How to do 100situps if you have muscle pain!?

Be careful here: If you exercise and have pain at the time of exercise, probably ok. If pain persists/worsens for more that 20-30 minutes after, you may be overdoing. If the pains are low back (lumbar), limit sit-ups to nearly "isometric" (with- out much motion). Here, assume a sit-up position, but only elevate the shoulder blades off the surface. Hold this position for 12-15 seconds. Gradually work up to 20 reps.