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Department of Exercise Science, The University of Iowa, Iowa City, Iowa 52242
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ABSTRACT |
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 Abstract
 Introduction
Methods
Results
Discussion
References
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Werremeyer, Mary M. and Kelly J. Cole. Wrist action affects precision grip force. J. Neurophysiol. 78: 271-280, 1997. When moving objects with a precision grip, fingertip forces normal to the object surface (grip force) change in parallel with forces tangential to the object (load force). We investigated whether voluntary wrist actions can affect grip force independent of load force, because the extrinsic finger muscles cross the wrist. Grip force increased with wrist angular speed during wrist motion in the horizontal plane, and was much larger than the increased tangential load at the fingertips or the reaction forces from linear acceleration of the test object. During wrist flexion the index finger muscles in the hand and forearm increased myoelectric activity; during wrist extension this myoelectric activity increased little, or decreased for some subjects. The grip force maxima coincided with wrist acceleration maxima, and grip force remained elevated when subjects held the wrist in extreme flexion or extension. Likewise, during isometric wrist actions the grip force increased even though the fingertip loads remained constant. A grip force "pulse" developed that increased with wrist force rate, followed by a static grip force while the wrist force was sustained. Subjects could not suppress the grip force pulse when provided visual feedback of their grip force. We conclude that the extrinsic hand muscles can be recruited to assist the intended wrist action, yielding higher grip-load ratios than those employed with the wrist at rest. This added drive to hand muscles overcame any loss in muscle force while the extrinsic finger flexors shortened during wrist flexion motion. During wrist extension motion grip force increases apparently occurred from eccentric contraction of the extrinsic finger flexors. The coactivation of hand closing muscles with other wrist muscles also may result in part from a general motor facilitation, because grip force increased during isometric knee extension. However, these increases were related weakly to the knee force. The observed muscle coactivation, from all sources, may contribute to grasp stability. For example, when transporting grasped objects, upper limb accelerations simultaneously produce inertial torques at the wrist that must be resisted, and inertial loads at the fingertips from the object that must be offset by increased grip force. The muscle coactivation described here would cause similarly timed pulses in the wrist force and grip force. However, grip-load coupling from this mechanism would not contribute much to grasp stability when small wrist forces are required, such as for slow movements or when the object's total resistive load is small.
When we manipulate objects we preserve a stable grasp by parallel changes in the grip and load forces at the digit contact surfaces. For example, when we lift an object vertically from a support surface, the grip force (force normal to the gripped surfaces) rises in parallel with the developing load force (vertical force tangential to the gripped surfaces) (Edin et al. 1992 Subjects and general procedure
A total of 25 healthy adults (20-28 yr) participated in these experiments. Informed consent was obtained from all subjects according to the Declaration of Helsinki.
Wrist motion experiment
Six subjects (4 females, 2 males) produced wrist movement (dorsiflexion and palmar flexion) in the horizontal plane while holding the object (Fig. 1). The arm and forearm were elevated so that they were parallel to the floor. The forearm was stabilized midway between pronation and supination in a cast secured to a table. The ulnar side of the hand rested on a lightweight aluminum platform (8 × 3 × 0.5 cm) connected to an axle set in a bearing that allowed the wrist to rotate. The three extended fingers were positioned between upright supports to stabilize the hand for flexion and extension movements of the wrist. A potentiometer attached to the axle transduced the angular position of the wrist.
Isometric wrist force
Nine subjects (8 females, 1 male) exerted isometric wrist flexion or extension forces at varying rates while holding the test object with a 200-g weight attached. Subjects sat with the right forearm rigidly restrained, as previously described, except that the hand touched only a flat, rigid bar that was oriented vertically. For flexion trials the palmar surface of the hand was placed against a the bar so that the bar did not overlie the long flexor tendons of the index finger. For extension trials the hand dorsum contacted the bar. Subjects produced target wrist forces of 10%, 25%, and 50% of their maximum voluntary contraction (MVC). Target forces were displayed to the subjects on an oscilloscope. Subjects were instructed to produce wrist force at slowly, over several seconds (slow trials) to the target force, and to hold that force for ~4 s. Three trials at each isometric target force were performed. Six of the nine subjects in the isometric wrist force experiment also produced the force rapidly (fast trials) for five trials each of 50% MVC flexion and extension.
