The influence of wrist position on individual finger forces during forceful grip

The Influence of Wrist Position on Individual Finger Forces During Forceful Grip Zong-Ming Li, PhD, North Canton, OH Nine healthy subjects sustained maximum grip of an instrumented handle while voluntarily moving the wrist joint within their available range of motion of the wrist in a continuous and random manner. Individual finger forces and wrist angular positions in flexion/extension and radial/ulnar deviation were recorded simultaneously. Wrist position had a significant effect on individual finger force and total force production. Peak finger forces were produced at 20 ° of wrist extension and 5 ° of ulnar deviation. At this position, a mean total grip force of 114.9 (+- 12.8) N was produced with force-sharing percentages of 32.2% (+-3.8%), 32.6% (_+4.3%), 23.5% (+-4.5%), and 11.7% (_+4.9%) among the index, middle, ring, and small finger, respectively. As the wrist was moved farther away from this position, the forces produced by individual fingers decreased incrementally; however, the decreases in individual finger forces were not proportional, leading to a dependence of finger force-sharing patterns on wrist position. (J Hand Surg 2002;27A:886-896. Copyright © 2002 by the American Society for Surgery of the Hand.) Key words: Wrist, motion, finger, force, hand.

People use their hands and fingers extensively during daily activities. The execution of m a n y manual tasks depends on well-coordinated finger forces, specifically the magnitude of individual forces and distribution of force among the fingers. A number of factors play a role in finger force production. These factors include variations in muscle length, r muscle and tendon compliance, 2 joint conditions, 3 neurologic problems, 4 pinch type, 5

From the Division of Physical Therapy, Walsh University, North Canton, OH. Received for publication November 9, 2001; accepted in revised form May 7, 2002. Although the author or authors have not received or will not receive benefits for personal or professional use from a commercial party related directly or indirectly to the subject of this article, benefitshave been or will be received but are directed solely to a research fund, foundation, educational institution, or other non-profit organization with which one or more of the authors are associated. Reprint requests: Zong-Ming Li, PhD, Musculoskeletal Research Center, Departmentof Orthopaedic Surgery, Universityof Pittsburgh, E1641 Biomedical Science Tower, 210 Lothrop St, Pittsburgh, PA 15213. Copyright © 2002 by the AmericanSocietyfor Surgery of the Hand 0363-5023/02/27A05-0019535.00/0 doi: l 0.1053/jhsu.2002.35078

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pinch width, s and the relevant body/joint configuration. 6 The hand is an end-effector of the multilink kinematic chain of the human body, and therefore a positional change in any of the proximal segments may have an effect on hand/finger force production. It has been reported that hand grip strength is dependent on body posture, that is, standing or sitting, and angular positions of the shoulder, 7 the elbow, 7-9 the forearm, m the wrist joint, 7"~1-~6 and the metacarpophalangeal and interphalangeal joints. ~7 A m o n g the aforementioned factors, wrist position has been shown to be one of the most important determinants for grip and pinch strength capabilities. Lamoreaux and Hoffer ~2 found a substantial loss of total grip strength with the wrist in maximal ulnar and maximal radial deviation as compared with the anatomic neutral position. Halpern and Fernandez 7 reported that at deviated wrist postures, pinch strength was reduced up to 33%. O'Driscoll et al J5 found m a x i m u m grip strength output at a self-selected optimal wrist position of 35 ° of extension and 7 ° of ulnar deviation. With the wrist in 15 ° of extension or 0 ° of radial-ulnar deviation, grip strength was reduced two thirds to three fourths of the

