Star Trek: The Next Generation made several design changes to its version of the Starfleet uniform after the 2nd season, mainly because the original uniforms were causing significant problems for the actors. Star Trek: The Next Generation was the second series in the Star Trek franchise. It ran from 1987 to 1994 and focused on Captain Jean-Luc Picard and the crew of the USS Enterprise-D. While initially not as popular as Star Trek: The Original Series, The Next Generation steadily gained favor with fans over the course of its 7 seasons and today is arguably the most popular Star Trek series of all time.
Although TNG was conceived and produced by TOS creator Gene Roddenberry, it was an entirely new take on the Star Trek universe, with a new ship, crew, and time period to explore. In keeping with this, the Original Series uniform was also redesigned for TNG; costume designer William Ware Theiss created a jumpsuit instead of TOS's shirt and pants combination. The jumpsuit was form-fitting, and the shoulders and pants were black while the chest and stomach were the color of the officer's division of service. There was corresponding piping of the same color on the shoulders, and the jumpsuit was equipped with a foot strap to go over the wearer's shoe so that it could be pulled taut when worn. This was to eliminate wrinkles, something Roddenberry and the creative team felt would not be an issue in the 24th century.
However, by the end of the 2nd season of TNG, this uniform began to be phased out, replaced with a similar but subtly different design. While the look and placement of the colors were similar, this new Star Trek uniform got rid of the piping on the shoulders, added a collar around the neck, and most importantly, changed the one-piece jumpsuit to a two-piece jacket and pants combination. Although the jumpsuit style continued to be worn by background actors and some of the women in the main cast, it was mostly phased out going into the 3rd season, and the new design became the standard uniform worn by the cast for the remainder of the show.
The changes to the uniform design occurred because of some important behind-the-scenes reasons having to do with the comfort of the actors. While the idea of a uniform that did not wrinkle was an interesting one, the foot strap on the original uniforms pulled the jumpsuit so tight that it caused several actors, including Brent Spiner, Jonathan Frakes, and Patrick Stewart, significant back problems. The issues were so bad that Stewart's chiropractor reportedly told him he should sue the production for the damage done to his back. The original uniform was also made of a material that was reportedly not very breathable, causing actors to overheat under the studio lights and some not insignificant issues with body odor. When the uniforms were redesigned by new costume designer Robert Blackman, the material was changed to eliminate the odor issues and the switch to a two-piece suit put much less strain on the actors' backs.
Over the years, the redesigned uniform has become one of the most recognizable Starfleet uniform styles in the Star Trek franchise, perhaps only second to the uniforms worn on Star Trek: Deep Space Nine and Star Trek: Voyager. While the jumpsuit looked great on paper and has several design elements that were carried over, the issues with it were too numerous to warrant anything other than a redesign. Ultimately, Star Trek: The Next Generation made the right decision in phasing out its original uniform.