Isometric leg experiment
Ten subjects (5 males, 5 females) participated in an experiment involving isometric knee extension. Subjects held a 200-g test object and produced knee extension forces with the leg ipsilateral to the object. Subjects sat with the knee flexed ~30° (with 0 equal to alignment of the tibia and femur). The leg was positioned against a padded, horizontal steel bar at the distal aspect of the tibia. Each subject's MVC for leg extension was established. Isometric contractions consisted of 10% MVC at slow, medium, and fast rates; 25% MVC at a medium rate; and 50% MVC at slow, medium, and fast rates. These rates were decided by each subject. Eight trials of each condition were produced.
Electromyographic procedures
Electrical activity was recorded from selected muscles during the wrist motion and isometric wrist force experiments. During the wrist motion experiment, surface electromyographic (EMG) signals were collected from first dorsal interosseous (1DO), flexor digitorum superficialis (FDS), flexor carpi radialis (FCR), and extensor carpi radialis (ECR). The FDS electrodes were placed distal on the forearm, typically within a few centimeters of the distal wrist crease, to minimize cross talk from wrist flexor muscles. During the isometric force experiments, fine wire electrodes (75-µm wire) in bipolar pairs (1-cm interelectrode distance) collected EMG signals from 1DO and FDS. Surface electrodes collected signals from flexor carpi ulnaris and ECR. Surface EMG signals were collected with the use of bipolar silver-silver chloride surface EMG electrodes filled with conducting jelly. Interelectrode distances were 1 cm for 1DO and FDS and 3 cm for FCR and ECR. The common-mode reference electrode for use with the differential preamplifiers was placed over the olecranon process. Muscle activity signals were filtered during amplification (30 Hz-2.5 kHz).
Data sampling and signal processing
All data were digitized at 12-bit resolution with a personal computer with the use of the "SC/ZOOM" system (Department of Physiology, University of Umeå, Umea, Sweden). Force and position signals were sampled at 400 Hz. EMG signals were sampled at 1,600 Hz for the wrist motion experiment and at 3,200 Hz for the isometric force experiment. These were full-wave rectified digitally. The beginning of force increases and motion were determined by visual inspection with the use of force rate or wrist angular velocity signals, which were calculated as a function of time with the use of a ±5-point numerical differentiation method. Wrist acceleration was computed with the use of this numerical differentiation method also.
Data analysis
Repeated-measures analysis of variance (ANOVA) was used for statistical analysis of data, unless otherwise noted. Post hoc comparisons were tested with a Tukey's Honestly Significant Difference Test. Values expressed in the text are means ± SD averaged across subjects, unless otherwise noted.
Effects of wrist motion
The grip force increased during wrist flexion or extension, and during fast wrist flexion in particular (Figs. 2-4). For both flexion and extension movements the grip force began to increase slightly before the start of wrist motion for fast movements (32 ± 44 ms; P < 0.0175) and about at wrist motion onset for medium speed movements (18 ± 147 ms after wrist motion onset; P < 0.65). Also, for both movement directions the increased grip force often was maintained after the movement ended.
Effects of isometric wrist force and force rate
All subjects increased the grip force when they flexed or extended their wrists isometrically against a rigid bar while holding the test object (Fig. 5). In this experiment the object's total tangential load on the fingertips remained constant at 200 g of weight (see METHODS). The additional grip force was associated with increased activity of FDS and 1DO.
Effects of isometric leg contractions on the grip force
We were interested in whether the grip force increases that were observed in the previous experiments could arise from a generalized motor facilitation (i.e., a Jendrassik effect). Subjects produced isometric contractions of the quadriceps muscles while holding an object with a precision grip. Compared with the upper limb experiments, only small increases in grip force occurred during isometric leg extension (5, 7, and 15.7% change for 10, 25, and 50% MVC at slow rates of force production. Moreover, the grip force did not depend on the rate of knee extension force (Fig. 8). The grip force increased slowly, even on trials with rapid knee extension, and reached a maximum at variable times well after peak knee extension. Only 1 of the 10 subjects showed a grip force that increased with the rate of isometric leg contraction.
One purpose of these experiments was to examine how well the commands for precision grip force account for the muscle length and velocity changes that occur during wrist angular motion, and also for the moment that the extrinsic finger muscles produce at the wrist. Subjects did not maintain a constant grip force either in flexion or extension directions. This finding is consistent with a report from Johansson and Westling (1984)
INTRODUCTION
Abstract
Introduction
Methods
Results
Discussion
References
; Johansson and Westling 1984, 1988a; Westling and Johansson 1984). This parallel fluctuation of grip and load force occurs when we accelerate and decelerate objects held aloft with the use of arm movements, walking and running, or jumping (Flanagan and Tresilian 1994; Flanagan and Wing 1993, 1995; Kinoshita et al. 1996), pull on elastic loads (Johansson and Westling 1984), and pull or push on immovable objects (Johansson et al. 1992). Indeed, the coupling between the grip and load forces at the fingertips is a defining characteristic of lifting objects with a precision grip and appears by the age of 2 yr in humans (Forssberg et al. 1991). In theory, coupling the grip force with load forces reduces the degrees of freedom for controlling these forces, and may simplify the process of achieving coordinated fingertip forces.