The Journal of Hand Surgery / Vol. 27A No. 5 September 2002

strength found at the self-selected position. Pryce ~3 measured power grip strength in 9 wrist positions of flexion/extension and ulnar deviation and found that the strongest grip strength occurred at 15° of wrist extension and 0 ° deviation whereas changes in directions in either flexion/extension or radial/ulnar deviation resulted in a marked decrease in grip forces. Hazelton et al l~ found that the greatest total finger flexion force was achieved in wrist ulnar deviation, followed in order by the wrist positions of anatomic neutral, radial deviation, extension, and flexion. The finger force distribution has been reported to be relatively constant regardless of hand size, IS hand dominance, ~8 magnitude of force, t9-2~ and wrist position.~'18'22 Some studies show the relative contribution of individual finger forces can be modified by internal volitional alteration 23 or external mechanical constraints, in particular, the thumb position. 24 Previous studies have focused on the influence of wrist postural changes on force output of a single finger, for example, pinch, or the total force output of multiple fingers, for example, handgrip.~5 Few studies ~ examined how the force production capability of individual fingers is influenced by wrist position. The execution of many tasks that involve multiple fingers may depend not only on total force output, but also on the distribution of the total force among individual fingers. In addition, previous attempts to evaluate the effect of wrist position on grip strength have concentrated on limited static wrist positions. TM 13.25 Many activities of daily living, industrial work, and recreational tasks require forceful gripping while the wrist travels through an arc of motion. To date it remains unclear as to how wrist position affects force production of individual fingers in functional grip tasks involving mutiple fingers that require forceful gripping while the wrist travels through an arc of motion. The purpose of this study was to determine the influence of wrist position on individual finger forces and force sharing among fingers during maximum grip of an instrumented handle while voluntarily moving the wrist joint continuously. It was hypothesized that varying wrist position would lead to changes in individual finger forces, total force production, and the pattern of force sharing among fingers.

Methods Subjects Nine male, right-handed, college students (age, 26.5 _+ 5.9 y; height, 1.793 _ 0.105 m; body mass, 77.5 _+ 11.3 kg) volunteered to participate in the

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study. The subjects had no known trauma or neuropathy in the hand or upper extremity. Each subject signed a consent form that was approved by the Institutional Review Board. Apparatus Four force sensors (M208A03; Piezotronic, Depew, NY) and 1 biaxial electrogoniometer (XM110, Biometrics Ltd, Cwmfelinfach, UK) were used to record finger force production in compression and wrist position, respectively. Each force sensor has a force range of 2,224 N and a resolution of 0.09 N. Cotton covers were attached to the surface of the plates to increase friction and reduce the effect of temperature. ~9'24 A handle made of clay (Laguna Clay Company, Laguna, CA) functioned as a grasping object (Fig. 1A). The clay material allowed the handle to be molded to a desirable size and shape and became rigid after it dried. The force sensors were mounted to the distal side of the clay handle by mounting tape. The handle together with the sensors weighed 2.92 N. When the handle was grasped within the hand the proximal side of the handle was buttressed by the proximal portion of the palm, while the distal side of the handle (ie, sensor surfaces) was in contact with finger palm pads. The whole hand functioned as a rigid body that articulated about the wrist joint. All 4 fingers were placed in a closed kinematic chain, which allowed stabilization of the metacarpophalangeal and interphalangeal joints of the fingers. The distance between the centers of neighboring sensors measured 25 mm in the radioulnar direction. The height of each sensor was adjusted so that the handle was grasped comfortably within the hand with the metacarpophalangeal joints at approximately 20 ° of flexion, the proximal interphalangeal joints at approximately 50 ° flexion, and the distal interphalangeal joints at approximately l0 ° of flexion. This configuration of finger joints formed a nearly perpendicular orientation between the sensor surfaces and the direction of finger forces. To ensure the perpendicular contact and accommodate the length difference of individual fingers, the height of each sensor surface was carefully adjusted during trial runs. The electrogoniometer was used to monitor wrist joint position in the directions of flexion/extension motion (FEM) and radial/ulnar deviation (RUD). The force signals from the sensors were conditioned by 4 channels of conditioners (M482M66; Piezotronic). Both force and goniometry signals were

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Zong-Ming Li / Wrist Position Effect on Finger Forces

A

Figure 1. Experimental setup. (A) Palmar view of the instrumented handle within a hand. (B) Lateral view of body posture and electrogoniometer attachment.

