MUSCLE INJURY IN REPETITIVE MOTION DISORDERS

Documentation of causality between repetitive motions and musculoskele tal disorders calls for detailed understanding of the exposure variabl es and the corresponding physiologic responses in the biologic tissues . Quantification of the kinetics in some jobs characterized by repetit ive motions is summarized with the physiologic responses in the muscle s. Muscle activity pattern was studied in different shoulder muscles o r muscle parts, and in elbow and wrist flexor muscles. Activity patter n was dependent on the kinetics of the work requirements. This holds t rue for the compound electromyographic signal and for single motor uni ts. Low threshold motor units have been identified that are recruited continuously, the so called Cinderella fibers. The physiologic consequ ences of prolonged muscle fiber activity are reviewed, revealing mecha nisms for the development of necrotic changes in the muscle, which sup port the likelihood of work relatedness for musculoskeletal disorders.