intervention can produce valid claims if coupled with other evidence about the change and with probes concerning the presence or absence of other plausible explanations.

In complex domains, single studies can seldom provide a conclusive verification of a causal proposition. It is through replication and synthesis of evidence across studies, preferably with studies that use a variety of methods (each with different strengths and weaknesses), that causal claims gain their inferential strength. In performing such syntheses, greatest weight should be given to the evidence from studies that most completely satisfy the five criteria specified above. Poorly conceptualized and executed studies may have little to offer for assessments of causal claims. In contrast, the evidence from a few well-conceived and well-executed studies can strongly outweigh the ''noise" created by a large number of studies that do not satisfy the five criteria for causality. (A similar argument was made in deliberations about the evidence related to the effectiveness of a set of complex public health programs; see Normand et al., 1995).

Finally, inferential strength is gained by examining the evidence from a variety of theoretical perspectives (as well as a variety of research methods), as specified in the framework provided in Figure 1. Establishing that biological and biomechanical processes influence tissues, that these forces are present in some work environments or work-related tasks, that their presence is associated with musculoskeletal disorders, and that their influence can be reduced by workplace redesign (or other interventions) should provide a greater understanding of the evidence than can be gained by considering each factor separately. The findings presented in the rest of this section reflect the steering committee's application of the criteria to the research literature.

Soft Tissue Responses to Physical Stressors

Several well-established findings are supported by the papers of Rempel et al. (1998) and Ashton-Miller (1998), presented at the workshop. While certain loads can be tolerated and adapted to, all soft tissues, including muscle, tendon, ligament, fascia, synovia, cartilage, intervertebral disc, and nerve, fail when subjected to sufficient force. Data from cadaver studies provide ranges within which such failures occur, and animal models of some tissue provide support for the laboratory data. Even at levels of force clearly below the failure level, however, there is scientific evidence, from these types of studies, that tissue response to deformation can produce inflammation, failure at microscopic levels, and muscle fatigue.

Injuries to muscle from single-event and repetitive contractions have been documented in humans and in animal models. Local muscle fatigue occurs at low contraction levels when maintained for long periods. Inflammatory muscle responses have been documented in humans subjected to repetitive or prolonged loading. Muscles also are affected by individual factors, such as age and level of conditioning. These effects involve not only the tissue, but also the neuromuscular control system.

Ligaments and tendons also fail from single or repetitive loading. For tendons, disorders can occur at the insertion into the bone, in the tendon proper, or at the junction between tendon and muscle. Animal models have demonstrated the development of inflammatory responses in the tendon sheath and insertion areas (tendinitis and tendinosis).

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