collected by a 16-bit analog-digital converter (PCI6031E, National Instruments, Austin, TX) and a personal computer. A LabVIEW (National Instruments) program was designed for data acquisition and processing. Experimental Procedures Each subject was seated in a chair with their feet placed comfortably on the floor. The chair height was adjusted so that the hips, knees, and ankles were at approximately 90 ° . Seat cushions were used to adjust the back rest so that the shoulder was in a relaxed position with little abduction or flexion, and the elbow was positioned at 90 ° of flexion. The electrogoniometer was taped over the dorsal side of the right wrist to measure wrist FEM and RUD. The 2 tails of the goniometer were aligned and attached along the third metacarpal on the distal end and along the midline of the posterior aspect of the forearm on the proximal end. The center of the goniometer was aligned approximately at the level of the head of the capitate, which was identified by palpating the interval between the radius and base of the third metacarpal. The goniometer readings were zeroed with the forearm and hand resting on a flat table surface and the wrist in a functional neutral position, that is, the long axis of the third metacarpal was aligned with the midline of the posterior aspect of the forearm. 15 With the goniometer attached, the ulnar surface of

the forearm was positioned on the armrest of the chair with the forearm at 0 ° pronation/supination. The forearm was stabilized with a Velcro strap (McMaster-Carr, Cleveland, OH). The end of the armrest was placed proximal to the head of the ulna to not impede free motion of the wrist. The instrumented handle was placed within the right hand with the index, middle, ring, and small fingers comfortably placed on individual sensor surfaces (Fig. 1). The main task was to grasp the instrumented handle as hard as possible in varying wrist positions while maintaining maximum voluntary contraction. The subject was instructed to select an arbitrary starting position of the wrist and then perform a maximum voluntary contraction at that position. When the maximum voluntary contraction was attained, the subject started to slowly move (<20°/s) the wrist to different positions while maintaining a maximal grasping effort, even though the subject might experience difficulty in producing force in some extreme positions. The experimenter started to collect data when the subject initiated wrist movement. The data collection lasted for 5 seconds and ended with a beep sound, signaling the stopping of the current trial. The data sampling frequency was 100 Hz. The stretching capability of the goniometer was not enough for full range of motion (ROM) of the wrist, in particular in the direction of FEM. To overcome this limitation the total movement arc of the

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wrist was broken into extension and flexion halves, and the goniometer was attached to facilitate flexion or extension separately. The sequence of 2 motion arcs was randomized across subjects. During the extension arc session the subject was instructed to move the wrist in the region of extension combined with arbitrary RUD. In contrast, during the flexion arc session, the wrist range of motion was in the flexion region combined with arbitrary RUD. The subject was encouraged to explore all unconstrained wrist positions in the directions of FEM and RUD. The subject was instructed not to rotate the forearm. After each trial a cumulative positional coverage of the wrist (FEM angle vs RUD angle graph) for the subject was shown on the monitor, guiding the subject to explore data-lacking areas so that data for a relatively even distribution of wrist positions within the whole ROM would be obtained. A total of 20 trials were performed: 10 in the extension session and 10 in the flexion session. Four channels of force data and 2 channels of goniometer data were collected simultaneously; therefore, the wrist FEM and RUD angles and the corresponding individual finger forces at each specific position were recorded. The data of each channel for 20 trials were concatenated to form one single data file for each subject. A 2-minute break was given between trials to avoid intertrial fatigue. The whole experiment lasted about 70 minutes. Data Processing Each subject produced finger force data at randomly selected wrist positions. To obtain finger forces at the same positions for different subjects

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interpolation procedures were performed for the data of each subject. To reduce data for interpretation based on ROM, force data for selected FEM and RUD angles were calculated (Fig. 2). First, meshgrid data from flexion ( - 8 0 °) to extension (80 °) and from ulnar deviation ( - 4 0 °) to radial deviation (30 °) with an increment of 1° in each direction were generated. Second, 2-dimensional cubic interpolation was used to obtain estimated forces of each individual finger at the positions corresponding to the meshgrid data. Therefore force data maps are available for all subjects at identical wrist positions. Force sharing of a finger at any position was defined as the percentage of force generated by this finger as compared with the total force produced at this position. Note that the sum of the 4 force-sharing percentages is equal to 100%. Statistical Analysis The force data interpolated at the 18 representative wrist positions were compared statistically (Fig. 2). The FEM and RUD angles were independent variables; individual finger forces, total force, and forcesharing percentages were dependent variables. Oneand 2-way repeated-measure analysis of variance (ANOVA) were used for statistical analyses with a significant level of o~ < .05, followed by post hoc Tukey's Honest Significant Difference testing.

