Power yoga can require a lot of muscle, especially Shiva Rea's, but it's not necessarily the best way to build it. I'd think of it more as a complement to your strength training, not a replacement for it.
While a power yoga workout contains many of the same types of movements you would include in a full body strength workout, it lacks the specific heavy weight adaptations to failure at 10-12 reps that a good strength training workout would provide.
It would therefore actually be a poor warmup for a strength workout. A better warmup is a cardio one that gets your HR up and starts pumping blood around your muscles rather than a stretching one. Stretching before working out is now becoming less popular as it doesn't really help - it doesn't warm you up and it doesn't prevent injury. So do cardio for 5-10 mins, then the strength workout. Leave the power yoga for a different day altogether.
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While you do get some strength and cardio benefits performing yoga, SparkPeople considers yoga more for flexibility and should not be a substitute for weight training.
Below is a link to a SparkPeople article. I hope this helps!
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I oftentimes use yoga as a warmup before working out with weights. Yesterday, I had used Shiva Rea's Fire and Earth routines from her Daily Energy Vinyasa Yoga Dvd. The routine was very strength-oriented, with lots of pushups, squats, and arm balances -- more than I usually do in yoga. I just could not manage to do a full strength workout afterwards. I wound up just doing the triceps/thighs circuit from Jillian Michael's No More Trouble Zones, and skipped the rest. Can some power yoga routines be considered full-body strength workouts?