"ANW" often focuses on strong hands and quick feet, but no one gets very far without some heavy lifting from the legs. The rolling-log obstacle usually takes a four-limb bear hug, for example, and several others — namely the frame slider, body prop and spider climb (pictured) — feature a gripless gap too wide for arm leverage alone.
As with most physical feats, the best practice is the act itself. It's hard to find a spider-climb-sized training space, though, and even if you do, weight-bearing midair leg splits aren't for beginners. Lunges, butterflies and other stretches increase leg flexibility over time, as can yoga, gymnastics and martial arts. For more powerful legs, options range from weight lifts like squats to swimming, cycling, running and skiing.