; Johansson and Cole 1992; Johansson and Westling 1988b; Lacquaniti et al. 1992). This is consistent with observations that, during transport of grasped objects, the grip and load force change in parallel and without phase differences regardless of the grip style (e.g., 1- or 2-hand grip) or mode of transport (arm motion vs. jumping) (Flanagan and Tresilian 1994; Flanagan and Wing 1993, 1995; Kinoshita et al. 1996).
; Bunnell 1944; Snijders et al. 1987). Thus recruiting the finger flexors to assist with a desired wrist action could contribute to force coupling at the fingertips if the wrist action increased fingertip load, for example, during wrist angular motion. However, this also opens the possibility that the grip force could vary independent of fingertip load.
). O'Driscoll et al. (1992) reported that maximum grip strength occurred at ~35° wrist extension, with reductions of grip strength up to 73% at less favorable wrist positions. Thus during angular wrist motion a constant grip-load force ratio would require an appropriate modulation of the neural drive to hand muscles. Johansson and Westling (1984) in an anecdotal report noted little variation in grip force for slow wrist rotation (<90°/s), indicating that appropriate adjustments occurred in the activation levels of hand muscles. Grip force increased substantially at faster speeds, but the rising inertial loads on the fingertips during hand motion could account for this observation.
METHODS
Abstract
Introduction
Methods
Results
Discussion
References
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FIG. 1.
Apparatus used for wrist motion experiment. Low-mass object (28 gm) minimized centrifugal force on fingertips. Elastic load provided vertical force on fingertips that required moderate grip force to hold object. Elastic load was maintained at constant level by allowing it to rotate with wrist flexion-extension.
where m is object mass (0.024 kg), r is distance (m) from the object's center to the estimated center of wrist rotation in the flexion/extension plane (dorso/palmar), and w is wrist angular velocity (rad/s).
where m and r are as defined above, and a is angular acceleration (rad/s2).
RESULTS
Abstract
Introduction
Methods
Results
Discussion
References
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FIG. 2.
Examples of signals obtained from 1 subject on single trial of "rapid" speed wrist flexion (left) and extension (right) movements. Centrifugal load was calculated with the use of angular velocity (see METHODS). Vertical load is force obtained from load cell in series with test object and spring. "Inertial" force is portion of grip force due to linear acceleration of object (see METHODS). Total load is sum of centrifugal and vertical loads. Shaded region depicts grip force that is in excess of total load. Baselines of grip force and total load signals were aligned before movement start. The 2 calibration bars adjacent to grip force and total load traces are for grip force (left bar) and total load (right bar). 1DO, 1st dorsal interosseus; FDS, flexor digitorum superficialis; FCR, flexor carpi radialis; ECR, extensor carpi radialis.
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FIG. 3.
Examples of signals from 2 subjects (top vs. bottom) performing 5 trials (superimposed) of wrist flexion (right) and wrist extension (left) movements. Bottom traces in each panel: ratio of grip force divided by total load.
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FIG. 4.
Examples of signals obtained from single subject during single trials at "medium" speed for wrist flexion (right) and wrist extension (left) movements. Calibration bars for total load are located at right of each total load trace.
1.0 (mean = 1.6). This indicates that adequate frictional forces could be produced during centrifugal loading with grip force responses that were smaller than those observed.
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FIG. 5.
Examples of signals obtained from single subject producing "fast" isometric wrist flexion (right) and extension (left).
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FIG. 6.
Wrist force and grip force signals along with their 1st derivatives from single subject producing 3 trials (superimposed) of isometric wrist extension at rapid rates (right) vs. slightly slower wrist force rates (left). All trials are aligned with start of wrist force production.
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FIG. 7.
Wrist force and grip force signals from repeated trials of rapid isometric wrist flexion during (A) no visual feedback of grip force and no instructions to subject to suppress grip force increase, and (B) visual feedback of grip force, and instructions to subject to attempt to suppress grip force increase.
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FIG. 8.
Examples of signals obtained during knee extension from 1 subject. A: slow extension. B: fast extension. Note that grip force does not depend on knee force rate.
