Chair Pose (Utkatasana), when done with proper alignment, works the core while strengthening your legs. Your quadriceps, hamstrings, and calves must engage. You must strengthen your hip, knee, and ankle joints. At the same time, you stretch your Achilles and calves if you keep your heels down and send your hips back. Raising your arms will add more complexity because it requires both strength and flexibility in your lower, mid, and upper back. To keep your shoulders from rising up to your ears, your trapezius and erector spinae must be strong. This pose is frequently used by fitness trainers to make assessments about muscular imbalances through the body.