Psychological and social factors at work may contribute to musculoskeletal disorders by the following processes: (i) Direct pathogenic effects by affecting physiological mechanisms, such as muscle blood vessels and hormonal secretion; (ii) by altering work procedures and thereby altering the biomechanical loads through changes in posture, movements, and exerted forces; (iii) by altering sensations, mood, and cognitions, and thereby influencing symptoms, consequences of symptoms, and functional impairment; (iv) by interfering with buffer mechanisms, reducing the tolerance to other exposures. Almost all hypotheses of direct pathogenic effects (i) of psychological "stress" on muscle pain, maintain that the pain results from muscle-cell activation. The mechanisms proposed for the generation of pain are related to effects of energy deficit or intracellular calcium accumulation, leading to muscle-cell damage. However, it has not been possible to find reliable causal associations between muscle activation and pain. Furthermore, during active coping behaviours, muscle blood flow generally increases, rendering hypoxia less probable. Other hypotheses propose that increase in muscle-cell activity in musculoskeletal disorders is a consequence of the pain or that the pain originates from interactions between blood vessels and nociceptive nerves of the muscle. Explanations of the pathogenesis of pain generally do not yet account for the activation of sensory nerves (the nociceptors) that mediate information of potential tissue injury to the nerve system. Psychophysiological mechanisms determine whether pain become chronic and the consequences of pain. Mechanisms of the spinal medulla may amplify or inhibit transmission in the nociceptive circuits. Attention and perception are determined by the appraisal of the threat value of sensations. Pain sensations that are appraised to signal threat of injury or disability, are maintained and amplified. Pain beliefs contribute to the cognitive appraisal process. Health care personnel play a central role in forming pain beliefs by the way they inform of potential risk factors at the workplace and by the way they perform interventions and prevention measures. Therefore, the occupational health personnel must possess specific knowledge of which psychological and social factors contribute to musculoskeletal disorders and how their own recommendations may prevent or promote chronic disabilities.