Defined, toned abs — commonly called a six-pack — are an often sought-after goal in the gym. But not all toned abs look the same. Some people sport a four-pack, while others may have an eight-pack.

Toned abs — but in particular the infamous six-pack — are often seen as the Holy Grail of fitness. Experts say they're not only much harder to achieve than many people realize but some caution that ...