Anyone who's ever taken a yoga class knows it's not as easy as it looks. Sure, stretching and standing around in Lululemon gear sounds pleasant, but a long Vinyasa, Bikram or Ashtanga class can be seriously sweaty and physically challenging. And a strong core is essential to balancing on one leg, twisting like a pretzel or just getting into a sturdy triangle pose. The core muscles stabilize two of the body's most important structures -- the spine and the pelvis -- and help maintain flexibility throughout the torso. While certain asanas challenge the core more than others, nearly every movement in yoga requires well-developed abs and a strong back. The following five muscle groups are especially relevant for flexing, posing and stretching.