Presentations

Discussion

The papers provide evidence for several conclusions about soft tissue response to physical stress. Although certain loads can be tolerated or adapted to, all soft tissues, including muscle, tendon, ligament, fascia, synovia, cartilage, intervertebral disc, and nerve, fail if subjected to sufficient force. Data from cadaver studies provide ranges within which such failures occur, as do animal models of some soft tissues tested in laboratory studies. Even at levels clearly below these failure ranges, however, there is scientific evidence from laboratory studies that soft tissue responses include inflammation, muscle fatigue, and ultrastructural degeneration that does not heal without cessation or restriction of the provoking force. As Figure 1 illustrates, intrinsic factors, such as age and conditioning, can influence soft tissue response and recovery, as can extrinsic factors, including the work environment and life-style characteristics.

The discussants of the Ashton-Miller and Rempel et al. papers did not generally dispute the conclusions presented, though many believe that greater attention to some aspects of the scientific evidence on soft tissue responses to stress could provide additional, important insights on tissues responsible for work-related musculoskeletal disorders. Suggested research areas included:

• studies of the synovium (the tissue lining the tendons and joints);
• recent biological work on cytokines and growth factors considered important in tissue inflammation, healing, and repair;
• more scrutiny of studies using electromyography (EMG) to monitor muscle fatigue and pain (EMG measurement is noninvasive, objective, and can be used to establish biomarkers for muscle injury); and
• studies of the effects of peripheral tissue inflammation on the central nervous system, especially in cases where muscle pain lasts much longer than expected.

Several other themes emerged from the discussions of these papers. The need for more research and more integrative research was a major theme. The long-term or chronic effects of soft tissue responses to stressors was one area in which discussants believed further research was critical. Steven Lehman, University of California at Berkeley, raised this issue in discussing the need to better understand the physiology of low-force, long-duration work and its effects on motor control, muscle fatigue, pain, and the recovery process. Carlo de Luca, Boston University, suggested the use of electromyography as a method for studying muscle fatigue.

The clinicians among the discussants stressed that, although most of the laboratory evidence presented in the papers corroborated their own case observations, that laboratory evidence deals primarily with acute stressors while they find the most difficult cases to be those involving chronic stressors. Susan Mackinnon, Washington University School of Medicine, noted that her experience indicates that an understanding of patient symptoms requires an understanding of chronic nerve compression. She described a model of chronic nerve compression in rats and primates that closely mimics the pathological changes seen in humans. These pathological changes (ranging from edema to degeneration) are paralleled by the symptoms of patients in clinical testing. Patient complaints typically begin with intermittent numbness brought on by specific postures, but not apparent when the patient is resting. As compression continues or increases, numbness becomes more persistent and, eventually, permanent. There are also neural and physiological symptoms which begin with complaints of aching and progress to weakness and finally, to muscle atrophy.

Robert Szabo, University of California, Davis, reinforced Mackinnon's comments, noting that the Rempel et al. paper's discussion of laboratory studies on nerve compression over time did not highlight the clinical relevance of the findings. The findings show a progressive degeneration of nerve health under sustained compression, which suggests that nerve compression effects can be viewed on a spectrum divided into early, intermediate, and late categories. In clinical terms, early stages respond most favorably to conservative treatment (such as steroid injections and splinting for carpal tunnel syndrome). For intermediate stages of nerve compression, involving numbness and parasthesia, patients respond well to nerve decompression via the surgical release of the carpal tunnel. In late stages, patients often have permanent sensory loss with long-standing edema and fibrosis: these patients may benefit from carpal tunnel release, but this will not eliminate all their symptoms.

Putting these clinical views on chronic nerve compression in an even broader context, Thomas Mayer, PRIDE, Dallas, Texas, reported that even when all forms of acute nerve compression and musculotendinous sprains and strains are considered, they account for a small portion of medical care and medical costs (see box).

Another common "need for research" theme among the paper discussants was the need to generate links between specific anatomically distinct disorders (as they are clinically observed) and specific underlying patterns of pathology. J. Steven Moore, Texas A&M, summarized this view by noting that models that relate (1) specific patterns of musculoskeletal loading to (2) distinct internal stress in specific anatomical structures that (3) elicit tissue responses that (4) lead to observed tissue changes—are critical to diagnosis, treatment, and epidemiological tracking of musculoskeletal disorders and the circumstances under which people are exposed to such disorders. Echoing these remarks, Kai-Nan An, Mayo Clinic, noted that inconsistencies in the definitions and diagnosis of work-related repetitive stress injuries makes the true incidence of such injuries and the factors that provoke them difficult to identify.

Overall, the paper authors and discussants agreed that sound research shows that muscles, tendons, ligaments, and nerves can fail when subjected to sufficient force and that they can heal and repair themselves over time under the appropriate conditions (restriction or cessation of force and gradual reconditioning). However, there was also agreement that work remains to be done in establishing the links between impetus to injury to soft tissue response, on one side, and optimal conditions (either physical or environmental) for healing and repair of injury, on the other.