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. Author manuscript; available in PMC: 2011 Jul 1.
Published in final edited form as: J Hand Surg Am. 2010 Jul;35(7):1097–1104. doi: 10.1016/j.jhsa.2010.04.021

Carpal and Forearm Kinematics During a Simulated Hammering Task

Evan L Leventhal 1, Douglas C Moore 1, Edward Akelman 2, Scott W Wolfe 3, Joseph J Crisco 1
PMCID: PMC2901240  NIHMSID: NIHMS212506  PMID: 20610055

Abstract

Purpose

Hammering is a functional task in which the wrist generally follows a path of motion from a position of combined radial deviation and extension to combined ulnar deviation and flexion, colloquially referred to as a dart thrower's motion (DTM). The purpose of this study was to measure wrist and forearm motion and scaphoid and lunate kinematics during a simulated hammering task. We hypothesized that the wrist follows an oblique path from radial extension to ulnar flexion and that there would be minimal radiocarpal motion during the hammering task.

Methods

13 healthy volunteers consented to have their wrist and distal forearm imaged with computed tomography at five positions in a simulated hammering task. The kinematics of the carpus and distal radial ulnar joint were calculated using established markerless bone registration methods. The path of wrist motion was described relative to the sagittal plane. Forearm rotation and radioscaphoid and radiolunate motion were computed as a function wrist position.

Results

All volunteers performed the simulated hammering task using a path of wrist motion from radial extension to ulnar flexion that was oriented an average 41 ± 3° from the sagittal plane. These paths did not pass through the anatomic neutral wrist position; rather they passed through the neutral hammering position, which was offset by 36° ± 8° in extension. Rotations of the scaphoid and lunate were not minimal but averaged 40% and 41% respectively of total wrist motion. The range of forearm pronosupination during the task averaged 12 ± 8°.

Conclusions

The simulated hammering task was performed using a wrist motion that followed an oblique path, from radial extension to ulnar flexion. Scaphoid and lunate rotations were significantly reduced, but not minimized, when compared with rotations during pure wrist flexion/extension. This is likely due to the fact that an extended wrist position was maintained throughout the entire task studied.

Keywords: Carpal, Kinematics, Hammering, Scaphoid, Lunate

INTRODUCTION

Hammering is accomplished via tightly coordinated motion of the shoulder, elbow, forearm, and wrist1. The general sequence of events during the swing phase of hammering includes combined shoulder and elbow extension, followed by forearm supination and “snapping” of the cocked wrist1 from radial extension to ulnar flexion to generate high driving forces.

While there have been several studies on the ergonomics of hammering1–3, the specific bony motions that occur at the wrist and forearm have not been characterized in detail. At this point it is generally acknowledged that the hand moves along a path from radial extension (a wrist position of combined radial deviation and extension) to ulnar flexion (a wrist position of combined ulnar deviation and flexion)1,4, colloquially referred to as the Dart Thrower's Motion1. It has also been reported that the wrist is held in a position of extension throughout the hammering motion1,4, and that the forearm naturally pronates during the windup phase (as the wrist is moved into radial extension) and supinates during the swing phase (as the wrist is snapped into ulnar flexion)5. However, the specific motions of the carpal bones during the hammering task have not been quantified.

The previous kinematic studies of hammering have measured joint motion using electromechanical devices affixed to the surface of the distal forearm and hand1,4. Although these studies have provided important insight into the gross motion of the forearm and hand during hammering, their use of surface-based measurement methods precluded analysis of carpal bone kinematics due to the movement of the skin relative to the underlying bone. Moreover, previous hammering studies have focused on the relative motion of the hand, with little if any emphasis on forearm pronosupination.

