EVALUATION OF THE FINGER AND PHALANGE FROCE DISTRIBUTIONS IN PULLING TASK
Yong-Ku Kong and Andris Freivalds
Department of Industrial and Manufacturing Engineering
The Pennsylvania State University, University Park, PA 16802
The pulling finger/phalange force distributions were investigated with various handles. Generally, oval handles required higher forces than double frustum handles in pulling task. The finger force distributions of the pulling task were similar to those of the gripping task. Middle (28%) and index (27.2%) were the strongest and little (20.8%) showed the lowest, followed by ring (23.9%). However, the phalange force distributions of the pulling task were different from the gripping task. Proximal (37.6%) produced the largest and middle (33.6%) also exerted more forces than distal (28.8%) phalanges. As the handle size increased, the forces of index and middle fingers showed increasing trends while ring and little fingers had decreasing trends. The phalange force was being moved more on distal phalanges than on proximal and middle phalanges. The understanding of force distributions may help to develop biomechanical finger models and to design hand tools for reducing related hand injuries.
INTRODUCTION
A major concern of many industries is the high percentage of injuries that occur annually. It has been reported that 6% of all compensable work injuries in the U.S. are caused by hand tools (Ayoub et al. 1975, Aghazadeh and Mital 1987, Karwowski and Salvendy 1998). In addition, about 20% of overexertion injuries have been associated with pushing and pulling tasks (Garg and Beller, 1990). Therefore, excessive used of improperly designed hand tools involving pushing or pulling tasks may cause acute and chronic injuries of the hand, wrist and low-back.
Measurements and predictions of finger/phalange force are important for developing functional biomechanical model and for designing hand tools, work equipment and manual activities (Radwin et al. 1992). Individual finger and phalange force distributions have been studied in numerous investigations involving total grasp force in cylindrical gripping tasks (An et al., 1978; Ejeskar et al., 1981; Amis, 1987; Chao et al., 1989; Lee and Rim, 1990; Radhakrishnan and Nagravindra, 1993).
An et al. (1978) studied forces imposed during cylindrical grasp actions, using a Grasp meter, which measured the force exerted by each individual phalange. They found 31.7%, 32.9%, 21.5% and 13.9% contributions for index, middle, ring and little fingers, respectively. It was found that the contributions of distal, proximal and middle phalanges to the total grasp force of the index finger were 43.8%, 33.8% and 22.3%, respectively.
Ejeskar and Ortengren (1981) found the middle finger was strongest, followed by the index, ring and little fingers in men. Women had almost equal strength in the middle and index fingers, which were significantly stronger than ring and little fingers.
Amis (1987) developed a rig for simultaneous measurement of both normal and shear forces during gripping actions. The contributions of the index, middle, ring and little fingers to the overall grip force were 30%, 30%, 22% and 18%, respectively. He presented the middle and proximal phalanges exerted similar forces, whereas the force imposed by the distal phalanges was always significantly higher than those imposed by middle and proximal phalanges.
Lee and Rim (1990) measured maximum gripping forces using pressure-sensitive films. They found that the mean finger contributions were 22.6%, 32.5%, 29.5% and 15.4% for the index, middle, ring and little fingers, respectively, which were fairly constant for the five different grip sizes. They also found that 50% of total finger force was exerted by the distal phalanges, 32% by the proximal, and 18% by the middle phalanges.
Radhakrishnan and Nagaravindra (1993) found that the mean contributions of finger forces to total grip strength, from index to little fingers, were 31%, 33%, 22%, and 14%, respectively. Also, the distal phalanges always exerted more force than the other two phalanges for all four handles.
Although they found finger and phalange contributions for the cylindrical handles, they were limited to varying the sizes of cylindrical handles, and did not vary the shapes of the handles. One of the reasons for this limitation was the lack of an appropriate measurement system. In addition, these previous studies have been conducted only during maximum gripping tasks. Thus few data are available concerning finger and phalange force distributions in other tasks such as a pulling activity.
The objective of this study was to investigate finger and phalange force distributions with various handle shapes and sizes in the pulling task and to compare with those of the gripping task.
METHODS
Force Measurement
A portable hand force measurement system was developed by overlaying force sensitive resistors (FSRs, Part #400) on a thin cotton-knit glove. Twelve FSRs were placed on the pulpy parts of each phalange to analyze force distributions.
The output signals from the FSR were sent to a custom-made voltage division circuit box, designed to provide 0V ~ ± 5V DC to the A/D converter.
Hook Handles
Ten meat hooks (Figure 1) of different sizes, shapes, and hook positions, i.e., small, medium and large-double frustum shapes, and medium and large-oval shapes with the hook inserted at the center and off-center, respectively were tested in the study.
(a)
(b)
Figure 1. Handle types [(a) double frustum handles: (b) oval handles]
Subjects
Thirty subjects (15 female and 15 male) between 18 and 45 years, with an average of 28.4 years, were recruited from the student population at the Pennsylvania State University. All subjects were screened for any hand and wrist injuries or any hand surgery, which may have limited their physical activities.
