By now most music-loving athletes are aware of the potential dangers of wearing -- and being distracted by -- walkmen on the run.

But a recent issue of American Health magazine presents a new hazard for the headset set: "cross-modality masking," a phenomenon in which one set of bodily sensors obliterates another. "It's the same principle," notes the magazine, "dentists use when they give patients music by headphones to block out the pain of the drill."

Skiers, runners and others trying to perform a task requiring concentration or coordination, the article contends, can be dangerously disoriented by loud noises. Headsets create "such a cocoon of sound, few other sensations filter through. Equilibrium -- monitored by the central nervous system, as is sound -- can be lost in the noise. The result: minor, and not so minor, failures of coordination."

* Agent on the Run: Three years ago, travel specialist and runner Tom Gilligan mixed business with pleasure to form Marathon Tours, Inc., in Cambridge, Mass. Today he grosses about $400,000 annually, says Inc. magazine, by leading groups of runners to races in exotic places like Shanghai, London, Montreal and Stockholm.

* On the Rebound: If you're tired of just tramp, tramp, tramping on your mini-trampoline, try doing "the canary flap," "the tummy tumble" or "the buttock jog," a sampling of rebounder routines in The Rebound System by Robert Appel (Harper & Row, 144 pages, $7.95). For safe aerobic benefit from rebounding, Appel says, "Exercise as fast as you can without straining, at a pace that you can maintain for about 15 to 20 minutes without exhausting yourself."

To determine your ideal exercise pulse rate (taken during peak exercise or within 60 seconds thereafter), he offers this formula: Beginners should equal 220 minus their age, times .6. Avocational athletes should equal 220 minus their age, times .7. Professional athletes should equal 220 minus their age, times .8.

Among Appel's other tips on rebounding:

* Do: Warm up and cool down, include at least 10 minutes of activity in each session, exercise in a well-ventilated area, keep track of your pulse rate, use a mirror to check posture and positioning, stop if you get dizzy.

* Don't: Eat just before, wear shoes on the mat, start too vigorously or force any exercise, take a hot bath or sauna immediately after exercise or jump off the mat.