Despite the lack of kinematic data on the hammering task, the kinematics of the carpus have been evaluated for less demanding tasks that employ the DTM in both living subjects and in vitro cadaver experiments6–10. These studies have generally reported that radiocarpal motion is reduced during the DTM, compared to that seen with pure flexion and extension of the wrist. However, the findings regarding the relative contribution of radiocarpal motion to overall wrist motion have varied. In particular, scaphoid rotation during the DTM has been reported to be as little as 26% and as much as 50% of overall wrist motion6–8,10, while the range of reported lunate rotations has varied from 22% to 40% of overall wrist motion6–8,10. Analyzing in vivo carpal kinematics using data from 28 normal volunteers and 504 wrist positions, Crisco et al.9 predicted near-zero scaphoid and lunate rotations for DTM motion paths oriented at angles of 33° and 20° to the sagittal plane, respectively.

Data on the intricacies of wrist and forearm bone motion during hammering is critical to the understanding of work-related injury, and necessary for the rational design of ergonomic tools, rehabilitation protocols, and wrist implants. The purpose of this study, therefore, was to determine the overall path of wrist motion during hammering, as defined by the third metacarpal motion, and to determine the specific three-dimensional (3-D) motion of the radius, ulna and carpal bones that occur during hammering. Given the existing data in the literature and the high-demand nature of the hammering task, we hypothesized that wrist motion would follow the path of the dart thrower's motion, and during hammering motion of the proximal carpal row with respect to the distal radius would be minimal.

METHODS

Volunteer Selection and CT Scanning

With IRB approval, 13 healthy, right-hand dominant volunteers (6 male, 7 female; average age 24.8, range [21–31]) were enrolled in the study, after a brief wrist exam performed by a board certified hand surgeon (including plain films). A prioi exclusion criteria included any history or findings of prior wrist disease or injury, or any soft tissue or metabolic disease that could affect carpal motion. All of the enrolled volunteers were `neophyte' hammerers; none had ever held a job or had hobbies that required significant amounts of hammering.

Computed tomography (CT) volume images were generated of each volunteer's dominant wrist as they gripped a wooden hammer handle and performed a simulated hammering task. During scanning the volunteers were positioned prone on the CT table (chests supported with a pillow), with their dominant arm near full shoulder flexion (overhead elevation), parallel with the center axis of the gantry. A custom-designed jig was affixed to the scanner table to facilitate wrist positioning and minimize artifactual motion during scanning. The jig included a wrist support and pegs that served as stops for the hammer handle at five targeted positions along the hammering path: −40° (full windup), −20°, 0° (“hammering neutral,” which was defined as the position where the hammer handle was oriented vertically, perpendicular to the forearm), 20°, and 40° (“impact”) (Figure 1). The term “wrist motion” is used in this study is used to describe the motion of the wrist along this hammering path.

Figure 1.

Figure 1

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Illustration of a subject's hand and forearm position at −20° in the radiolucent jig used to assist in wrist positioning and limit artifactual motion during CT scanning. The jig included a wrist support and stops for the hammer handle at five evenly-spaced hammering positions: −40° (windup), –20°, 0° (hammering neutral, hammer handle perpendicular to forearm), 20°, 40° (impact). (The head of the hammer was not included during scanning in order to limit scatter. It is included here for illustration purposes.).

CT scanning was performed with a GE LightSpeed 16 scanner (General Electric, Milwaukee, WI) at tube settings of 80kVp and 80mA, slice thickness of 0.625mm, and a field of view that yielded and in-plane resolution of 0.5mm × 0.5mm. A sixth, higher-resolution scan (0.3mm × 0.3mm × 0.625mm) was acquired with the hand flat on the CT table (wrist near neutral, forearm pronated) to provide an image dataset that was used to generate the 3-D bone surface models used for visualization and markerless bone registration.

Carpal Kinematic Analysis

The 3-D kinematics of each of the carpal and metacarpal bones were calculated using CT-based markerless bone registration (MBR) methods11. Briefly, all of the imaged bones were manually segmented from the high-resolution pronated neutral scans and 3-D surface models were generated using Mimics 9.11 (Materialize, Leuven, Belgium). Custom C++ (GNU gcc, Free Software Foundation, Boston, Massachusetts). Custom Matlab (The MathWorks, Natick, Massachusetts) code was then used to calculate the positions of the radius, ulna, carpal and metacarpal bones in the five CT volume images acquired during the hammering task..