Results R O M and Raw Data

Figure 3 shows an example of the scattering data of wrist position. The mean ROMs of wrist in FEM

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Zong-Ming Li / Wrist Position Effect on Finger Forces Table 1. Wrist ROM (in degrees) in FEM and RUD FEM

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23.5 5.8 17.2 33.9

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Data from 9 subjects.

and RUD are shown in Table 1. The total mean ROM of wrist flexion/extension was 144 ° , 75 ° in flexion and 69 ° in extension. The total ROM of radial/ulnar deviation was 75 °, 54 ° in ulnar deviation and 24 ° in radial deviation. The arc in ulnar deviation exceeded radial deviation by 31°. In addition, the angle-angle mapping graph was not symmetric with respect to the anatomically defined axes of FEM and RUD (Fig. 3). The overall map shows a pattern of an oblique ellipse, and not a rectangular shape. Principal component analysis 26 revealed that, on average, the major principal axis was oriented 26 ° ± 6 ° (mean _+ SD) with respect to FEM direction. Individual Finger Forces and Total Force Figure 4 is an example of an interpolated surface plot of index finger force with respect to wrist FEM and RUD. The landscape in the graph shows a strong dependence of finger force production on the wrist joint position. The humps are located in the area of

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wrist extension. The altitude declines as the wrist deviates from this area. The landscapes of the other finger forces and the total force show similar patterns. The dependence of individual finger forces on 18 representative wrist positions is shown in Figure 5. The peak individual forces and peak total force occurred at the wrist position of 20 ° of extension and 5 ° of ulnar deviation (not shown in Fig. 5). At this position the index, middle, ring, and small fingers produced mean forces of 37 (_+9.2), 38 (+ 10.7), 27 (±8.8), and 13 (+7.7) N, respectively, with a mean total force of 115 (± 12.8) N. As the wrist deviated farther and farther away from this position, the forces generated by individual fingers and total force decreased incrementally. In general, force generated in extension was higher than force generated in flexion. Moreover finger forces varied more with wrist flexion than extension. See also Figure 4 for an example. Two-way ANOVA (FEM X RUD) performed on the selected 18 wrist positions showed that individual finger forces and total force production was dependent on FEM (p < .001), but the effect of RUD was not significant except for the middle finger (p > . 10, except p < .01 for the middle finger). In addition, no significant interactions between FEM and RUD were found for individual finger force or total force (p > .62). Post hoc analysis revealed that wrist flexion resulted in a significant decrease in total force production as compared with extended positions; the total force at any flexed position was less than any extended position (p < .05). The total forces were not different from each other at wrist positions of 0 °, 20 ° , and 40 ° in the direction of flexion/extension with the same amount of RUD (p > .24). Among the selected positions, the highest individual finger forces and total force are produced at 20 ° extension with neutral RUD, that is position coordinate (20, 0) (Fig. 5). For example, the total force was 104 N at

The Journal of Hand Surgery / Vol. 27A No. 5 September 2002

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position (20, 0). When the wrist assumed position ( - 6 0 , - 3 0 ) , the total force was 39 N, only 37.9% of the total force produced at position (20, 0). The total force produced at wrist position (20, 0) was 90.6% of the peak total force at wrist position (20, - 5 ) . Force Sharing The force-sharing percentages among the index, middle, ring, and small fingers at peak force position (20, - 5 ) were 32.2% (+3.8%), 32.6% (_+4.3%), 23.5% (+4.5%), and 11.7% (_+4.9%), respectively. The effects of FEM and RUD on force sharing of individual fingers were different on different fingers (Fig. 6). Among the selected positions the index finger force sharing was significantly affected by RUD (p < .001) but not by FEM (p = .47). When

the wrist deviated from (0, 0) to a radial position at

(0, 15) the force sharing of the index finger showed an increase from 31.0% to 42.6%, an absolute increase of 11.6%. Both FEM and RUD significantly affected middle finger sharing (p < .01). Flexion/ extension motion significantly affected the ring finger force sharing (p < .01 ), but the effect of RUD on the ring finger was not significant. Force sharing of the small finger was not significantly affected by either FEM or RUD (p > .71 ). Again, there were no FEM × RUD interaction effects for force sharing of any finger (p > .78). Discussion

