Planks are very effective at targeting the abs. These static exercises are excellent for improving your body's overall balance and protecting the spine, provided they're done with proper form. And if you're not sure how long you need to hold them to see real results, we've got you.However, planks don't just work your abs. Thanks to certain progressions, you can target other muscles or muscle groups. All you have to do is modify your static exercises -- the possibilities are (almost) endless. Proof: this particular progression targets the quadriceps. It's a great way to finish off your leg day strong, while also working your abs.How do you do this quad-targeting plank variation?1. Get in a classic plank position, with your arms straight.2. Put your feet up on a step or any other solid surface, so your feet are off the ground.3. Bend your knees until they're as close to the floor as possible.4. Straighten your body using the strength of your quads to return to the original position."This exercise is a triple-whammy because it requires a lot of core strength and balance, but if you do enough reps of them you really start to light up the quads," personal trainer Charlie Atkins told MensHealth.com. Indeed, as you perform this exercise, you have to make sure not to move your upper body. In order to achieve this, your abs must be engaged in order to keep your spine straight.Perform three sets of 12 to 15 repetitions of this exercise at the end of your workout sessions. As you progress, you can add weight to your legs or on your back to make it more challenging.Stretching to prevent abdominal painAfter putting this much stress on your core, you'll have to stretch your entire abdominal belt to avoid stiffness and injury. All you'll need to do is grab a swiss ball and lie on it, on your back.Then, put your hands behind your head and lean towards the ground as much as possible. Try to separate your rib cage from your pelvis for optimal results. This will allow you to fully stretch all the muscle fibres in your bust. If you want to kick your stretches up a notch, control your breathing and take a slow, deep breath, while relaxing completely.