Electromyographic Study of Postural Muscles in Various Positions and Movements

Electrical activity of ‘postural’ muscles (sacrospinales, hamstrings, gastrocnemii, quadriceps femoris) in normal subjects during different positions and movements of the body varies in different individuals. The sacrospinales become active whenever slight displacements of the center of gravity occur. The sacrospinalis of both sides participate in lateral flexion and extension, and rotation of the vertebral column. In leaning forward, these muscles (except the quadriceps) are working essentially under isometric conditions and their electrical activity is prominent. Stretch, then, appears to be a major factor for the activation of these muscles. In flexion of the trunk, the sacrospinales cease to function at a ‘critical point,’ however, further trunk flexion occurring mainly at the hip joints continues to be accompanied by the activity present in the hamstrings. In more complex movements, such as sitting and standing, the same basic mechanisms related to stretch and displacement of the center of gravity appear to hold true.