It has been a given for some years that smooth, repetitive, non-competitive exercises like swimming or sometimes biking are beneficial to arthritis victims. But there is some new evidence that highly stressed or anxious people can derive special benefits from this type of exercise as well, and perhaps even more so than their laid-back friends and relations.

The activity categories for arthritics usually do not include running or jogging, because of their potential for excess wear and tear on sensitive joints. However, for your everyday uptight jock, they apparently rank with the best.

This added incentive for smooth exercise programs derives in part from some research done by Brad Hatfield, a sports psychologist, runner, ex-weight lifter, biology teacher and ex-part-time Baltimore Colts public-relations assistant.

Now the 31-year-old Canadian-born Hatfield is assistant professor of physical education at the University of Maryland. His research centers on the effects of exercise on brain activity. Hatfield uses the electroencephalograph -- EEG -- as a measure, with the brain's so-called "alpha" waves designating a state of low arousal, a deeply relaxed mental state.

Some of his most recent work with volunteer students at Maryland's College Park campus has produced these preliminary findings:

*Everyone participating in the test activity -- running on a treadmill -- demonstrated some measurable mental relaxation a few minutes after the exercise was completed.

*People who tended to be so-called "high anxiety" or highly stressed personalities, derived more mental relaxation from hard exercise than people who are lower-keyed.

*Both high- and low-anxiety people received more benefit when they were exercising at 60 percent of maximum -- as measured on a treadmill -- rather than at the maximum they can do, suggesting that the stress of pushing themselves can adversely affect the psychological benefits. "When you think about it," says Hatfield, "an exercise stress test at maximum is not a very relaxing thing to go through."

Hatfield began his research (which formed the basis for his doctoral dissertation) a few years ago at Pennsylvania State University, where he demonstrated that world-class sharpshooting riflemen had a few seconds of deep relaxation before they actually pulled a trigger. At the same time that the EEG readings showed mental relaxation, other measures showed that their heart rates were elevated.

Hatfield speculated then that this sort of apparent psychophysiological phenomenon might be even more intense when it was applied to an activity "where you really got the heart beating." For instance, he wondered, "what about running?"

Based on his findings, and those of other researchers, Hatfield hypothesizes that certain forms of activity produce an effect on the cardiovascular system which, through a series of physiological feedback and negative feedback loops between body and brain, finally set off a rebound relaxation response.

Says Hatfield, "It appears that an elevated heart rate can cause a depressing effect on the central nervous system by activating a center in the brain stem. This then causes the cardiovascular activity to decrease but also goes up into the brain and causes a reduction in mental arousal as well." Read: mental relaxation. Read, perhaps, "runner's high."

Hatfield says he doesn't believe in the so-called "runner's high," but that he does see "an awful lot of people who run to relax and who report that all the 'garbage' in their minds seems to empty following the running."

On the other hand, he says, "There are those who just say, 'Hey, I know it's good for my body, but it doesn't do anything special for my head.' "

To see if he could further refine the phenomenon, Hatfield and other Maryland researchers screened 200 or so students before they found a small group who were "cognitively stressed," and fit into his "high-anxiety" category. They found the same number who were opposite personality types.

Each of eight volunteers was tested for a total of six hours on two separate visits to the Maryland exercise physiology lab at a period when no examinations or overdue term papers could affect the normal stress reactions.

After baseline testing, the volunteers ran on the treadmill, first at 100 percent of their maximum ability and then at 60 percent. Their brain waves were measured five minutes after the exercise and uniformly supported Hatfield's rebound thesis.

Further support came from the participants' verbal reports. "The people who were the high-anxious types seemed to get more of a rebound relaxation from this somatic excitement and were the ones who reported that the exercise made them feel better."

Explanations for this difference remain to be proven, but Hatfield says that "perhaps what is happening is that the high-stress person is in more of a chronically agitated state to begin with, so they are more likely to reach this critical level which then shuts everything down and rebounds into relaxation."

On the other hand, Hatfield also speculates that "broken" or "intermittent" exercises such as tennis or baseball or racquet ball, although they might provide aerobic benefits, probably would not produce the state of relaxation he has observed. "Oh," he says, "lots of people say they feel relaxed after a tennis match, especially if they've won, but I don't think that's the same mechanism. That is a feeling self-satisfaction, of having achieved a goal. But suppose they are frustrated in terms of the goal. In fact they may be very agitated, especially if they lose . . . "

Perhaps even more intriguing to researchers, says Hatfield, is a phenomenon that showed up during the study of the sharpshooters and to some extent in the latest running project. There were, he found, clearly observable differences in the arousal levels of the left and right hemispheres of the brain. In the shooters, measured before, during and after they pulled the trigger in a marksmanship contest, the EEGs showed a period of left-brain arousal, followed, a few seconds before the trigger was pulled, by left-brain inactivity or relaxation so deep it was as though the left brain was, as Hatfield put it, "turned off."

Generally, the left hemisphere is seen as the logical, mathematical side of the brain, with the right side the center for intuition, imagination and probably autonomic functions. The two sides are linked and need to work together almost constantly, but sometimes activity centers more on one side or the other and people vary in terms of which side is more dominant more often. This is an area still imperfectly defined or understood by brain specialists.

Some studies have suggested that skilled runners who can "tune in" to the pain and other bodily feelings associated with long distance running may be processing more through the right side of the brain, and other runners, usually less skilled, who try to "distract themselves so as not to feel what is coming up from down below," may be using their left hemispheres more.

This year Hatfield and his colleagues will measure brain waves and heart rates during the treadmill testing as well as afterward, including right- and left-brain activity (as measured by beta waves) and inactivity (as measured by alpha and theta waves) and, says Hatfield, "Who knows? It may be at some point we can actually teach people mental strategies and see if they can get into those purposefully and see if it has an effect on sustaining exhaustive exercise."