This study investigated individual finger force production during gripping tasks with varying wrist

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positions in real time. The experimental design in this study holds several advantages over previous studies. First, the tasks used in the current study simulated the dynamic nature of wrist motion during gripping tasks as opposed to previous studies with a few static wrist positions. In addition, dynamic grip strength may better represent the true functional capacity of a hand, in particular for clinical assessment of an impaired hand. Clinically, grip strength has been traditionally measured with the wrist in standard static positions; however, static grip strength may overestimate the functional dynamic grip strength by as high as 30% in some wrist positions. 16 Second, the instrumented handle with multiple force sensors enabled the measurement of force production of individual fingers, as opposed to the commonly used grip dynamometer, which provides an overall index of grip strength of all digits. Although LaStayo and Hartzel ]6 studied dynamic total grip strength in conjunction with wrist angular motion monitoring, and Hazelton et al 1~ investigated individual finger pulling strength at various discrete wrist positions, this study examined how individual finger forces change during real-time movement of the wrist. Several limitations, however, should be noted for this study. First, only healthy, right-handed, male, college subjects were tested, which may restrict the

generalization of the findings to other populations. Second, the custom handle, although being advantageous for the isolation of the wrist joint, may also limit the extension of the current results to other objects with different size or shape. Third, the rotation of the forearm was not monitored during wrist FEM and RUD. Finally, the division of wrist motion space into 2 halves during wrist movement might have hindered the natural wrist movement in a continuous manner. Wrist Kinematics

Although this study was not meant to investigate wrist kinematics, wrist ROM and the associated principal axes of motion deserve attention and discussion. The ideal range of wrist motion required to perform activities of daily living has been investigated in several studies. 27'2s The available ROM in the current study (mean of 144 ° in FE, 75 ° in RUD) was higher than that of Ryu et al, 27 in particular in the direction of ulnar deviation (a difference of 13°). The discrepancy may be attributable to task differences. It has been reported that powerful grip is associated with an increase in ulnar deviation, 29 which helps explain the higher ulnar ROM in the current study with maximum voluntary grips. In general, the ROM in FEM is about twice as

The Journal of Hand Surgery / Vol. 27A No. 5 September 2002 893 large as the ROM in RUD. Of the components making up the ROM in FEM, the flexion arc was comparable with the extension arc. In contrast, the movement arc in radial deviation was about half of the arc in ulnar deviation. The uneven distribution in radial and ulnar deviation is mainly related to the buttressing effect of the radial styloid process because the carpus becomes mechanically blocked in radial deviation. 3° Another kinematic feature of the wrist joint observed in this study was the elliptic shape with oblique orientations of major principal axes, which implies that ROM in one direction (eg, RUD) is dependent on the wrist position in the other direction (eg, FEM). The wrist could accomplish 60 ° extension with 0 deviation, or 40 ° ulnar deviation with 0 flexion, but a combination of 40 ° of extension and 30 ° of ulnar deviation was not attainable. This motion interaction in FEM and RUD directions has also been recently reported by Marshall et al 3~ who found that radial deviation capacity at wrist flexion of 45 ° was 30% lower than radial deviation capacity at wrist extension of 45 ° . The oblique orientation of major principal axes indicates that the preferred wrist motion does not follow the anatomically defined axes of rotation for FEM in the sagittal plane or RUD in the frontal plane. A plane oriented 26 ° with respect to the sagittal plane is more congruent for wrist motion. This finding agrees with other studies. TMI6'28"32 Palmer et al 2s investigated activities of daily living and professional tasks and found that consistent flexion and ulnar deviation were shown for certain activities, for example, surgical operations; but no group of tasks showed predominantly flexion and radial deviation of the wrist. In addition Palmer et al 2s found that the centroid of wrist motion was ulnar, which helps explain the fact that in this study maximum grip strength was generated with the wrist in ulnar deviation, j3"~6 The amount of obliquity of the current in vivo measurement is close to the corresponding incline angle of 23 ° found by Ishikawa et a132 in cadaver studies. Functional Neutral Position When a person is asked to perform grip tasks without specifying the wrist joint angle, a reproducible wrist joint angle is selected for optimal performance, such as comfort or maximum force production. The optimal wrist position for maximum force production in the current study was about 20 ° of extension and 5 ° of ulnar deviation, which was con-

