THE MUSCLES in Courtenay Padgett's 5-foot-1, 113-pound frame are well concealed, but the 17-year-old T.C. Williams High School junior knows how to use them.

Ask Andrea Schreiner of Lakeport School in St. Catharines, Ontario, whose dimensions are something like 5-11, 170 pounds. Before the Canadian girl could shake the spray from her eyes, Padgett had crossed the finish line to take home the 1977 Stotesbury Cup women's single scull gold medal this spring in Philadelphia.

Schreiner recovered to turn the tables in the next weekend's Scholastic Rowing Association of America regatta in Princeton, N.J. But even the second place finish Padgett managed there was a success few who knew her when she joined the Williams crew program in eighth grade expected.

"She has no excuses to make," said Steve Weir, her coach at Williams. "She's sensible enough to realize there's no way she can compete with that big girl. She beat all the American girls."

Padgett is not disturbed by her physical limitations. "I like being short," she said. "I got stronger because I lifted weights all fall, much of the regret of my mother. And I rowed last fall. I'm glad not to be puny like I used to be."

Now her mother is Padgett's biggest fan.

"At first I thought she is so little, it was unrealistic. But I was wrong," Mrs. Padgett said. "I'm a total, 112 per cent convert. I'm of a different generation and girls took ballet, for heaven's sake. I've learned it's a different world and I think this one is better."

Sculling differs from standard sweep rowing. In a standard boat an oarman uses both hands to manage one oar on one side of the shell; a sculler handles two oars. Sculling competition includes singles, doubles, and quads.

Padgett's first experience with sculling came at the end of her freshman year at Hammond, a feeder school to Williams, when she started rowing a double with weir, who works as an architect when he's not helping with the Williams girl's crew program.

In the fall of 1975, she and Weir built a single-scull from a school-bought kit. That boat started her in singles and later she borrowed a much lighter model for her big races.

Padgett also has served as coxswain for Williams crews, including the boys' junior four which took second in the 1974 Stotesbury and the boy's lightweight eight which took third in the scholastics of 1975.

Padgett entered her first singles race in the 1976 Stotesbury Cup and placed second in a field of three. This year she took seconds in the single scull and lightweight single dash races of the Women's Rowing Association Middle Atlantic Region regatta, which is an open competition with college women and boat club members.

There were no singles races during the regular spring season, so Padgett filled in by rowing in the Williams girls' second and third eights.

"It's not real important that I race all the time," Padgett said. "I just like to row. I like the sport. I like to row around and goof off."

She credits Weir for her success.

"He taught me how to row right.He helped me by just training me. I couldn't have done it if he gave me a watch and said do this many two-minute sprints and this many three-minute sprints. I need him yelling at me."

Padgett is realistic about her future in rowing singles. Schreiner, though presently the equivalent of an American high school senior, will be eligible for scholastic competition next season because Canadian schools have a 13th grade. Padgett hopes to lighweight eights will be introduced to girls rowing next year.

"It would be difficult for me to find someone my size to row against," she said. "The girl was an Amazon. I seem to run up against things like that all the time.

"I don't think I'll ever be able to beat her again unless I grow five inches and all the weight I gain is pure muscle. There's not much chance of that. That's why if they start a lightweight, I'd be interested.