Do you guys think it's possible to maintain muscle mass just with yoga alone? (Power yoga, that is)
This depends on the level of strength you are trying to maintain, and also on your flexibility. Yoga is great for a lot of things but isn't necessarily the best option for everything. Some examples:
1. Cardio. A vigorous power yoga session is absolutely a cardio workout. It will give you more cardio strengthening than simply going for a walk or a jog. But not as much as a skilled cyclist or runner is getting from cycling or running at speed.
2. Lower Body Strength. Power yoga is bodyweight strength training. For a body well-adapted to the yoga poses, the benefits can be very similar. For a body not so well-adapted to yoga, they're not so much so. For example: I used to teach a class called BodyFlow (in Canada and the US, elsewhere it's called BodyBalance). A BodyFlow/BodyBalance class includes 5-6 minutes of "standing strength" poses--basically warriors, lunges, and chair pose. For me, this is a good lower body strength workout. But a friend of mine who came to my classes a couple of times did not feel she got any lower body strength training benefit out of the class. What was the difference? My friend was (and still is) a cyclist. But as well as being a yogi, I was (and still am) a runner. We were both decently good at our respective sports back in that day and I would say had similar lower body strength profiles. But I had a lot more flexibility. In my lunges and warriors I & II, the thigh of my bent leg is parallel to the ground. My friend could not get anywhere near that low. Warriors with a horizontal thigh are a lot harder than warriors where one's thigh is say 45 degrees to the ground. Ditto for a deep chair pose versus one with knees barely bent. So my friend had to pump iron to get the same lower body strength training benefit as I could get from doing yoga alone.
3. Upper Body Strength. If you're doing power yoga, you're most likely doing a lot of planks and chaturangas. So you're working your upper body. If your only strength training is yoga, I would suggest at least some of the time pushing from chaturanga straight up into plank. You're not getting the maximum benefit from the concentric phase of that exercise if you always push up through up dog. Down dogs, dolphins, forearm balances, and handstands help to train strength with the arms in an overhead position. Add in some turbo dogs and transitions between down dog and dolphin for dynamic strength training with arms overhead. We don't do any pulling in yoga, however; so that's something to consider as an add-on exercise if you're doing primarily yoga for your strength-training.
4. Core Strength. Power yoga is generally pretty awesome for core strengthening, but there is a lot of variance depending on how one practises. This is an area where enhanced flexibility can make a lot of the exercises easier, so it's important not to lean into that too much if you want maximum strength training benefits. For example: we often think of up dog as a more "advanced" pose than cobra. But practising a hands-free cobra is going to give you a lot more back-strengthening work than up dog. (For that matter, using your arms to get into a higher cobra will give you more arm-strengthening work than up dog. Anytime you're bearing your weight on vertically stacked bones your skeleton is doing a lot to decrease the workload on your muscles.)