My experience is that heavy sweating by itself is not too much of a problem - the adhesive is a bit weaker when you get hot and sweaty, but certainly not so much that there is any danger of the piece slipping or coming off, and it returns to full strength when you cool down. BUT I think it would be risky to get hot and sweaty and then take part in a sport which involves actual head contact - either contact from an opponent or contact with the mat. The piece is not going to come off, but if you were unlucky the hairline could be ripped loose or pushed back. I would only do those activities if I was able to wear head protection to keep everything safe. I know a guy who is a Brazilian Ju-jitsu instructor who wears a hairpiece, and that is what he does - he wears an ear protection cap which has Velcro straps across the top of the head.