Two lines of joggers flow in opposite directions. Suddenly, a small, hairy figure disrupts the continuity of the two streams, running out of pace, jumping and wagging a tail.

The scene is the C&-O Canal towpath, and the oddity among the crowd turns out to be a four-year-old male golden retriever named Dustin, who happens to be a veteran of the distances and one of the few canines that, thanks to their owners, could be seen practicing a sport monopolized by human beings.

Since dogs are supposed to be man's best friends, it's surprising that not too many runners take their dogs along for an exercise that would seem more enjoyable when done with company. The question that comes to mind, however, is why those who do take their dogs along do it.

Dustin's owner, Jeff Serrell, who coversa respectable four miles every day, said: "If he doesn't run he gets fat. He needs the exercise. He keeps me going, I have to catch up to him."

Serrell recognizes, however, that taking Dustin along sometimes means putting up with problems he otherwise wouldn't have to deal with.

"Sometimes he chases dogs, gets into fights. It breaks up my runing. I don't like it," he said.

Phil Stuart, associate editor of Running Times magazine, cities such disturbances as one of the reasons most joggers prefer to do without the company of their pets.

"I personally wouldn't take mine because it's a distration," he said.

According to Stuart, of the approximately 8,000 to 10,000 joggers in the area, "I would say a very small percentage of them do that. As a rule, most runners don't like to deal with other people's dogs," he said.

Juan Nieves, of Forest Knolls, Maryland, seems to have found a solution a leash. However, in this specific instance security rather than experience gave rise to the idea. Neives' dog, a cross-breed of German sheperd and Labrador retriever, has had an "aggressive background," according to his owner.

"He has been arrested twice by the police for harassing people. Although he is less aggressive because of the energy he spends running, I don't think he is less anxious to chase cars," Nieves said.

Less aggressiveness, loss of weight, more endurance and "80 per cent less" barking than before are some of the changes Nieves has noticed in his dog, Caesar, since he started jogging.

"At the beginning he started with a fast rhythm, but at the end of 1.8 miles he ran out of air due to the lack of training. I practically had to drag him to finish his two miles," Nieves said. "I have to keep to my pace so he can endure the distance. Otherwise he would run free and would get tired too soon."

According to Dr. Irving G. Cashell of Georgetown Veterinary Hospital, common, sense is all owners have to use when jogging with their dogs.

"There is a problem and it's that sometimes people have fixed ideas such as that the dog needs to run. They should be observant of what effect jogging has on their dogs, not on anybody else's dog, and see what the dog's tolerance is," said Cashell, who recommends that owners condition their dogs on short distances.

Dr. William Page of the Alaska Avenue Animal Hospital agrees with Dr. Cashell in that the phsysical demands and needs of dogs are different and so is the effect of jogging on them. Although bigger dogs obviously make better joggers and "smaller dogs usually . . . get adequate exercise in the houe," Dr. Page did not exclude small dogs from being benefited by the exercise.

"It depends on the dog. The exercise is good to maintain a good muscle tone, cardiovascular tone and to keep them in good top shape. There are exceptions. If the dog is too old or has some kind of degenrative disease the exercise would aggravate his condition," Page said. "But for healthy would have the benefit of the companionship, the exercise and the walking he needs to do, all in the same exercise."

"You kill two birds with one stone," said Claire Conkling, who runs a mile everyday in the company of Huff, her eight-year-old Labrador."I have to walk him anyway, so I might as well jog him.

"In the beginning I had to pull him along, he would start sniffing everything. But now he runs ahead of me," said Conkling, who started jogging with Huff more than a year ago. "He's a good incentive, because he makes me run."

Incentive is one of the advantages those joggers who run with their dogs think they would not have otherwise. Although Nieves admits Caesar sometimes gives him some trouble - "Sometimes I feel like keeping up training but I have to go back to the house because of him. Sometimes I have to jerk him because he wants to wander out of the course. I waste a lot of necessary energies" - he still thinks the advantages outweigh the inconveniences.

"Some joggers are into serious training and don't want to have the distration of taking care of the dog, but for myself, companionship, protection from people and from other dogs, competition and getting the dog to like and know me more as a friend are some advantages of runing with him," Nieves said. "I would miss it very much."

Conkling said she would not run without Huff now. "People would never bother me with him around. And I don't have to worry about running by myself," she said. "It's nice to run with somebody, even if it's a dog."