I'll start by saying that every yoga pose can be physically and mentally difficult, depending on the approach the student takes. As soon as you find resistance — whether it's physical, emotional, or mental — there is a challenge or lesson that needs to be learned. One of the hardest things that we as humans learn is the ability to adapt to our surroundings — and to do so in an optimally peaceful and healthy way. We tend to fight and resist change, which is what creates an internal and external struggle. So, as we move into some of these poses, a lot of our “reality” is questioned, thus creating a shift within us that transforms much more than we can imagine. I am astonished as to what my body and mind are able to do now at the age of 34 that I was unable to do at the age of 16.
A commonly challenging pose is the Scorpion Pose, otherwise known as Vrschikasana B in Sanskrit. This is a back-bend handstand with your legs angled behind you towards the crown of your head. This can be difficult because it requires your body to be in perfect synchronicity and balance. Achieving this level of synergy and alignment takes both practice and time. A student will not master this without having patience and perseverance — both qualities required in yoga. In addition, one must overcome the fear of falling and letting go, which are huge parts of the yoga practice .
Another difficult pose is the Tortoise, otherwise known as Kurmasana. This pose is seated in a forward, bending position with both legs behind your head and your hands interlaced behind your back. I would actually say that this is more difficult than some of the fancier, leg-behind-the-head poses because it can be considered a “gateway” for the other very challenging ones that follow. The difficulty stems from a level of surrender and flexibility that only the mind and the body working together can provide safely. It can be dangerous if you push yourself too hard here because it is easy to over-stretch if you're not being careful. You must have a lot of external rotation in your hips and very open hamstrings in order to move your legs behind your head.
The Mountain pose, known as Tadasana in Sanskrit, may look easy, because you are just standing, but in fact this is the base of all the yoga poses, including the more “difficult” ones above. Having a strong and properly aligned Tadasana is rather challenging because we tend to fall into our unconscious tendencies of improper posture. The difficulty in this pose first comes from being conscious and aware that we need to make the change, which is not always easy to accept. Then the pose is challenging because to make the proper changes takes a level of focus and dedication to constantly remind your muscles to contract in the most optimal way.
Also known as Padmasana, Full Lotus is the most important of all the poses. The reason for this is that the intention behind all of the yoga poses is to become flexible and strong enough to be able to sit in a cross-legged position for an extended period of time. This is accomplished in order to meditate without obstacles. Sigh. This is also the hardest position because we have so many distractions and “vrittis” (mental modifications in Sanskrit) that pull us away from our focus. Our mind is the most difficult "muscle" to train, especially when you add the element of sitting still for an extended period of time.
Asana: Any pose that you resist. In the Yoga Sutras of Patanjali, it states “Sthira Sukham Asanam,” which means that one must achieve any pose that is comfortable and steady. In reality, how many poses are we really comfortable with? Even when we sit in a chair at work we are moving constantly — always needing to adjust to find our sweet spots. What's comfortable for one person is challenging for another. A seated forward bend for a man whose hamstrings are tight from playing sports all his life will definitely be a difficult for him both physically and emotionally. This same pose could be comfortable and steady for someone who has trained their mind and body for a few years. Therefore, anything that feels limiting on any level will give the student the challenge they need in order to learn how to become limitless. Thus, any challenging pose must be approached with a strong, focused and flexible mind along with a strong, centered and flexible body. An open heart and a stable sense of core are always helpful as well.