The first basic exercise for core strength is a press-up or push-up. Follow the advice in this video and do them properly and you will feel the benefits in the dojo.
Crunches are another good exercise for hard dojo floors. Sit-ups can cause problems if you're not careful. Here's a video demonstrating correct form for crunches.
Do these exercises first thing in the morning and in the evening. Start by doing 5 of each correctly and slowly, paying attention to maintaining good form. Then build upon the number you can do in
increments of 5.
increments of 5.
You'll notice the difference in yourself within a week ... so will your sensei!
Villayat 'Wolf' Sunkmanitu