Which of the of these fitness events happened last

To determine which of these fitness events happened last, let's look at each one:

1. PE becomes part of American schools: Physical education (PE) started being integrated into American schools in the late 19th century and became more standardized in the early 20th century.

2. Title IX prohibits gender discrimination: Title IX was enacted in the United States in 1972.

3. The YMCA opens a gym in America: The first YMCA in the United States was established in Boston in 1851, but specific gym openings would have occurred thereafter.

4. The adjustable plate-loaded barbell is invented: The adjustable barbell was popularized in the mid-20th century, with significant developments happening in the 1950s.

Based on these timelines, Title IX (1972) is the latest event among the options provided.