I would argue -
Modern Dance can currently be defined by the different techniques bearing that name - i.e. Graham, Cunningham, Hawkins, Humphrey, Limon, Taylor, etc.
The pioneers of Modern were Isadora Duncan (1877-1927) and Ruth St. Denis (1877-1968). At this point modern had none of the structure and technique known today; it was instead a complete freedom of expressive movement. Isadora Duncan based her movement on the free and natural, inspired by the classical Greek arts, folk dances, nature and natural forces as well as athleticism (i.e. skipping, running, jumping, leaping, tossing). Duncan gave dancing new vitality using the solar plexus and the torso as generating forces for all of her movement.
In 1915, Ruth St. Denis founded the Denishawn company with her husband Ted Shawn. The company nurtured forerunners of American modern dance including Martha Graham. Prompted by a belief that dance should be spiritual instead of simply entertaining and technically skillful, St. Denis brought new emphasis on meaning and communication of ideas busing themes previously considered too philosophical for theatrical dance. Although she was never concerned with technique for its own sake, her extensive use of Oriental forms and abstract "music visualizations" encouraged her students to develop nonballetic movements that became known as modern dance.
St. Denis and Shawn separated maritally and professionally in 1931 and St. Denis retired from performance for a time, founding the Society for Spiritual Arts and promoting the use of dance as worship. She resumed performance at Shawn's Jacob's Pillow dance festival in 1941, and remained active until the 1960s.
Another little known pioneer of expressive movement in the early 20th century was Mary Wigman (1886-1973), a German dancer, choreographer, and teacher. Wigman opened her own
school in 1920 and became the most influential German exponent of expressive movement and toured extensively. Her school was the center of European modern dance for 20 years.
Many of the second generation of modern dancers were those artists who still have functioning schools and companies today.
Martha Graham is certainly among the most influential of these. She began dancing at the ripe old age of 22 with the Denishawn Company, and ten years later in April 1926 she appeared as a solo dancer in New York City, premiering her own works. After withstanding considerable ridicule she
began to acquire an audience of discriminating and
enthusiastic admirers of her avant-garde style, and in 1927 founded the Martha Graham School of Contemporary Dance in New York. Graham used sculpture as stage decor and with the 1935 piece Frontiers began her collaboration with sculptor-designer Isamu Noguchi. Graham's pieces are unusual in the contemporary world of modern dance for their narratives; frequently she would interpret legends such as that of Medea (in Cave of the Heart, 1946) or Joan of Arc (Seraphic Dialogue, 1955). Her most famous works include Lamentation, Appalachian Spring, and Night Journey (the tale of Jocasta).
Graham based her technique on the contraction and release of the torso. Her movements are strong and well defined, and marked by their ability to summon emotion. Many members of her company went on to become famous in their own right, including Merce Cunningham, Paul Taylor, and her husband (for a time) Eric Hawkins.
Graham is quoted "In 1980 a well-meaning fundraiser came to see me and said, "Miss Graham, the most powerful thing you have going for you to raise money is your respectability." I wanted to spit. Respectable! Show me any artist who wants to be respectable."
Martha Graham died on April Fool's Day, 1991, in New York City.