Wrist position at each of the five hammering positions was defined by the orientation of the third metacarpal with respect to the axes of a standardized, radius-based coordinate system12,13. Anatomical neutral wrist position is defined as the long axis of the third metacarpal oriented parallel to the long axis of the radius5,14. Simple gripping of a hammer handle placed the wrist into a “hammering” neutral position in which the third metacarpal was offset from anatomical neutralinto extension and ulnar deviation. Wrist position at hammering neutral was calculated for each volunteer, and the rotations of the scaphoid and lunate at each of the four targeted hammering positions (−40°, −20°, 20°, and 40°) were calculated with respect to the targeted neutral hammering position. The three-dimensional (3-D) components of radioscaphoid and radiolunate kinematics were then described with respect to the fixed radius-based coordinate system using helical axis of motion (HAM) variables.

The range of wrist motion was calculated as the overall difference in wrist position between the full windup (−40°) and the impact (40°) positions. This path of wrist motion was visualized by plotting wrist flexion/extension versus wrist radial/ulnar deviation. The path of motion was then described in terms of its angular orientation with respect to the sagittal plane15. For example, a path of 90° represents a motion of pure radial/ulnar deviation, a path of 45° describes a wrist motion involving equal coupling of flexion/extension and radial/ulnar deviation, and a path of 0° represents pure flexion/extension motion.

Pronation/Supination Kinematics

Forearm pronation/supination was also determined using markerless registration methods11. However, true neutral forearm pronation/supination position could not be calculated because only the distal ends of the forearm bones were imaged: determination of absolute pronosupination requires information about the position of the radius and ulna with respect to the humerus. Therefore, forearm rotation was defined to be 0° at the neutral hammering position and the pronation/supination motion of the forearm was calculated relative to this position. Forearm rotation was then analyzed as function of wrist motion.

Statistical Analysis

All of the statistical analysis was performed using mixed linear models. A single hierarchical linear model (HLM)16 was used to calculate the overall path of wrist motion for the hammering task, as well as the wrist position offset during hammering. The HLM provided estimates of the between-subject variability (standard error), as well as the significance of the coupling ratio. The coupling ratio, or ratio of extension-to-flexion to radial-to-ulnar deviation, was used as independent variable within the HLM to allow for simultaneous modeling of individual and overall paths of wrist motion in a single statistical model. The average coupling ratio was converted to the angular orientation of the plane of wrist motion with respect to the sagittal plane using standard trigonometric functions. A second HLM was used to test for a relationship between forearm pronation/supination and wrist flexion/extension, and a third HLM was used to determine the relative contribution of the scaphoid and lunate to overall wrist motion (radio-scaphoid and radio-lunate rotation).

A piecewise mixed effects model17 was used to determine how the direction of wrist motion affected the amount of radiocarpal rotation. In this model, the direction of wrist motion was compared with the percentage of radio-scaphoid and radio-lunate rotation independently for each subject. For all comparisons, the alpha level for statistical significance was set at 0.05.

RESULTS

Image data was successfully acquired and analyzed for all five wrist positions in each of the thirteen volunteers, for an analysis of a total of 65 unique hammering positions.

Wrist and Forearm Motion

The average range of wrist motion, from full windup to impact, was 70 ± 10°. The position of the wrist at the targeted neutral hammering position was offset in extension an average of 36° ± 8° and in ulnar deviation an average of 14° ± 10° from the anatomical neutral wrist position (0° flexion/extension and 0° radioulnar deviation).

There was a significant correlation between wrist flexion/extension and radial/ulnar deviation (p < 0.01). The resulting coupling ratio of the path of wrist motion employed by our volunteers during the hammering task was oriented an average 41° from the sagittal plane (Standard Error: 3°, Range: 29°–54°) (Figure 2).

Figure 2.

