A midseason upset epidemic is felling some of the best teams in the National Football League.

I don't believe upsets in the NFL occur on Sunday. The upset is born early in the preceding week, sometimes in the locker room immediately after the preceding game. That upset embryo then flourishes and grows stronger all week until by Sunday's game it is virtually a falt accompli.

A wise coach will see the signs and try to counter them. But sometimes the tide is too strong.

Here are some potential upset altuations:

Badly beaten opponent - a team beaten decisively the week before will be fired up and the favored team will be relaxed. I often won't show my team the game films of an opponent's whipping the week before. Players tend to think, "There's no way these guys can beat us," But they can, and often do.

Loose practices - regardless of the coming opponent, or what we did in the previous game. I always try to set a businesslike pace and mood at the beginning of each week's practice, usually on Tuesday. There should be no horseplay. I don't believe in loose, carefree attitude for fun. The fun is in winning. If a practice is loose, mistakes are made. Worse of all, concentration is broken. If Tuesday's practice is sloppy, the coach must communicate this to the team and its leaders, and redouble the efforts on Wednesday.

An earlier efforts win, if a team plays an opponent twice in a season, and beats them the first time - especially on the opponent's home fielf - an upset is brewing. "We beat them in their own backyard" is the thinking. "Certainly we can whip them in our own yard." This breeds overconfidence, lack of concentration and defeat.

Believing the media - though many coaches and players deny it, the fact is all of them read the newspapers, and read them closely. And they tend to believe what thet read. If a writer says a team is good, the players often believe it and become overconfident. The same is true of negative articles saying the team hasn't a chance.

Looking back - If a team has just won a big game, it can allow itself no more than it must be forgetten. Likewise, upsets often run in pairs. Looking back and brooding over a surprise defeat will kill concerntration and lead to another defeat the following Sunday.

Looking ahead - Some coaches, especially if they have a lot of collegiate coaching experience, look ahead to a big game some weeks in the furture. In college competition a coach can afford to do this, because there are a certain number of sure wins on the schedule. In the NFL a coach can't do this. Any team can geat you. The players have finely tuned radar. If the coaches are looking ahead, the players will too. That weak opponent in the interim can win a upset.

Petty distractions - The little things can kill concentration and lead to an upset. If buses aren't on time and players are left nervously standing around, or if the driver gets lost or makes a wrong turn on the way to the hotel, players' minds are taken off the game. (I used to have a man seated next ot the bus driver giving him precise directions on all routes to the stadiums, hotels, airports, etc.) Noisly hotel lobbies or hotels allowing outsiders access to the players' rooms must be avoided. Travel arrangements should be precise. It is deadly to have a gan of nervous players crowding up to Gate 38 in an airport when the plane is at Gate 5. Again, it breaks concentration and causes a questioning of the efficiency of the whole program.

This, incidentally, is why we rarely have a good Super Bowl. The endless distractions kill player concentration and the play in the game reflects this.

Injuries. This problem is peculiar to football.If a key player is injured before or during the game and his substitute must come in, everyone on the field imagines a weakness. It gives the other team a lift even if the substitute is 99 percent as good as the man he replaced. This is one reason why each man on a team must be confident of his own worth to the team, even if he has been a benchwarmer. His teammates must contribute to his sense of value and ability.

Being too smart-frequently, against an opponent it "knows" it can beat, a team will try grandstand stuff it would never try against Dallas or the redskins. Things like fake punts with the punter passing or giving the ball to the blocking back while the best ball carrier blocks a big linebacker.Let punters punt, runners run, blockers block.

Rivalries - long-time trivals like Chicago-Green Bay, Dallas-Washington and lately, Oakland-Denver play inspired football against each other. The odds go out of the window, regardless of each team's record that year.

The elements - certain teams, like the Rams of the early '60s, were known as fair-weather clubs who rarely won on the road when the weather was bad. This has been largely corrected, but an otherwise good team can fall apart in the mud while a plodding underdog pervails.

When a coach spots more than a couple of the above symptoms, he has to take action immediately. Even the coach with the best-laid, long-term program must become a hard nose. He must jump on players, halt practives. Even the most dedicated team leaders must be confronted and better leadership demanded of them.

The upset virus is contagious, and sometimes even the best assistant coaches contract it. They and the entire organization must be alerted to what is happening and be involved in remedying it. Trainers, equipment men, public relations people, secretaries, everyone, must be told that a letdown had occurred and that each has a responsibility to try correct it.

Sometimes the letdown extends through the warmup on game day. A coach must never give up. He must tell the team leaders that there is trouble ahead.

Sometimes this works, but usually the damage has been done. It's already Sunday, and the upset has been a week in the making.