THE door to Writer John Koffend's office was closed. Researcher Nancy Atkinson, a sheaf of material for this week's Essay in hand, knocked and then tentatively turned the knob. Koffend was lying on the floor -- but there was no reason for alarm. "Pardon me," he said, "I was just doing my push-ups."

Writing about exercise was for Koffend a jog across familiar terrain. Hardly a day passes when he does not do his pushups, sit-ups and jogs-in-place either at home or at the office. His expertise in the subject is matched by that of Senior Editor Robert Shnayerson, who is often involved with Essay, and for this one was a prime source. Shnayerson runs for his life every morning along the shoulder of Manhattan's car-clogged Henry Hudson Parkway, or in nearby Riverside Park, averaging 20 miles a week. One hazard of running in the park, he finds, is that dogs instinctively pursue a running man -- and he has been paced by Great Danes and poodles.

One of the more determined exercisers who reported for our Essay is Boston Correspondent Bill Marmon, who three mornings a week at 7:15 takes off from his apartment near Harvard Square, jogs across the Charles River on the Weeks Me morial Footbridge, trots on to the Harvard Graduate School of Busi ness Administration, which is about halfway on his route. There he pauses for 30 pushups, 30 situps, and occasionally a dozen chin-ups on a convenient tree branch. Then he heads home, sprinting the last 200 yards to "make the blood flow into the fin gers and toes and lungs and head." In Cambridge, his jaunt brings only looks of tolerant amusement from those he passes.

After citing these energetic in-the-family examples of the exercise era, we hasten to add that some--well, many--of our staff are quite remote from this enthusiastic level. A number are in a category with Essay Reporter George Taber, a sometime exerciser whose weekend effort now consists of lifting the Sunday New York Times. Others are on a par with (but few as lucky as) the Bonn bureau's lean Burton Pines, who says, "Eating hard-frozen chocolate ice cream is all the exercise I get--and that's all I need." At any rate, it can be said that we have enough opinion-and expertise on that side of the question to provide a broad and balanced perspective.

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