upper limb kinematics and the role of the wrist during .... williams,1,2* a.d. gordon,3 and b.g....

Upper Limb Kinematics and the Role of theWrist During Stone Tool Production

E.M. Williams,1,2* A.D. Gordon,3 and B.G. Richmond2,4

1Hominid Paleobiology Doctoral Program, Department of Anthropology, The George Washington University,Washington, DC 200522Center for the Advanced Study of Hominid Paleobiology, Department of Anthropology,The George Washington University, Washington, DC 200523Department of Anthropology, University at Albany – SUNY, Albany, NY4Human Origins Program, National Museum of Natural History, Smithsonian Institution, NW, Washington, DC

KEY WORDS extension; flexion; kinematics; mechanical work; stone tool production

ABSTRACT Past studies have hypothesized thataspects of hominin upper limb morphology are linked tothe ability to produce stone tools. However, we lack thedata on upper limb motions needed to evaluate the bio-mechanical context of stone tool production. This studyseeks to better understand the biomechanics of stonetool-making by investigating upper limb joint kinemat-ics, focusing on the role of the wrist joint, during simpleflake production. We test the hypotheses, based on stud-ies of other upper limb activities (e.g., throwing), thatupper limb movements will occur in a proximal-to-distalsequence, culminating in rapid wrist flexion just prior tostrike. Data were captured from four amateur knappersduring simple flake production using a VICON motionanalysis system (50 Hz). Results show that subjects uti-lized a proximal-to-distal joint sequence and disassoci-

ated the shoulder joint from the elbow and wrist joints,suggesting a shared strategy employed in other contexts(e.g., throwing) to increase target accuracy. The knap-ping strategy included moving the wrist into peak exten-sion (subject peak grand mean 5 47.38) at the beginningof the downswing phase, which facilitated rapid wristflexion and accelerated the hammerstone toward thenodule. This sequence resulted in the production of sig-nificantly more mechanical work, and therefore greaterstrike forces, than would otherwise be produced. To-gether these results represent a strategy for increasingknapping efficiency in Homo sapiens and point to aspectsof skeletal anatomy that might be examined to assesspotential knapping ability and efficiency in fossilhominin taxa. Am J Phys Anthropol 143:134–145,2010. VVC 2010 Wiley-Liss, Inc.

Modern humans are the most eurytopic primate spe-cies the planet has ever hosted, capable of inhabitingregions that our closest living relatives, the Africanapes, would find inhospitable and even hostile to theirsurvival. Our success has been attributed in part to ourelaborate relationship with technology, of which stonetools represent the earliest evidence in the archaeologi-cal record (Stiner and Kuhn, 1992; Schick and Toth,1993; Foley, 1995; Semaw et al., 1997; Wood and Brooks,1999; Wrangham et al., 1999; Ambrose, 2001; Lutz andQiang, 2002; Wood and Strait, 2004; Wrangham, 2007).Consequently, the ability to produce and use tools isrecognized as a key adaptation in hominin evolution.Hominin hands and wrists have undergone numerous

changes over the course of human evolution, many ofwhich occurred soon after the origin of early stone tooltechnologies (Tocheri et al., 2008). While major gapsremain, the fossil record documents changes includingbroader apical tufts of the fingertips, a more robustthumb, and a rearrangement of carpal and radiocarpalanatomy. Due in part to the temporal proximity of thetwo events, researchers have hypothesized that stonetool production was a major selective pressure inducingsome of these changes in upper limb anatomy (Napier,1962; Marzke and Shackley, 1986; Susman, 1988, 1994;Marzke and Marzke, 2000; Ambrose, 2001; Richmondet al., 2001; Panger et al., 2002; Tocheri et al., 2008).With few exceptions (Marzke et al., 1998; Biryukova

et al., 2005), we currently lack the quantitative data onupper limb kinematics necessary for evaluating hypothe-

ses on the mechanical context of stone tool production.The current project was undertaken to begin rectifyingthis issue by investigating upper limb kinematics duringthe production of simple flakes. We examined upper limbmotion patterns associated with stone tool production asa means of testing basic assumptions underlying hypoth-eses about the functional demands and selective pres-sures that may have been acting on the upper limb dur-ing early hominin evolution. Susman (1998) noted thatbecause experiments using modern humans cannotaccount for the mosaic combinations of primitive and

Grant sponsor: National Science Foundation’s Integrative Gradu-ate Education and Research Traineeship; Grant numbers: IGERTDGE 9987590; Grant numbers: DGE 0801634. Grant sponsor: TheGeorge Washington University’s Research Enhancement Fund.Grant sponsor: The George Washington University’s Sigma Xichapter.

