This possibility is irrelevant, because the conclusion concerns only what is more effective at relieving lower back pain. What is better healing the underlying injury is a separate issue from what’s better at relieving pain.

This possibility shows that the athlete’s relief from lower back pain does not have to be a result of the heat pad. It could have been due to the additional time after the athletic competition.

This points out that the athlete’s experience may not be representative of the general population. So the author cannot conclude what is “generally” more effective at relieving back pain simply based on this one athlete.

If back pain from some other cause — such as moving heavy objects — might be affected differently from pain relief methods, then we cannot conclude what is “generally” more effective from this one athlete’s experience after an athletic competition.

This points out that the athlete might not have tried the most effective methods of stretching. This shows that we cannot conclude that stretching is generally less effective simply from this one athlete’s experience stretching. Maybe the athlete didn’t stretch in the right way.