The straight-arm plank is an exercise targeting the core muscles. It is basically the top of a push-up position, with elbows fully extended and palms on the floor. The position is held for time to target the muscles of the core, as well as the shoulders.
Targets the core muscles, including the abdominals and lower back
Builds muscular endurance in the core and shoulders
Can help build up to more demanding core movements like stir-the-pot on a ball