Figure 2

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Wrist positions measured during the simulated hammering task demonstrate that wrist motion was performed along the general path of the “dart thrower's motion”, but with an offset in extension and ulnar deviation from the anatomical neutral wrist position. The mean path for all thirteen volunteers was oriented at an angle of 41° from the sagittal plane (solid line), which corresponded to a functional coupling ratio of 1.2 (flexion-extension motion/radial-ulnar motion). In this plot, the individual positions for each volunteer were normalized to the average neutral hammering position (N).

From windup to impact, forearm pronation/supination rotation with respect to the neutral hammering position was minimal and the total range of motion averaged 12 ± 8° (Figure 3). There was no significant (p = 0.12) correlation between forearm rotation and wrist motion during the hammering task.

Figure 3.

Figure 3

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Forearm pronation/supination as a function of wrist motion along the hammering path (Figure 2). There was minimal pronation/supination during hammering, with no significant correlation between forearm pronation/supination and wrist motion (p=0.12).

Radiocarpal Motion

During hammering, radiocarpal rotation accounted for approximately 40% of overall wrist motion (Figure 4). The hierarchical linear model used to evaluate scaphoid and lunate rotations confirmed that rotation of the scaphoid with respect to the radius was non-zero (p<0.01), and that radioscaphoid rotation was responsible for approximately 40% (standard error 5%) of the overall wrist motion (i.e. for every 10° of overall wrist rotation along the hammering path, the scaphoid rotated on average of 4°). Similarly, lunate rotation was also non-zero (p<0.01), and lunate rotation contributed 41% (standard error 4%) towards the overall wrist rotation. By extension, these very similar radiocarpal motions indicate that approximately 60% of the total wrist motion occurred at the midcarpal joint.

Figure 4.

Figure 4

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Carpal bone rotations at the radiocarpal and midcarpal joints plotted as a function of motion along the hammering path. During hammering, scaphoid and lunate rotation accounted for 40% and 41% of the overall wrist motion, respectively. Rotation at the midcarpal joint was 60% of wrist motion.

Scaphoid and lunate translations were 0.5 ± 0.4 mm and 0.6 ± 0.5 mm respectively, throughout the entire task, confirming, as expected, that the vast majority of the radiocarpal motion during hammering was rotational.

As a subject's path of wrist motion deviated from a DTM angled 32° from the sagittal plane, radioscaphoid motion increased (Figure 5a). Radioscaphoid motion decreased as the angle of wrist motion decreased from 60° to 30° (with respect to the sagittal plane), and then increased again as the angle decreased further, from 30° to 15°.

Figure 5.

Figure 5

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Scaphoid rotation was found to depend significantly on the path of wrist motion relative to the sagittal plane for this study (a), as in Werner et al. 2004 (b). a) Our findings indicate that the path of wrist motion, or the coupling ratio, influenced the amount of radioscaphoid rotation during hammering. Radioscaphoid rotation was minimized when the path of wrist motion was oriented 32° from the sagittal plane; as subjects moved away from that path, radioscaphoid rotation increased. These findings were in agreement with those of Werner et al. (b) who used a cadaver model.

The mixed effects model failed to converge for the lunate data set, which implies that there was no clear inflection point, or local minimum, for rotation of the lunate as a function of hammering path (Figure 6a). In this case, regression of the lunate rotation data against hammering path indicated that radiolunate rotation decreased monotonically as the path of wrist motion decreased from 60° to 30° (with respect to the sagittal plane).

Figure 6.

Figure 6

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Lunate rotation, as with scaphoid rotation (Fig. 5), was found to depend significantly on the path of wrist motion relative to the sagittal plane for this study (a) and in Werner et al. 2004 (b). a) Our findings indicate that increasing wrist flexion/extension was associated with a decrease in the amount of radiolunate rotation. b) When compared to the results of Werner et al., the wrist motion path of our subjects appear to lack sufficient wrist flexion/extension to minimize radiolunate rotation.

