By Frances Yoon

SEOUL, Feb 26 (Reuters) At 74, there are several things that warm the heart of Ji Bok-hyun, such as seeing her grandchildren smile and pounding her fist through wooden tiles.

Ji is one of a group of 23 South Korean grandmothers, all of whom are 70 or older, practising the high-kicking and hard-punching martial art of taekwondo at a gymnasium in Inchon, about an hour west of Seoul.

The traditional Korean martial art is widely taught in South Korean schools and most young men in the country learn taekwondo as part of compulsory military service.

But for these grandmothers, taekwondo is a way to keep themselves happy and healthy.

''Smashing! I really like smashing the slabs. It releases my stress that I get from home,'' said Ji, who has been practising the martial art for nine years.

Ji, who has five grandchildren, is a black-belt holder and leads the class.

Almost half of the grandmothers have reached the highest-level ranking of holding black belts.

While most taekwondo athletes aspire to smash wooden or ceramic tiles with their feet, hands or head, the grandmothers mostly break plastic tiles with prefabricated fissures.

There is no granny-on-granny fighting during practices but several of the septuagenarians said they were ready to fend off an attacker.

HOWL LIKE BRUCE LEE While practising her high kicks, 74-year-old Cho Jong-jae howls like kung-fu legend Bruce Lee, and the other grandmothers break into laughter.

''I haven't been sick once since I started coming here three years ago. I'm here because it's fun. If it wasn't, I wouldn't want to do it,'' says Cho, who has a red belt.

Yet 77-year-old Kim Hee-bok, who is the oldest of the group, focuses more on slower movements and stretches. Before she began learning the sport, she had trouble simply moving.

She had been suffering from diabetes and arthritis for years.

''I couldn't even walk up five flights of stairs before.

Now, I can go up more than 50-60 stairs,'' she says.

She heard about the lessons from her fellow classmates and asked her son and daughter to walk her to class every day until she was finally able to walk on her own.

''Coming here is much better than going to the hospital.

Even my doctor says to attend the class every day.'' MORE REUTERS SB BS0855