DISCUSSION
Abstract
Introduction
Methods
Results
Discussion
References
, who described increases in grip force for rapid wrist rotations, but not for slow rotations(<90°/s). Furthermore, the present results indicate that the increased grip force exceeded the added tangential fingertip loads from hand and object motion. The failure to regulate the grip-load ratio was demonstrated also when grip force rose while subjects voluntarily produced isometric forces at the wrist. These data indicate that the production of voluntary wrist forces can affect the coupling between the grip force and tangential fingertip load.
; Maier and Hepp-Reymond 1995a).
; Kilbreath and Gandevia 1993; Long 1970; Maier and Hepp-Reymond 1995a,b).
; Johansson 1996). This interpretation is favored because grip force is modulated automatically in anticipation of fingertip loads (Johansson and Westling 1988a), and the function describing grip-load coupling can be independently controlled with regard to its gain and offset (Flanagan and Wing 1995). A high-level controller may have generated the rise in grip force during wrist motion to generate the frictional force needed to offset the rising centrifugal loads on the fingertip. Recent experiments demonstrate that, compared with holding an object motionless, the motor system adopts a more conservative strategy (i.e., a higher grip-load ratio) during intended object acceleration (Flanagan and Tresilian 1994; Flanagan and Wing 1995) and when responding automatically to unexpected object acceleration (Cole and Johansson 1993). This strategy may be especially useful to avoid slips during wrist angular motion, when predictions of grip force are further complicated by the changing lengths and velocities of the extrinsic finger flexors.
). These thumb muscles, if recruited as discussed previously, could contribute to the dual tasks of contributing to wrist flexion torque and pinch force.
). We expect that intrinsic thumb muscles likewise increased their activity, assuming that extrinsic thumb muscles increased their activity during grip force increases, as discussed previously. This general muscle coactivation during grasp is consistent with previous studies of muscle function during pulp pinch (Johansson and Westling 1988b; Kilbreath and Gandevia 1993; Long 1970; Maier and Hepp-Reymond 1995a,b; Smith 1981).
).
; Delwaide and Toulouse 1981; Hagbarth et al. 1975; Watanabe et al. 1994). This type of remote muscle facilitation may be responsible for the weak grip force increases that occurred during isometric leg contractions, and for at least some of the increased grip force during wrist muscle activation. For example, the easily suppressed static phase of the grip force increase may reflect a Jendrassik effect. It is debatable whether the grasp force pulse that developed during wrist muscle activation results from a general motor facilitation. On knee extension the grip force did not depend on knee force rate, in contrast to the grip force pulses that arose during wrist actions. However, these contrasting findings may reflect differences in the proximity of motoneuronal pools or premotoneuronal pools used for knee extension versus wrist action. If so, a general motor facilitation also may account for the coactivation of finger flexors and other wrist flexors that we observed during wrist flexion.
suggest that a portion of the grip-load coupling they observed when subjects moved grasped objects with the upper limb may result from an obligatory or "low-level" mechanism. In one of those experiments subjects grasped and lifted a test object with a large voluntary grip force and then moved the object back and forth in the horizontal plane. Flanagan and Wing observed that the grip force modulated in parallel with the fingertip tangential loads even though the grip force that they employed before starting the horizontal object motion was large enough to maintain a stable grasp without further increases in grip force. This seemingly obligatory coupling of the grip and load forces may reflect a low-level coupling of the grip force with tangential fingertip loads, as suggested by Flanagan and Wing (1995), and it also may reflect the more general coactivation of finger flexors with other wrist muscles, as observed in the present experiments.
; Koshland et al. 1991), apparently to stabilize the wrist. The resulting coactivation of hand closing muscles with wrist muscles will contribute partially to coupling the grip and tangential load forces at the fingertips. However, our observations indicate that the potential contribution to grip-load coupling from these mechanisms will weaken considerably for some movements. The task of coupling the grip forces to the fingertip loads may fall exclusively to the "higher-level" mechanisms suggested by Flanagan and Wing (1995) and Johansson (1996) for slow movements, or whenever small muscle forces at the wrist can be used to maintain a stable wrist position. This may be advantageous for tasks that are best performed with minimal grip-load ratios, for example when manipulating delicate objects or when dexterity must be maximized.
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ACKNOWLEDGEMENTS |
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The SC/Zoom data acquisition and analysis system was made available to us through the generosity of Professor Roland S. Johansson and the Department of Physiology, Umeå University, Umea, Sweden.
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FOOTNOTES |
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Address for reprint requests: K. J. Cole, South 501 Field House, Dept. of Exercise Science, The University of Iowa, Iowa City, Iowa 52242.
Received 30 January 1996; accepted in final form 25 March 1997.
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REFERENCES |
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Abstract
Introduction
Methods
Results
Discussion
References
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