World Sports Encyclopedia (2003) estimated that there are about 8,000 sports. Of course, physical activities are a superset of that. The number of pairings given n things is n*(n-1)/2. Even if we restrict our interest to, say, the 400 most popular physical activities, there are still about 80 thousand pairs. Moreover, we need at least 10-20 participants per pair. That’s a lot of data.
We would like to thank the following sites for listing our survey: