Lots of people use yoga to tone and strengthen their bodies. It's a natural system of using the whole self, so it works pretty well for the purpose. I notice, though, how effortful this is when my students are pushing towards goals rather than being where they are. Just staying with a fairly simple asana, or posture, can be strenuous and fierce. Lately my instructions have been including this one, "soften your mind." Well, that's a strange thing to say, but I know that the mind clenches tightly when the body is muscling through something, and I see the results in the whole body when a student can release that hard grip. All the fundamentals of yoga asana come into play as I soften the mind: releasing into my foundation, finding the space in the body, relaxing effort that is not necessary to the pose, letting go of judgment about oneself, benefiting fully from the breath, and finding freedom in the moment itself.
For me Utkatasana, known also as chair pose or fierce pose, is a good asana in which to explore softening the mind, releasing resistance and enabling strength from that place of ease. Basically, this standing pose is folded as though sitting in a chair, without the chair. Weight rests in the way the feet connect to the earth, tailbone balancing lightly over the heels with knees bent, body extending through an elongated spine, energy flowing from the earth through the top of the head. We can do lots of different things with our arms in this posture -- shoulders remain easy, neck relaxed. That seems like a joke! Most people tighten everything - especially the shoulders and neck, but it's not necessary at all for the pose. The state of mind is often reflected in the state of the jaw -- clenched tightly! This is a total waste of energy.
I recommend drinking in the breath in Utkatasana. Drawing a full breath up from the deepest place brings a sense of buoyancy to the body, releasing tension on the exhale in the form of consciously letting go of the shoulder and jaw gripping. It is an exploration of keeping the belly soft enough to expand with breath, while drawing the core muscles up and into the energy center. This brings a wonderful feeling of the body hovering over the base rather than that crunching tight gripping in the thighs and lower back. Imagine that your breath is energy flowing through you, bringing ease throughout the body.
Yoga practice can redefine fierceness as well as softness. Warrior pose (Virabhadrasana) can be a light balance of core body over extended legs, and it is the drawing of energy through the whole being that creates that sense of fierceness as well as grace and ease. Relaxed muscles respond much faster than gripping ones, making ease in the warrior a vital trait. The same is true for Utkatasana, that ease will make this "fierce pose" one of lightness and joy. Allow the mind to let go of judgment and attachment to performance and the body can release the associated anxieties and fears. With much amazement, you may find Utkatasana a welcome space for your heart energy to expand!