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Biomechanical effects of industrial eccentric exertions.

Radwin-RG; Sesto-ME; Chourasia-AO; Block-WF
Atlanta, GA: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control and Prevention, National Institute for Occupational Safety and Health, R01-OH-007793, 2007 Dec; :1-31
The mechanical and magnetic resonance imaging (MRI) changes following eccentric and concentric exertions were studied. Prolonged mechanical and physiological changes in the upper extremity following short-term simulated power hand tool use were also investigated. Prior work has shown that changes in mechanical parameters and MRI parameters occur following submaximal eccentric activity but it was unclear whether similar changes occurred following submaximal concentric activity. This study compared mechanical response parameters and MRI relaxation parameters following submaximal concentric or eccentric exertions. This single site, randomized study investigated in-vivo changes in human upper limb dynamic mechanical properties following exposure to short term repetitive submaximal eccentric or concentric exertions. Eighteen subjects were assigned to either an eccentric or concentric group and exercised for 30 minutes at 50% of isometric forearm maximum voluntary contraction. Changes in strength, symptom intensity, MRI T2 relaxation measurements, which are indicative of edema, and dynamic mechanical parameters ( stiffness, effective mass, and damping) were ascertained prior to exercise, one hour after, and 24 hours later. Strength decreased following exercise (P < 0.01), however only the eccentric exercise group exhibited a reduction in mechanical stiffness (55%, P <0.01) and damping (31%, P < 0.05), and an increase (17%, P < 0.05) in MRI T2 relaxation time. The changes in mechanical parameters and MRI findings following repetitive submaximal eccentric activity could negatively impact the ability of the arm to react to rapid forceful loading during repetitive industrial work activities and may result in increased strain on the upper limb. Similar changes were not observed following concentric exercise. In-vivo changes in upper limb dynamic mechanical properties and MRI parameters following short term power hand tool operation were also investigated. Previous studies have found reduction in mechanical properties following short term power tool usage at long buildup times. This study advances that work by having participants operate a simulated pistol grip power hand tool and evaluating changes in mechanical properties, strength, discomfort level and MRI prior to tool operation and daily for three days after tool operation. Twenty four participants were randomly assigned to operate a simulated power hand tool for either a high reaction force of 123 N (peak torque = 8 Nm, build-up time = 250 ms) or at a low reaction force of 5 N (peak torque = 2 Nm, build-up time = 50 ms). Subjects operated the tool for 60 minutes at the rate of six times per minute. A reduction in stiffness (27%, p<0.05) was observed 24 hours after tool operation for the high force group and this change persisted (26%, p<0.05) up to 72 hours after tool operation. Similar changes were not observed for the low force group. No changes were observed in mass moment of inertia, damping, isometric strength and damping for either group (p>0.05). There was a signal intensity increase (12%, Cl = 19%, 5.06%) in the supinator muscle MR images for both groups 24 hours after tool operation but only the high force group remained elevated (10%,Cl = 13.7%, 0.06%) 72 hours after tool operation. Persistent short term changes in mechanical and MRI parameters at high force levels could indicate increased strain on the upper limb and may negatively affect ability to react during rapid forceful loading of the upper limb.
Biomechanics; Magnetic-fields; Physiology; Physiological-effects; Musculoskeletal-system; Muscles; Muscle-function; Repetitive-work; Humans; Men; Women; Musculoskeletal-system; Power-tools; Hand-tools; Statistical-analysis
Robert G. Radwin, Ph.D., Department of Biomedical Engineering, University of Wisconsin-Madison, 1550 Engineering Drive, Madison , W I 53706-1608
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National Institute for Occupational Safety and Health
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University of Wisconsin, Madison