*Correspondence to: E.M. Williams, Hominid PaleobiologyDoctoral Program, Department of Anthropology, The George Wash-ington University, Center for the Advanced Study of HominidPaleobiology, 2110 G St., NW, Washington, DC 20052.E-mail: [email protected]

Received 31 July 2009; accepted 17 February 2010

DOI 10.1002/ajpa.21302Published online 14 May 2010 in Wiley InterScience

(www.interscience.wiley.com).

VVC 2010 WILEY-LISS, INC.

AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 143:134–145 (2010)

derived features in fossil hominin upper limbs, theirrelevance to understanding the tool behaviors of earlyhominins is limited. While this critique may apply tosome studies, it is not applicable here. The goal of thepresent study was to examine upper limb kinematics ofknappers and the role of the wrist during stone tool pro-duction as currently practiced, enabling a more informedevaluation of some of the functional hypotheses linkingthe derived upper limb and wrist conditions to stone toolproduction. We concur with Lauder’s (1995) and Marzkeand Marzke’s (2000) argument that functional dataobtained through direct observation are necessary forevaluating functional hypotheses.Compared with our closest living relatives, the African

apes, human wrist anatomy differs in many respects (seeRichmond et al., 2001; Tocheri et al., 2008 and referen-ces therein). Chimpanzees and gorillas possess a suite oftraits related to knuckle-walking, which are thought tostabilize the wrist transversely and improve the resist-ance of compressive stresses experienced during the sup-port phase of knuckle-walking (Tuttle, 1970; Jenkins andFleagle, 1975; Richmond and Strait, 2000; Richmondet al., 2001; Kivell and Schmitt, 2009). A component ofthis suite is a distally projecting dorsal ridge of the dis-tal radius (Tuttle, 1967; Richmond and Strait, 2000).During the support phase, as the wrist extends the dor-sal portion of the distal radius reaches a close-packedposition with maximal articular congruence with thelunate and scaphoid that, in conjunction with palmar lig-aments, maintains a stable joint and prevents the wristfrom further extension (Tuttle, 1967; Jenkins and Flea-gle, 1975; Richmond and Strait, 2000). In this manner,the dorsal ridge of the radius plays a key role in mini-mizing the degree of maximum wrist extension and con-tributing to a stable support column during knuckle-walking. Empirically, average ranges of maximum wristextension are greater in modern humans (x 5 708) thanin Gorilla (x 5 588) and Pan (x 5 348) (Table 1).An African ape-like projecting dorsal ridge is present

in Australopithecus anamensis and Au. afarensis, sug-gesting a lower range of wrist extension compared withlater hominins (Richmond and Strait, 2000). Lovejoyet al. (2009) argue that the carpal anatomy of Ardipithe-cus ramidus indicates that the wrist was capable ofgreater degrees of extension than that observed in Afri-can apes. However, their analysis unfortunately does notdemonstrate a link between midcarpal morphology andranges of motion in extension, and lacks sufficient com-

parisons with primate taxa other than African apes andhumans to permit clear functional interpretations aboutwrist mobility. Although data were not presented, Love-joy et al. (2009; see also White et al., 1994) imply thatthe distal radius of Ar. ramidus has an African ape-likedistally projecting dorsal ridge. Therefore, the functionalsignificance of the Ar. ramidus wrist remains open toquestion and requires further analysis. Furthermore, Ar.ramidus probably post-dates the Pan-Homo last commonancestor (LCA) by over 1 million years (White et al.,2009). Without direct evidence of the LCA, wrist mor-phology in the LCA will remain a matter of debate.What can be concluded with certainty is that over the

course of human evolution the morphology of the wristhas undergone significant modifications, including achange in the distal radius from an African ape-like mor-phology in Au. anamensis and Au. afarensis to a rela-tively flat, modern human-like distal radius in laterhominins (Richmond and Strait, 2000; Richmond et al.,2001). Researchers have hypothesized that such anatom-ical changes in wrist anatomy may have offered laterhominins, including Homo, increased wrist mobility thatfacilitated a variety of behaviors including throwing andstone tool production (Marzke, 1971; Richmond andStrait, 2000; Ambrose, 2001; Richmond et al., 2001).However, this hypothesis rests on the assumption thatwrist mobility, particularly in extension, plays an impor-tant role in stone tool production.To examine the role of the wrist in stone tool-making,

this study tests several hypotheses. Previous research onupper limb activities such as writing, throwing, andpiano playing has demonstrated that subjects main-tained general kinematic uniformity (e.g., sequence of ki-nematic events) within a given task. Specific patterns(e.g., kinematic values), however, varied among subjectsand across competency levels (Newell and van Emmerik,1989; Hore et al., 1996; Fleisig et al., 1999; Minettiet al., 2007). We predict that during stone tool produc-tion knappers will similarly demonstrate consistentgross upper limb motion patterns.Biomechanics research on activities such as pitching