sistent with previous studies. 33 Some studies, however, have reported larger extension positions such as 30 ° of wrist extension.15'l~' The discrepancy may be attributable to the size of the object being grasped. 15'~7'2~'34 For example, O'Driscoll et al ~5 reported that the degree of self-selected optimal wrist extension is inversely and linearly related to the setting size on the Jamar dynamometer (Asimov Engineering Co, Santa Monica, CA). A difference of 11 ° in extension was found between the largest and smallest dynamometer setting sizes, with the smallest extension position associated with the largest dynamometer setting. This relationship is reasonable considering the optimal functioning of the musculotendinous units under length-tension relationship. 2 When a smaller object is being grasped, fingers are more curved with increased flexion angles at the metacarpophalangeal and interphalangeal joints, and a more extended wrist position is needed to preserve the optimal musculotendinous length of the extrinsic flexors, and vice visa. The gripping posture used in the current study can be considered to be similar to the hand posture associated with large dynamometer gripping used by O'Driscoll et al. ~5 In addition, Blackwell et a134 found that grip span affected maximal grip force, and that middle grip sizes allowed for greater absolute forces than the small or large size. The finding in this study that the optimal position was at slight ulnar deviation agrees closely with a number of previous studies. 11.15.16,25.29 For example, O'Driscoll et al ~5 reported that the optimal radialunlar deviation position is relatively robust, regardless of grasping size. 15 Friedman et al 2'~ found that a slight increase toward ulnar deviation is accompanied with increased grip force. Decreased Force in Flexion and Extension In this study fingers produced progressively smaller forces as the wrist was deviated away from the position of 20 ° of extension and 5 ° of ulnar deviation. The effect of wrist deviation in FEM was more remarkable than in RUD. In the direction of flexion/extension, the deviation toward flexion affected finger forces more strikingly than extension deviation. There are a few possible explanations for force decreases associated with deviated wrist position away from the peak position. The first explanation is related to the well-known length-tension relationship of the active contractile elements within a muscle. 2 It may be that when the wrist is positioned at 20 ° of

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Zong-Ming Li / Wrist Position Effect on Finger Forces

extension and 5 ° of ulnar deviation the muscular compartments for individual fingers are at optimal length for maximum active force production. As the wrist joint moves in any direction combining FEM and RUD, the associated muscular compartment for each finger becomes less optimal, leading to an impairment of flexion force production. Furthermore, a large muscle length change is more likely to occur with musculotendinous units that cross more than 1 joint, such as extrinsic finger flexors (digitorum superficialis, flexor digitorum profundus), which are primarily responsible for powerful finger force production. When an external force is applied at a distal phalanx during gripping the profundus is the only flexor that balances the external extension torque at the distal interphalangeal joint. 35 The torque balance at the proximal interphalangeal and metacarpophalangeal joints is progressively assisted by the flexor digitorium superficialis and intrinsic muscles. The flexor digitorum profundus originates outside the hand, inserts into the distal phalanx, and crosses the wrist, the metacarpophalangeal, proximal interphalangeal, and distal interphalangeal joints. Therefore, decreased grip force at a deviated wrist position may be primarily caused by the weakened force production capability of the flexor digitorum profundus. The factor related to the length-tension relationship describes an isolated agonist, however, finger flexion force production, in particular with wrist flexion, is a result of interaction among flexor and extensor muscles. Therefore, a second explanation for decreased finger force production may be related to tension development in the antagonists, in particular the passive stretching of extensors owing to wrist flexion. With the wrist in a flexed position, the primary finger extensor (ie, extensor digitorum communis) may be passively stretched and produce tension in the direction of finger extension, causing a reduction in active finger flexion force. 14 This opposing (ie, extension) force can be considerably large at an excessive flexed wrist position. The decrease in active contraction of flexors plus the accumulation of passive stretching of extensors helps explain the severely weakened grip force observed at positions of large wrist flexion. On the other hand, when the wrist deviates toward extension, passive stretching of the flexors facilitates flexion force, compensating for the attenuating effect of sacromere lengthening of flexors. This may be why a less severe decrease in total flexion force was observed owing to wrist extension. Third, friction force within the carpal tunnel may also contribute to the decreased finger flexion force

