Wai Lana Yoga and Ashtanga Yoga

I've been doing wai lana yoga for the past two years but I recently attended a ashtanga yoga class or otherwise known as power yoga. There's no doubt about it, it was a good workout but i saw so many people struggling so much with the poses. One guy even injured his hip. I'm thinking i'll stick with doing Wai Lana yoga and if i want a more physical workout I'll head to the gym haha.