Flutists and oboists can be ethereal and esthetic creatures, but to play the double bass you have to be an athlete -- as Hans Roelofsen and Rudolf Senn, double-bass soloists from Holland, both know.

From St. Louis (one of the stops on the duo's five-week American tour), Roelofsen spoke about the physical requirements of double-bass playing: "The position of the musician behind the instrument is very important, epecially for the back. Many players do not pay enough attention to position, curl over and end up with back pains. You have to think about everything, including the way you sit or stand, if you want to play top-quality music."

Roelofsen also spoke about the relationship between the musician's physical size and that of his instrument: "We both have quite long arms, which is a positive feature for tackling the bass, but to fit the size of the body we've had double basses made specially. You could say it's the same with a bicycle: If you want the best results, you adjust the bicycle to fit the length of your legs."

Saturday the Grand Duo Concertant, as the soloists call themselves, will join the Montgomery Chamber Orchestra at 8:30 p.m. at the National Bureau of Standards in Gaithersburg. The two will play Handel's Trio Sonata, Op. 2, No. 8, and Kurt Atterberg's "Suite for Two Double Basses."