A couple of points.
1. Burning fat is an all-body process, and you can't target wher you lose weight from. Energy is delivered to the muscles via the bloodstream, NOT from being absorbed from surrounding fat stores. Planks (or any other core exercise, for that matter) will strengthen the core muscles, but will do absolutely nothing about the overlying layer of fat. If reducing belly fat is what you mean by "tightening the abs", then the only way you can achieve this is by reducing your overall body fat through a combination of cardio, all-body strength training and watching your intake.
2. A strong core is important for posture and balance, and planks are a great core exercise, but the core is just like any other muscle group - you need to rest it for 48 hours between strength training sessions for the muscles to recover and get stronger. Strength training (including planks) 3 times per week is actually more effective than training every day.
The reasonable man adapts himself to the world; the unreasonable one persists in trying to adapt the world to himself. Therefore all progress depends on the unreasonable man.
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