I have been noticing recently that my abdominal area (over the stomach) has been getting really firm. I think it is the abdominal muscles.
I have been bycycling fairly hard for about an hour every weekday for the least 18 months. Its about the only serious exercise I get.
I didn't expect it to strengthen my abs. I have been getting the expected thigh muscles, and had noticeable change in <6 months.
I'm getting slightly concerned about the firmness. If it isn't exercise improving the muscles it may be some kind of medical issue, which warrants looking into.