Final answer:
Coleman Griffith started his research much earlier in the 1920s, not in the 1970s, which is when sport psychology was established with the study of competitive anxiety, development of a dedicated journal, and the creation of graduate programs.
Step-by-step explanation:
The event from the 1970s that did not mark it as the decade in which sport psychology became established as a legitimate subdiscipline in the study of physical activity was 'Coleman Griffith began his research at the University of Illinois.' While Griffith is known as the 'Father of North American Sport Psychology,' his seminal work and founding of the first sport psychology laboratory occurred much earlier, in the 1920s. Therefore, his contributions predate the significant events of the 1970s that characterized the establishment of sport psychology as a recognized field.
However, the 1970s did see key developments within the field, such as the systematic study of competitive anxiety, which began during this time. Additionally, the first academic journal focusing on sport psychology was developed to facilitate the dissemination of research findings, and the creation of graduate programs in sport psychology helped formalize its study and practice. These steps were crucial in legitimating the field within the broader academic and professional communities.