I'm going to fast forward to 1976 on the assumption that the basics of a modern task chair have been established in the last entry. Today's subject, the Ergon chair by Herman Miller introduced in 1976 reflects the refinement of the characteristics of the early desk chairs we looked at, and is a good example of an early modern task chair.
What we see here in the Ergon is a design that emerged out of extensive study of how people sat when they worked. Designer Bill Stumpf brought real research to bear on the design problem and the chair represented several breakthroughs. From the Herman Miller site:
The Ergon chair revolutionized office seating because it was designed for both comfort and user health. The chair quickly and easily adjusted to various body sizes and proportions, and it reduced physiological stress by providing exceptional spinal support and unrestricted blood flow. For the first time, businesses could provide their workers with seating designed for the way they really sit -- not for the way someone thought they should sit. With the Ergon chair, Herman Miller established the reference point for comfortable, healthful, and visually appealing ergonomic seating.
The two pad configuration is an element of the early task chair designs, but now the seat pads are not shaped like a traditional wood chair with upholstery on it - the pads are a specific shape designed to respond to the dynamics of seated task work. The seat is now supported on a hydraulic gas strut rather than a threaded shaft. The seat height can now be adjusted quickly, and while seated, allowing the chair to be quickly adjusted for specific tasks. Other adjustments on the chair have become similarly "quick". No longer the need to un-screw threaded clamps holding the seat adjustments in place. Chair arm pads have been positioned not in the traditional form and location, but are shaped and located to respond to desk work and support the arms for those tasks.
Still this chair has a 4 leg/castor spider base in 1976. Herman Miller has continued to manufacture this chair right up to today, updating the design in 1988 and again in the 1990's to add features on a par with newer chair designs. From the start the oval lozenge shaped pads of this chair design defined the image of an "ergonomic" office chair that still persists today.
The Ergon3 chair at Herman Miller (includes design history of the chair)