Most of those topics are already on topic for a physical fitness site (general exercising advice, injury prevention, rest & muscle recovery, nutritional advice).
As for the specific sports, in as much as someone interested in any sort of physical activity has a question about exercising or nutrition… their questions are welcome, too.
But enumerating all the activities that have a strong component of physical fitness, and covering it under the umbrella of a generalized "we all exercise" site just trivializes the broad interests of those groups.
Only a small fraction of these interests (martial arts, cycling, swimming, etc.) are about exercising or even training. These are competitive activities with a broad range of interests of their own:
- When a rock climber wants to ask about an effective avalanche kit;
- Or a competitive runner wants to know which is the best shoe to buy;
- Or a soccer enthusiast wants to understand the intricacies of going off sides;
- Or an archer wants to know the advantages of carbon core over wood core;
… you don't want to tell these passionate audiences that their proposal has been closed as a duplicate of an exercise site.
This theme keeps playing itself out in many, many subjectsThis theme keeps playing itself out in many, many subjects. My feelings about broadening Physical Fitness to subsume all sites about sports is essentially the same as these other cases: The preeminent author (or collector) of comic books does not want to hang out on a Graphic Designer's site. A Hindu looking for authoritative guidance about his deeply-held religious crisis does not want to go to a generic Religion site … And when an athlete wants to know how David Beckham curves the ball around a defensive wall into a goal, you don't want to tell them to go to the Fitness & Nutrition site.
You can take this merging thing too far until you end up with a half-dozen sites about everything.