How long should you plank a day?

Related questions

1. Is working out 30 minutes a day enough to lose weight?
2. What are supermans?
3. Are planks bad for your lower back?
4. Is cycling bad for your back?
5. Is cycling bad for your knees?
6. What is the healthiest exercise?
7. Is walking good for lower back pain?
8. Do Superman's work your back?
9. Can I do lower back exercises everyday?
10. Why does my back stiffen up when I walk?

Tags

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13. #seconds
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17. #wrong
18. #there