DISCUSSION

This study was performed to investigate wrist function during hammering, in particular the overall path of wrist motion, the specific contributions of the radiocarpal and midcarpal joints, and the role (if any) of forearm rotation. To do so we performed sequential CT scans of the wrists of thirteen normal volunteers performing a simulated hammering task and we used markerless registration techniques to quantify bone motion and generate kinematic data. We found that during the simulated hammering task for our volunteers the average path of wrist motion was oriented 41° from the sagittal plane and radiocarpal motion accounted for 40% of the overall wrist motion, with the remaining 60% occurring at the midcarpal joint. We also found that this simulated hammering task was performed with the wrist significantly extended, and with limited forearm pronation/supination.

We chose to use serial CT scans and markerless registration techniques because they are currently the most accurate non-invasive methods available for measuring the complex 3-D motions of the carpal bones in vivo. Sequential CT scanning and markerless bone registration (MBR) have been used by a number of groups to analyze carpal kinematics in normal individuals18–20 and in patients with wrist pathology21,22. While the technique necessarily infers dynamic kinematic behavior from static analytical methods, recent studies have demonstrated that in normal wrists, carpal kinematics can be studied equally well statically and dynamically23,24. Furthermore, in a small pilot study we found that simulated and dynamic hammering followed similar paths of wrist motion (Curran PF, Rainbow MJ, Moore DC, Crisco JJ. Hammering and Dart Throwing are Kinematically Different. Presented at: American Society of Biomechanics Annual Conference; Auguest 22–25, 2007; Stanford, California).

Our findings regarding the direction of the wrist's path during hammering are generally consistent with the paths reported in previous hammering studies1,4, and in other studies that have analyzed the dart thrower's motion6–10. These studies reported paths of wrist motion that were angled 23° to 50° from the sagittal plane1,4,6–10. However, we also found that our volunteers hammered with their wrists in extension, such that at mid-swing, when their wrists were in neutral radial/ulnar deviation, there was an extension offset of 54°. The fact that hammering is performed with the wrist extended has been reported by others1,4. This indicates that, from a kinematics perspective, hammering takes place on a dart thrower's-like path (or paths) of wrist motion, parallel to one that passes through neutral flexion/extension and neutral radial/ulnar deviation6. In other words, hammering motion occurs in a plane different but parallel to the plane of the classical dart thrower's motion, and this “hammering plane” is offset in wrist extension. The implications of this, as we have found, is that hammering involves carpal kinematics different from that associated with the dart thrower's motion that passes through the neutral wrist position (i.e. when the third metacarpal is aligned with the long axis of the forearm).

Our piecewise modeling (Figure 5a) demonstrated clearly that the path of wrist motion used during hammering affected radiocarpal motion. Both radioscaphoid and radiolunate motion decreased significantly in volunteers who selected paths that involved increasingly more extension-to-flexion motion compared to those who selected paths that involved more radial-to-ulnar deviation (Figures 5a, 6a). Scaphoid rotation appeared to be minimized at a path angled ~30° from the sagittal plane, and then it increased again as the path of wrist motion shifted to include proportionally more extension-to-flexion motion. We were unable to identify a minimum path for lunate motion; rather it appeared that lunate rotation decreased as the hammering path favored increasing amounts of wrist flexion/extension. However, for both the scaphoid and lunate rotation, our results were less definitive for hammering paths less than 30° from the sagittal plane given that we had limited data for those paths (only four of our volunteers hammered along paths that were angled less than 32° from sagittal, and the paths of these four subjects were within 5° of each other). Additional data will be required to confirm these findings.

Our findings regarding the relationship between the hammering path and radiocarpal motion are generally consistent with the findings of Werner et al.6 who measured radioscaphoid and radiolunate rotation for nine separate paths of wrist motion using five cadaver wrists. When we analyzed their scaphoid motion data using our piecewise model, the regression lines were very similar to ours (Figure 5b), with the exception that their minimum scaphoid rotation (26% of overall wrist motion) occurred along a path of wrist motion oriented 45° from the sagittal plane while ours occurred along a path oriented 32° from the sagittal plane. Similarly, piecewise regression of their radiolunate rotation data revealed a trend towards decreasing lunate rotation as the path of wrist motion shifted from pure radial/ulnar deviation to include more flexion/extension motion, as did ours. However, analysis of Werner et al.'s lunate rotation data also revealed a local minimum at a path oriented approximately 40° from the sagittal plane, with increasing lunate rotation along paths that that included more flexion/extension or radioulnar motion. With the finite number of subjects enrolled in our study, we were unable to detect a local minimum for lunate rotation.

