I have absolutely no experience in Karate (which I plan to remedy), but I took a class in Ashtanga Yoga (I had to quit because my teacher quit, and her successor was a total failure). Yoga, like Karate, has many variations, and it is not right to generalise saying that it is all about "relaxation". In Ashtanga Yoga, many postures include strength building and the intensity can (according to my teacher, ought to) climax to the point where one just cannot take it any more. Luckily, in my class, most of the people were beginners creating a mess with constant interruptions - nice coverage