You may do a plank hold to strengthen your abs and core but a plank, really, is a whole-body workout. It activates your rectus abdominis, inner and outer obliques, and transverse abdominis as well as the the spinal erectors (back muscles). Holding it also engages shoulders (deltoids), upper body (lats, triceps, traps, chest, biceps, rhomboids) and lower body (glutes, quads, hamstrings, calves, extensor muscles in feet). This makes it energetically expensive (which is why you get tired holding it) and physical adaptations catalytic (which is why you begin to feel fitter and stronger all over, after a while). The First Things Plank Hold Challenge helps you achieve all this, gradually, but persistently and consistently. By the end of the month you will have reached a new level.