A significant difference between the study by Werner et al.6 and ours was that theirs was designed to include anatomical neutral (0° flexion/extension, 0° radial/ulnar deviation) in the motion arc. In our study, radiocarpal motion accounted for an average of 40% of overall wrist motion. While this represents a substantial reduction (~50%) in scaphoid and lunate rotation compared to that seen with pure flexion and extension of the wrist25, it is much more than 26% reported by Werner et al.6 and the theoretical minimum of zero motion estimated by Crisco et al.9. We believe that this disparity is related to the 54° extension offset of wrist motion during the simulated hammering task.

Previous studies have suggested that the rotation of the scaphoid and lunate bones is minimized during tasks that involve wrist motions along the dart thrower's path4,6–10. In fact motion of the scaphoid and lunate have been reported to rotate as little as 26% of wrist motion during the DTM6. It has been proposed that the ability to selectively use motions that minimize motion of the proximal row conferred an evolutionary advantage on our hominid ancestors because it resulted a more “stable” wrist joint for high demand activities, such as clubbing and hammering26. In this study, most of our volunteers used simulated hammering paths that decreased but did not minimize rotation of either the scaphoid or the lunate. Our piecewise modeling results (Figures 5a, 6a) indicate that most of our subjects used paths that were biased slightly more to radioulnar deviation than the 45° path identified by Werner et al. Moreover, both our data and the data in the papers by Werner et al.6 and Crisco et al.9, suggest there may not be a single motion path that simultaneously minimizes scaphoid and lunate rotation.

It has been reported that the terminal motion in hammering involves supination of the wrist and forearm5,15. Our study was not able to confirm this because we were unable to satisfactorily determine the absolute position of forearm rotation. Based upon our previous accuracy studies we are capable of measuring forearm rotation with an accuracy of better that 0.5°11. However, our CT images contained only the distal portions of the radius and ulna, and without knowing their relationship to the humerus we were not able to define the absolute neutral forearm position. This challenge was further compounded by the fact that the volunteers were prone on the CT table, with their arm elevated, further confounding our ability to define forearm position. From our accurate measure of relative forearm rotation, we did find that there was consistently minimal pronosupination during the hammering task. By carefully aligning the elbow, and wrist with the hammering swing, we may have artificially reduced the amount of potential forearm rotation normally associated with dynamic hammering. It is possible, too, that by limiting our last wrist position to 40° impact we may have missed some of the terminal wrist motion and forearm supination.

Finally, it should also be noted that all of our volunteers were novices at hammering, none having worked at jobs or having had hobbies that involved substantial hammering. As such, it is possible that the wrist and forearm motion for more experienced hammerers would differ from what we report. However, within the context of our experiment we think the differences would be relatively small, in part because we used a positioning jig to steady the wrists during scanning, but also because we focused on the terminal activity in the task – the wrist snap from radial extension to ulnar flexion – which seem to be relatively invariant for these high-demand tasks. Additional data, acquired using high-speed dynamic data acquisition systems, will be required to confirm this.

In summary, we demonstrated that 1) the path of wrist motion during a simulated hammering task paralleled a dart thrower's motion from radial extension to ulnar flexion, 2) the path of wrist motion was offset in extension and ulnar deviation during this task, and 3) the task was performed with limited forearm pronation/supination motion.

Acknowledgements

This publication was made possible by grant number AR0053648 from National Institute of Arthritis and Musculoskeletal and Skin Diseases and the Brown University MD/PhD program.

Footnotes

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