Evaluation of handle design characteristics in a maximum screwdriving torque task.

The purpose of this study was to evaluate the effects of screwdriver handle shape, surface material and workpiece orientation on torque performance, finger force distribution and muscle activity in a maximum screwdriving torque task. Twelve male subjects performed maximum screw-tightening exertions using screwdriver handles with three longitudinal shapes (circular, hexagonal and triangular), four lateral shapes (cylindrical, double frustum, cone and reversed double frustum) and two surfaces (rubber and plastic). The average finger force contributions to the total hand force were 28.1%, 39.3%, 26.5% and 6.2%, in order from index to little fingers; the average phalangeal segment force contributions were 47.3%, 14.0%, 20.5% and 18.1% for distal, middle, proximal and metacarpal phalanges, respectively. The plastic surface handles were associated with 15% less torque output (4.86 Nm) than the rubber coated handles (5.73 Nm). In general, the vertical workpiece orientation was associated with higher torque output (5.9 Nm) than the horizontal orientation (4.69 Nm). Analysis of handle shapes indicates that screwdrivers designed with a circular or hexagonal cross-sectional shape result in greater torque outputs (5.49 Nm, 5.57 Nm), with less total finger force (95 N, 105 N). In terms of lateral shape, reversed double frustum handles were associated with less torque output (5.23 Nm) than the double frustum (5.44 Nm) and cone (5.37 Nm) handles. Screwdriver handles designed with combinations of circular or hexagonal cross-sectional shapes with double frustum and cone lateral shapes were optimal in this study.

Electromyographic and discomfort analysis of confined-space shipyard welding processes.

This study examined muscle fatigue and discomfort in a confined-space welding operation at a shipyard. Surface electromyography (SEMG) was recorded from seven upper extremity and torso muscles of welders welding in a mock-up of the work environment. Following spectral transform of the SEMG data the percentage of the total signal power in the 10-30 Hz frequency band was compared over time during welding. For the conventional stick electrode welding process (SMAW) several muscles exhibited an increase in the percent of the total signal power in the low-frequency band. Fewer muscles exhibited this fatigue-related spectral density shift with a wire welding process (FCAW) the shipyard has considered adopting. This finding suggests that localized muscle fatigue may be reduced by a change to the wire welding process. Subjectively reported discomfort was generally low for both processes, but confirmed the finding that discomfort in the low back and shoulder regions is experienced in this welding operation.

Precision grip force control of older and younger adults, revisited.

This study revisited the hypothesis that older adults lose some ability to efficiently control precision grip force. A previous study demonstrated such a decrement in older adults' performance in a vertical lift and support maneuver. This study employed a similar paradigm in which dynamic forces were applied with a simulated hand tool while measuring grip force and force applied with the tool. Measures of grip force control reflected subjects' modulation of grip force in parallel with force transmitted with the tool and their scaling of the ratio of grip to applied force. Nine older (> 65 years) and 9 younger (< 65 years) subjects' grip force control measures were compared with emphasis on recruiting active older individuals for whom upper extremity usage was high in their daily life. No statistically significant age effects were found in either force control measure, suggesting a smaller age-related decrement than reported in a previous study.

Effect of carpal tunnel syndrome on grip force coordination on hand tools.

This study investigated coordination of the grip force on and force applied with a hand tool using a precision pinch grip. A simulated hand tool was developed to measure grip force exerted on the tool as a function of the force transmitted from the tool to an external object in a dynamic force matching paradigm. Grip force coordination measures reflected subjects' abilities to modulate grip force in parallel with the tool application force and their abilities to minimize excessive grip force. These measures were calculated for seven subjects with a diagnosis of carpal tunnel syndrome (CTS) and seven age- and gender-matched controls. The absolute magnitude of excessive grip force (safety margin) was unreliable because of the high intrasubject variability in coefficient of friction measurements. Linear regression equations predicting coefficient of friction from pinch force magnitude had low r2 coefficients of determination and were generally not statistically significant (p > 0.05). Relative comparisons of grip force control showed that individuals with CTS exhibited a statistically significant (p < 0.05) increase in ratio of grip force to application force (54% higher than controls) and a significant (p < 0.05) decrease in modulation of pinch force with application force (12% lower than controls). These results suggest that individuals with CTS lose some ability to coordinate efficiently grip force on hand tools and exert higher grip forces on tools, at equivalent application forces, than controls. This is believed to be a result of tactile sensibility deficits associated with CTS. As a result, workers with CTS may be at increased risk of accelerating the progression of their musculoskeletal disorder.

Repeatability of magnitude production in isometric, hand-grip force estimation: a working-memory approach.

Magnitude production has been used in ergonomics research to quantify hand-grip and finger-pinch force exertions. The repeatability of this psychophysical method may be affected by a memory component if the subject must recall multiple, previously performed force exertions. This investigation tested the hypothesis that the number of different to-be-remembered hand-grip force exertions would affect the repeatability of force productions obtained via magnitude production for 12 male students. Four levels of force were examined (20%, 35%, 50%, and 65% of subject's individual maximum) at low and high memory loads (two versus four forces to be remembered concurrently). Amount of force had a significant effect on the variance among a sample of five observations; however, memory load showed no effect on the variance measure. Thus, it is concluded that four forces do not pose a significantly greater memory load than two forces for perceived force in hand-grip magnitude productions.