To determine which event happened last, let's look at the timeline of the events mentioned:
- The YMCA opens a gym in America - This occurred in 1851.
- PE becomes part of American school curriculum - This started happening around the late 19th and early 20th centuries, but was solidified in the 1950s and 1960s.
- Title IX forbids gender discrimination in sports - This was enacted in 1972.
- The adjustable plate-loaded barbell is invented - The exact date of the invention is less clear, but adjustable barbells began to be developed and become popular around the 20th century, with significant development taking place mid-century.
Based on this timeline, the event that happened last is Title IX forbids gender discrimination in sports (1972).