AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting
of the American Society of Biomechanics
Clemson University, South Carolina
September 24-27, 1997
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| AMERICAN SOCIETY OF BIOMECHANICS
Presented at the Twenty-First Annual Meeting |
Gloves are widely used in industry and affect the mechanics of force and torque production. Among the most common industrial tasks are grip, and torque production with wrist flexion. Understanding the mechanics of gripping and wrist flexion is necessary in order to reduce the incidence of cumulative trauma disorders (CTD). The objective of this study was to investigate these tasks by creating force development profiles and by finding the effect of gloves on maximum force and the rate of force development.
Studies on the use of gloves (Hertzberg, 1955; Cochran et al., 1986) have showed that they may lead to significant reductions (15-20%) in the maximum force that can be exerted. The reasons for the drop in grip strength when gloves are worn are not clear. Cochran et al. (1986) suggested that the reasons are "interference of the glove in closing the hand around objects, the possible decreased friction between the glove and object, and the interference of the glove in tactile feedback".
The accumulated evidence that use of gloves reduces maximum grasp force raises the question of whether similar reductions occur in twisting. Two studies that looked into wrist torque production found that gloves have a positive effect on maximum torque (Riley et al., 1985; Mital et al., 1994).
A second question that has not been addressed at all is how wearing gloves affects the rate of force development. A reduced rate of force development would mean either that the response time of the workers is reduced (slower performance), or that in order to avoid the performance deterioration the workers resort to increased levels of muscle activation (higher risk of injury).
Subjects: Fifteen healthy college students (twelve male and three female) volunteered for this study. All participants were right-handed.
Procedure: The subjects were asked to perform -with their self-reported dominant hand- three maximum effort trials for every task (grip, wrist flexion, and the 'dual' task which was a combination of wrist flexion and grip) with and without glove, in random order. Subjects were asked to start from an initial state of relaxation, develop their maximum force as fast as possible and maintain it. However, reaction time was not to be minimized, the emphasis was solely on maximum force and the rate of force development. In the 'dual' task the instructions were to try to produce both maximum grip and wrist torque.
Apparatus: A leather glove without lining was used in the experiments. (a) Grasp force: The apparatus for the grasping task consisted of a handle wrapped around by tubing. The diameter of the cylindrical handle -with the tubing- was 4.5 cm. The liquid-filled was closed on one end, on the other it was connected to a pressure transducer. (b) Wrist flexion (twisting): The apparatus was the same handle that was used for the grasp force measurements, connected to a torque transducer.
Data collection and analysis: The sampling rate was 200 Hz and the data were low-pass filtered at a 20 Hz. The following variables were examined: (a) maximum value, and (b) rate of force or torque development for the following parts of the data - time curve: 0 to 30%, 0 to 50%, 0 to 90%, 30 to 70%, and 50 to 90% of maximum force or torque. For instance to calculate the rate of force development in the '0 to 50%' period the maximum force of the particular trial was multiplied by 0.5 and divided by the time it took to reach 50% of the maximum force.
The data suggest that in all tasks and conditions the time to reach the maximum strength was approximately double the time to reach 90% and three to five times the time to reach 70%.
Grip: The results of the t-tests for the grip task reveal a strong negative effect of glove wearing for all variables. Maximum force dropped by 15% while the reductions in the rate of force development ranged from 24% to 38%. The p-values show a statistical significance (a=0.05) except the rate of grip force development in the intervals 0 to 30%, 0 to 50% and 0 to 70% of the peak value.
Wrist flexion: The main focus of that task was torque production but since grip pressure was also produced in the process, both variables were examined in an effort to see how their combination relates to performance.
(a) Torque: None of the results was statistically significant. The data show there was no change in performance when gloves were worn.
(b) Grip pressure during wrist flexion: Interestingly these results are very similar to those obtained in the grip task. Once again, the effect of wearing gloves is negative, both for maximal grip force and many aspects of the rate of its development (particularly in the late stages of development).
'Dual' task (wrist torque and grip combination): Figure 1 shows a sample trial of the 'dual' task. The effect of gloves was negative and concentrated primarily in the grip components of the task. In contrast, wrist flexion torque results in the 'dual' task showed no significant change.
Figure 1: Grip force (thick line) and torque (thin line) in sample 'dual' task trial.
The maximum grip force results, that showed a decrease in the gloved condition, are in agreement with the published literature (Cochran et al., 1986; Sudhakar et al., 1988). The highest rate of force development appeared in 30 to 70% period. It was interesting to record that wearing gloves leads to significant deterioration in the rate of grip force development. This deterioration could contribute to loss of performance and/or increased risk of injury. The drop in the rate seems to be concentrated in the late phases of exertion. In essence the only period unaffected by gloves is the first one (0 to 30%) and after that, the rate of grip force development starts to suffer. It is possible as compression forces on the glove increase, more and more of the work done by the hand is stored as elastic energy in the glove material and its creases.
Torque production was unaffected by gloves. As far as peak torque is concerned, this contradicts the findings of Riley et al. (1985) and Mital et al. (1994) who recorded improvements in peak magnitude as a result of wearing gloves. However, the differences can be attributed to slippery handles used by Riley et al., or to calibration differences in Mital et al. Grip force during torque production followed a very similar pattern to grip force in the dedicated grip task. This seems to imply that, although grip is a prerequisite for torque, the two variables, torque production and grip, are controlled independently by the central nervous system, at least in the task under examination in this study. For instance, even in the 'dual' task where equal emphasis was placed on the two tasks, torque development was much faster than grip force development especially in the periods up to 70% of maximum.
Cochran, D.J., et al. Proc. of Human Factors Soc., 30th meeting, 1986.
Hertzberg, T. Annals of NY Acad. of Sc., 63, 621-23, 1955.
Mital A., et al. Ergon., 37, 333-43, 1994.
Riley, M.W., et al. Ergon., 28, 441-47, 1985.
Sudhakar, L.R., et al. Proc. of the Human Factors Soc., 32nd meeting, 1988.