seen with wrist deviation. 36 As the wrist deviates, extrinsic finger flexors are bent against the structures within the carpal tunnel and the wall of the tunnel. As a result, a fraction of tendon force is lost owing to friction. In general, the effect of wrist RUD on the magnitude of finger force production is less remarkable as compared with wrist FEM. This may be owing to smaller changes in musculotendinous excursion of both flexors and extensors associated with RUD. It has been reported that there is less variation in the moment arm of wrist flexors and extensors with RUD as compared with FEM. 2 Force Sharing

Force-sharing percentages among the index, middle, ring, and small fingers at the position where peak total and peak individual finger forces were produced were 32.2%, 32.6%, 23.5%, and 11.7%, respectively. This percentage distribution is fairly consistent with previous findings. ~1'19-24 Overall, previous studies have shown force-sharing patterns with the index and middle fingers contributing the most (-30%), the ring finger contributing less (-25%), and the small finger contributing the least (-15%). Hazelton et al I~ studied how wrist position affected individual finger force production. The procedure used by Hazelton et al ~ involved a digital dynamometer, arm and forearm restraints, and a pulley assembly consisting of wire mesh straps attached to each digit. The wrist was placed in 5 representative positions: anatomic neutral, two thirds of maximal flexion, two thirds of maximal extension, 21 ° of ulnar deviation, and 14° of radial deviation. Hazelton et al ~1 concluded that force sharing among individual fingers was fairly constant at the selected wrist positions and no remarkable trends were observed. In contrast to the findings of Hazelton et al~ ~ this study showed that the force-sharing pattern was significantly affected by changes in wrist position. Deviation of the wrist from the optimum position resulted in deterioration of force production of all fingers; however, the decreases observed in individual finger forces were not proportional. Furthermore, it appears that a change in force sharing of a finger was compensated by one of the neighboring fingers. In particular, wrist radial deviation led to an increase in index finger force sharing and a compensatory decrease in middle finger force sharing. Likewise wrist flexion led to an increase of ring finger force sharing and a compensatory decrease in middle finger force sharing. The differential effect on individ-

The Journal of Hand Surgery / Vol. 27A No. 5 September 2002

ual fingers may be owing to dissimilar excursions of individual extrinsic flexor tendons associated with wrist deviation in a complex manner. First, the flexor tendons (ie, flexor digitorum profundus and flexor digitorum superficialis) of individual fingers cross the wrist joint at different locations, exhibiting different moment arms with respect to the axis of FEM or R U D . 37 Second, the moment arms of individual finger flexor tendons may change with wrist deviation. In a study of how wrist joint torque generation capability is affected by wrist joint angle, Loren et al 2 reported that moment arms of the wrist muscles vary substantially during flexion/extension and radial/ulnar deviation. Implications

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It is evident that moderate extension together with slight ulnar deviation is the most important functional position of the wrist joint. Extreme wrist position is not only detrimental to finger force production, but is also linked with high risk for cumulative trauma disorders of the wrist during repetitive manual exertion. 3~ It is important in equipment design and orientation to accommodate wrist positions that afford strong grip forces, especially for tasks that demand a strenuous exertion or repetitive operation. Clinically, wrist fusion is a common surgery to relieve pain caused by rheumatoid arthritis or after traumatic injury. Ls'~7 Wrist fusion at an extended wrist position (20 ° to 30 °) and slight ulnar deviation may be advantageous for a wrist fusion in terms of strength and aesthetics. In recreational and athletic activities excessive wrist deviation has been reported to be associated with an increased incidence of injury for golfers 29 and gymnasts. 3~'3~ Lastly, the grip dynamometer has been widely used for clinical purposes to assess total grip force at a self-selected gripping posture. An advanced apparatus to measure individual finger strength over a dynamic R O M may provide further beneficial clinical information about hand function. The author thanks David Wozniak, Steve Trocchio, Tim Porco, and Mike Flaherty for performing a pilot experiment and helping collect data, and VladimirZatsiorsky, Jamie Pfaeffle, and Steve Abramowitch for helpful comments.

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