Procedure
The subjects used the force measurement glove, which was outfitted with 12 FSR sensors. One force level, 50% of subject’s maximum pulling capacity, was employed. The force was applied horizontally through a pulley system with a hanging weight.
The subject was requested to pull the weight for 3 seconds with two trials for each hook. Subjects also were allowed a 3 min. resting time between each trial. All of the trials were completely randomized in a full factorial design.
Individual finger forces were taken as the sum of three phalange forces, and the total grip forces, which was the sum of all four finger forces were also computed.
Experimental Design
The measured finger and phalange forces for the pulling task were the dependent variables for an ANOVA procedure. The independent variables were subject, handle type, finger, phalange, and hook position, respectively.
RESULTS
Statistical data analysis showed handle type, finger and phalange (all p< 0.005) were significant.
Results indicated that oval handles (116.8~115.6N) and small-double frustum handles (113.7N) required more pulling force than medium (109.8N) and large-double frustum handles (106.2N).
Table 1 shows the middle and index fingers had the largest average forces and were significantly different from the ring and little fingers. However, there was no significant difference between index and middle fingers. The average force of the little finger showed the lowest force, followed by the ring finger. It also indicates that proximal phalange always exerted more force than the other phalanges in pulling task. Proximal phalange produced the largest pulling forces while middle phalange also exerted more force than distal phalange.
Table 1. Finger and phalange forces
Finger / phalange / Pulling Force (Means: N)Index / 30.67
Middle / 31.56
Ring / 26.95
Little / 23.42
Distal / 24.30
Middle / 28.32
Proximal / 31.75
Figure 2 presents the trends of the finger forces according to the handle types. As the handle size increased, the force was concentrated more on the index and middle fingers than the others. That is, index and middle finger forces had increasing trends while ring and little finger forces showed decreasing trends.
The interaction between the handle type and the phalange is plotted in Figure 3. From this figure, although proximal phalange force was always significantly higher than middle and distal phalange forces, the force was being moved more on distal phalange than on proximal and middle phalanges, when the grip size increased.
Figure 2. The interaction effects of handle type and finger
Figure 3. The interaction effects of handle type and phalange
As expected, center hook position handles indicated the index (25.2%), middle (27.8%) and ring (25.2%) finger forces evenly distributed, although middle finger was still the strongest finger. The contributions of index and ring fingers were very close to each other. In case of off-center hook handles, however, high forces of index (29.3%) and middle (28.2%) fingers were necessary for pulling tasks. Ring and little finger contributions of the off-center handles were much less than those of the center handles (Figure 4).
Figure 4. The interaction effects of hook position and finger
DISCUSSION
The results showed that an average of 55.2% of total pulling force was exerted by the index and middle fingers. The contribution of the ring finger was the next, followed by the little finger.
These finger force patterns, which are similar to the gripping task among the fingers, may be explained by the mechanical characteristics of fingers. The middle finger is at the center of the hand and longer than the others and thus may have the mechanical advantage over the other fingers. Since the index and the ring fingers are located about the same distances from the center of the hand, they can exert similar amount of forces on the handles. The little finger is the shortest and the farthest distance from the center of the hand, therefore, it may have the mechanical disadvantage over the others.
In addition, the differences of the muscle mass between fingers may have contributed to the different finger force distribution patterns. Brand et al. (1981) reported that the mass or volume of a muscle is proportional to its total work capacity and showed that the flexor digitorum superficialis (FDS) of the middle finger has the largest mass fraction of total weight, followed by flexor digitorum profundus (FDP) of the middle finger. The FDS of the little finger has the lowest mass fraction. Ketchum et al. (1978) also reported that the FDS of the middle finger was strongest and the combined force of both the superficialis and the profundus tendons was also the strongest in the middle finger, followed by the index, ring and little finger.
Overall high contribution of the index finger in off-center hooks can be explained by hook position effect. The off-center hook which is located between subject’s index and middle fingers requires the high forces of the index (29.3%) and middle (28.2%) fingers, while the center hook, which is located between subject’s middle and ring fingers relies mainly on the middle (27.8%) and ring (25.2%) fingers.
The force imposed by the proximal phalange was significantly higher than those imposed by the others in this study. These findings are significantly different with the results obtained from previous gripping studies (An et al., 1978; Amis, 1987; Lee and Rim, 1990; Radhakrishnan and Nagravindra, 1993). They reported that the distal phalange exerted the greatest force, followed by the proximal or middle phalange. The conclusions of the pulling task, however, showed the proximal phalange was the strongest, followed by the middle phalange and distal phalange was the least. This difference can be caused by the task types. It also may explain the high frequency of trigger finger injuries of meat packers using meat hooks to pull a cow. According to Karwowski and Salvendy (1998), the localized compression in an area of the A1 pulleys (between proximal phalanges and metacarpals) is one of the main factors implicated in trigger finger injuries.
These different findings of the phalange force distributions in the pulling task, which are different from the results obtained by the maximum gripping task, may indicate need of a different handle design for effective pulling tasks.
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