16 research outputs found
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Upper limb mechanical changes following short duration repetitive eccentric exertions
Background. Power hand tool use is considered a risk factor for upper extremity musculoskeletal disorders. It is unclear if submaximal eccentric activity inherent to power tool use adversely affects the mechanical properties of muscle.
Methods. This study investigated in vivo changes in human upper limb dynamic mechanical properties following exposure to short-term repetitive submaximal eccentric exertions that are similar to operating an industrial power hand tool. Eighteen subjects (12 males and 6 females) were assigned to one of three exercise groups (isometric, eccentric or control) and exercised 10 min for 60 repetitions at 50% of isometric forearm supination maximum voluntary contraction. Supination strength and dynamic mechanical properties (stiffness, effective mass, and damping) of forearm rotation, modeled as a single-degree-of-freedom system during maximal exertion, were ascertained prior to exercise, immediately following exercise and 24 h later.
Findings. Strength decreased for the isometric (17%) (
P
<
0.05) and eccentric (34%) (
P
<
0.01) groups following exercise. Only the eccentric exercise group had a reduction in mechanical stiffness (53%) (
P
<
0.01) and effective mass (58%) (
P
<
0.05). The other groups had no changes in mechanical properties.
Interpretation. The change in mechanical properties 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 indicate mechanical strain on the upper limb
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Upper Limb Dynamic Responses to Impulsive Forces for Selected Assembly Workers
This study evaluated the upper limb, dynamic, mechanical response parameters for 14 male assembly workers recruited from selected jobs based on power tool use. It was hypothesized that the type of power tool operation would affect stiffness, effective mass, and damping of the upper extremity; and workers with symptoms and positive physical examination findings would have different mechanical responses than asymptomatic workers without physical examination findings. Participants included operators who regularly used torque reaction power hand tools, such as nutrunners and screwdrivers, and nontorque reaction power hand tools, such as riveters. The mechanical parameters of the upper limb were characterized from the loading response of an apparatus having known dynamic properties while worker grasps an oscillating handle in free vibration. In addition, all workers underwent a physical examination, magnetic resonance imaging, and completed a symptom survey. Workers were categorized as controls or cases based on reported forearm symptoms and physical exam findings. A total of seven workers were categorized as cases and had less average mechanical stiffness (46%, p > 0.01), damping (74%, p > 0.01), and effective mass (59%, p > 0.05) than the seven workers categorized as controls. Magnetic resonance imaging (MRI) findings suggestive of muscle edema were observed for two workers classified as cases and who regularly used torque reaction power tools. No MRI enhancement was observed in the seven subjects who did not regularly use torque reaction power tools. The ergonomic consequences of less stiffness, effective mass, and damping in symptomatic workers may include reduced capacity to react against rapidly building torque reaction forces encountered when operating power hand tools
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Anatomical and mechanical changes following repetitive eccentric exertions
Background. Submaximal eccentric exertions occur occupationally when rapidly rising tool-generated forces exceed the operator’s capacity to react against them. The purpose of this study was to investigate the effects of short duration repetitive submaximal eccentric forearm exertions at levels comparable to industrial power hand tool use on dynamic mechanical properties (stiffness, effective mass and damping) and on forearm edema.
Methods. This study investigated changes following short term repetitive submaximal eccentric exertions comparable to occupational levels. Eight male participants exercised eccentrically for 30
min at 50% of isometric maximum voluntary contraction forearm supination in a posture and loading similar to power hand tool use in the workplace. Dynamic mechanical properties (stiffness, effective mass and damping) of the upper limb were measured before, immediately following, and daily for three days after the activity. An MRI scan to assess edema was also performed for five of the participants before, on day one and day three following the activity.
Findings. Mechanical stiffness decreased 51% (
P
<
0.05) and effective mass decreased 43% (
P
=
0.052) immediately following eccentric exercise. Average isometric strength also decreased 42% immediately following exercise (
P
<
0.01) and pain persisted for two days. The recovery of static strength however was not correlated with changes in mechanical stiffness (
r
=
0.56) or effective mass (
r
=
0.30). The exercised arms had a 360% increase (
P
<
0.01) in supinator–extensor
T
2 relaxation time difference, a quantifiable measure of edema, one day after exercise while the non-exercised arms had no significant changes.
Interpretation. Changes in both
T
2 relaxation time, indicative of edema, and forearm mechanical properties, were observed following short duration submaximal repetitive exercise. If similar changes in dynamic mechanical properties of the upper extremity occur following repetitive submaximal eccentric activity in the workplace, they could negatively impact the ability of the arm to react to rapid forceful loading during repetitive industrial work activities and increase mechanical loading of the upper lim