonomic chairs. The tractor seat, save for a few companies, is not standard.