core
The antagonist for plank exercises are the muscles on the front of the body, particularly the muscles of the hips and abdomen, such as the hip flexors and rectus abdominis. These muscles work in opposition to the muscles engaged during a plank, which are predominantly the muscles of the back, core, and shoulders.
A plank primarily engages the back, shoulder, and abdominal muscles. Most notably, these include the erector spinae, rectus abdominis, transverse abdominus, trapezius, pectorals, and deltoids.
Flappy Jacks
The traditional plank works out not only your abdominal area, but also your thighs and butt. :) A side plank does all that and works on the muscles in the arm supporting the plank.
Your use youe leg muscles
Because they stretch and use most of the muscles in your body.
I got a Herkules air jack, which has worked out pretty well. Yahoo shopping has some great deals on Air Jacks.
The Transversalis abdominus can be worked out with tummy tuck, planks, plank with leg lifts and transverse pulls. The single most effective exercise to isolate this area is the stomach vacuum.
They strengthen your legs for the most part. It might possibly strengthen your core muscles for keeping your balance, but the effect would be minimal. Jumping jacks basically targets only your leg muscles.
You use your quadriceps, hamstrings, gluts, abs, biceps, triceps, and obliques.
Abdominal muscles, among others.