You don't need to be ripped to be a ninja, but you do need "bodyweight strength," or the ability to carry and propel your own body through space. That doesn't just mean running and jumping — it also means pushups, pullups, pegboards and rope climbs. And while such tasks require full-body brawn, none of that matters without grip strength.
Durable hands can make a world of difference on "ANW." The show is big on upper-body obstacles, from the grip hang and cliffhanger to the flying nunchucks and salmon ladder. Do chinups and pullups to avoid fatigue below the elbow, and try mimicking "ANW" obstacles by climbing ropes, poles and pegboards. Lifts like wrist curls and farmer's walks may help, but since the goal is stamina more than power, don't go crazy with weights. Gymnastics, rock climbing, martial arts and many other sports also build grip strength.