Work-Related Musculoskeletal Disorders Report, Workshop Summary, and Workshop Papers (1999) / Chapter Skim
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Response of Muscle and Tendon Injury and Overuse
Response of Muscle and Tendon Injury and Overuse
Pages 73-97
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From page 73... ...
Finally, and probably more commonly, excessive tissue strain can be caused by some combination of the two single events superimposed upon a history of repetitive loading.
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From page 74... ...
followed bv a gradual increase in physical loading to stimulate healing arid subsequent tissue remodeling processes. The problem is not so much increasing physical loading again, but increasing it at a rate that does not ~ _ w ~ ~ ~ ~ 7 ~ ~ ~ _ ~ exacerbate symptoms or, worse, cause re-injury.
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From page 75... ...
For single stretches the threshold for injury in mice has been estimated as a work input of ~ 50 I/Kg muscle, corresponding to stretches in excess of 20% LO (optimal muscle length) strain, say, at a rate of 2Lf/s (muscle fiber lengths per second)
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From page 76... ...
Muscle Fatigue Sustained postural muscle activity can create significant musculoskeletal problems. Epidemiological studies in assembly workers, for example, indicate that redesigning the work place in order to reduce average trapezius muscle activity below 2% MVC over an eight hour day can significantly reduce sick leave (for example, (Aaraas, 1987)
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From page 77... ...
One of the leading theories proposed to explain such disorders is that of lohansson and Sojka (Iohansson & Sojka, 1991~. Briefly, this theory starts with evidence that muscle pain, inflammation, ischemia, or sustained static muscle contractions are known to lead to the release of KCL, lactic acid, arachidonic acid, bradykinin, serotinin and histamine in the affected muscle.
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From page 78... ...
These models include electrical stimulation of the target muscle, intramuscular infusion of chemical irritants into the target muscle, and the use of fatiguing muscle contractions that include the target muscle. Because the pain derived from electrical stimulation, and delayed muscle soreness from fatiguing contractions is variable and difficult to control, one of the more successful muscle pain models has involved the infusion of hypertonic saline into the muscle.
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From page 79... ...
Thirdly, acute pain at an intensity of 5 on a VAS scale of 0-10 caused autonomic changes such as sweating and blanching in these healthy subjects. Since painrelated autonomic changes such as sweating and blanching are rarely, if ever, observer} in patients with work-related overuse syndromes, yet pain scores of 5 on a ~ O point VAS scale are commonly reported by them, these patients may have an average level of pain that is an order of magnitude less than that fostered in the above acute pain model.
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From page 80... ...
Estimation of Muscle and Tendon Forces The performance of many physical activities requires humans to generate considerable muscle forces as they interact with equipment or their environment. It is these muscle forces, when combined with the external reaction forces applied to the body by its physical environment, that load the remaining soft tissues and skeleton with arrays of forces than can peak with values many times body weight.
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From page 81... ...
Given those uncertain tissue loading estimates, and given additional uncertainties in tissue material properties, attempts to predict tissue strain states are currently imprecise. Inaccuracies in these estimates of tissue strain states is one of the major impediments to identifying cause and effect relationships in tissue overuse syndromes.
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From page 82... ...
in addition, much of the work on ligaments has concerned responses to surgical insult which, although important, may have limited relevance to repetitive loading disorders. The vascular response of the ligament to injury has been measured in rabbit medial collateral knee ligaments using colored microspheres.
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From page 83... ...
Ligamentous response to repetitive loading Ligament viscoelasticity is evident under repetitive loading. In man, 1 hour of exercise has been observed to increase wrist laxity by approximately 30°/0 which then returned to baseline after 24 hours (Crisco, Chelikani, Brown, & Wolfe, ~ 9971.
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From page 84... ...
The most commonly-affected tendons are in the hand, the wrist, the forearm, and the humeral epicondyles. Conversely, sport-related tendon disorders are more common in the lower extremities, particularly in those who use running as their main form of conditioning.
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From page 85... ...
, paratenon inflammation together with intratendinous degeneration; histological findings include same as above, with loss of tendon collage, fiber disorientation, scattered vascular ingrowth, but no prominent intratendinous inflammation. Clinical signs include same as above with frequently palpable tendon nodule, swelling, and inflammatory signs.
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From page 86... ...
The resulting fiictional forces between the tendon and the paratendinous structures have been implicated in tendinitis. Wilson and Goodship, for example, have quantified the temperature at the tendon core as being 5.4 °C above the tendon surface temperature in the superficial digital flexor tendons of exercising thoroughbred races horses (Wilson & Goodship, 1994~.
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From page 87... ...
Under repetitive loading with an ~ s loading phase and 2 see recovery time, a significant (~40°/0) increase in strain occurred between the ~ 00 and 400th loading cycle, demonstrating viscoelastic creep behavior.
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From page 88... ...
, but a corresponding study of digital flexor studies failed to find similar increases- rather there was a slight decrease in tendon material properties, or collagen content (Woo, Gomez, & Amiel, 1981~. These and related studies indicate that tendon shows a modest, slow and variable adaptational response to increased levels of activity, suggesting that it is easy to increase loading demands more rapidly than the tendon remodeling process can keep up with.
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From page 89... ...
, the pathomechanics remain uncertain. A recent study has reported on the association between carpal tunnel hydrostatic pressures in viva and controlled fingertip loading in man (Rempel et al., ~ 9971.
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From page 90... ...
There is a need for studies to determine whether repetitive strain disorders might be initiated by single forceful events which trigger the injury, but might be maintained by repeated loading that prevents adequate healing/remodeling.
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From page 91... ...
More research is needed on how age and repetitive loading interact to affect the risk for an injury to become chronic in muscle, tendon or paratendinous structures, and ligament. Recent biomechanical studies of the factors leading to Diction between tendon and paratendinous structures yield opportunities for more quantitative studies of tendonitis and tenosynivitis at tendon pulley and retinaculae sites.
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From page 92... ...
(1973~. Localized muscle fatigue - Definition and Measurements.
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From page 93... ...
(1993~. Ultrasonographicallycletected changes in equine superficial digital flexor tendons during the first months of race training.
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From page 94... ...
(1997~. Compar~son of collagen-fibril populations in the superficial digital flexor tendons of exercized and nonexercised thoroughbreds.
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From page 95... ...
. The effect of experimental jaw muscle pain on postural muscle activity.
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From page 96... ...
(1990~. Pattern of muscle activity during sterotyped work and its relation to muscle pain.
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From page 97... ...
(1993~. A Closed-Loop Control System for Maintaining Constant Experimental Muscle Pain in Man.
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