(Debicki et al., 2004), dart throwing (McDonald et al.,1989; Jeansonne, 2003), hammering (Cote et al., 2005),and soccer kicking (Putnam, 1991) show that the consist-ent gross movement pattern of the limb occurs in a prox-imal-to-distal sequence. This pattern allows for greateraccuracy because movements of proximal joints havelarger effects at the end of the limb than do movements

TABLE 1. Wrist joint limits of Pan, Gorilla, Pongo and Homo

Taxon n Method Extension Flexion Radial deviationUlnar

deviation References

Pan troglodytes 51 Mixed 34 127 27 60 Tuttle (1967, 1969); Jenkins andFleagle (1975); Richmond (2006)

Gorilla 10 Anesthetized 58 117 44 70 Tuttle (1969)Pongo pygmaeus 27 Anesthetized 85 139 57 97 Tuttle (1967)Homo sapiens NA In vivo 70 80 20 30–40 Almquist (2001)Subject A 1 Muscular 58.79 49.8 20.53 17.68Subject B 1 Muscular 66.68 60.2 12.89 40.01Subject C 1 Muscular 62.05 67.14 24.01 25.89Subject D 1 Muscular 68.43 75.02 26.40 33.89Subject A 121 Knapping 48.51 125.84 7.96 10.76Subject B 261 Knapping 65.61 131.47 6.52 5.78Subject C 101 Knapping 36.43 16.59 12.19 10.01Subject D 181 Knapping 38.62 125.8 9.28 11.04

Knapping data are presented in degrees relative to each subject’s neutral position. The values for Pan represent the weightedmeans derived from reported range of motion values. n1 represents the number of knapping swing trials per subject.

135UPPER LIMB KINEMATICS IN STONE TOOL PRODUCTION

American Journal of Physical Anthropology

of distal joints (Hore et al., 1996). Therefore, the distaljoints are thought to act late to refine the final positionbefore object release or strike. Distal joint kinematics(e.g., position and velocity) have a demonstrated effecton accuracy in upper limb activities such as pitching abaseball and striking a target (Southard, 1989; Horeet al., 1996; Hirashima et al., 2007). In this study, wetest the hypothesis that during knapping upper limbmovements will similarly occur in a proximal-to-distalsequence, culminating in peak wrist extension followedby rapid wrist flexion just prior to strike.Finally we predict, based on previous upper limb kine-

matics research, that wrist motion will significantlyinfluence knapping mechanical efficiency and strike ac-curacy. In their EMG study of muscle recruitment dur-ing stone tool production, Marzke et al. (1998) reportedthat all subjects maximally recruited their flexor carpiulnaris (FCU) muscles (the only forearm muscle moni-tored whose chief action is motion at the wrist and/ormidcarpal joint) during downswing. FCU activity andwrist flexion may help knappers reach higher joint veloc-ities, thereby producing more mechanical work andgreater strike forces than could be achieved with a rigidwrist (Bunn, 1955; Putnam, 1991). Here, we test the hy-pothesis that during knapping wrist movements signifi-cantly increase mechanical work. We also test the hy-pothesis that wrist extension in particular plays an im-portant role in producing this increased mechanicalwork.

METHODS

Sample

Data were captured from four knappers; three males(Subjects A–C), and one female (Subject D). Two of thesubjects periodically participated in knapping (i.e., fewerthan five times per year, Subjects A and C), and two hadlimited-to-no prior knapping experience (Subjects B andD). All subjects were healthy, right hand dominantadults free from muscular and/or osteological conditionsthat may have compromised their motion patterns.

Raw materials

Experiments were conducted in cortex-free raw Texasflint (material toughness 5 1 Kc, unpublished data fromHerzl Chai). The mass and dimensions of the noduleswere initially similar (n 5 8, mean mass 5 5.85 kg; SD5 1.22; mean length 5 40.5 cm, SD 5 2.76; mean cir-

cumference 5 34.1 cm, SD 5 6.65). Flint was obtainedfrom Neolithics.com. A single quartzite hammerstonewas used for all flake production (0.765 kg).

Motion capture

Kinematics data were captured using the VICONmotion capture analysis system in The George Washing-ton University’s Motion Capture and Analysis laboratory.The VICON system uses high-speed cameras to digitallyrecord reflective markers applied to subjects as theymove across a calibrated space, allowing extraction ofmotion data such as landmark coordinates, joint angles,joint (e.g., wrist) angular velocity and acceleration, andsegment (e.g., forearm) velocity and acceleration. Multi-ple cameras are linked and simultaneously record thesubject’s motion from various angles, thereby providingmultiple views of the same movement. Six to eight infra-red cameras and one digital video recorder were usedto capture each subject’s knapping motions, recorded at50 Hz.Each subjects’ dominant hand (i.e., hammer hand) was

fitted with a tight, fingerless glove (rayon/cotton/rubberblend) with separate holes for the thumb and index fin-ger, and a single large hole for the three remaining dig-its. Six 10-mm diameter reflective markers were affixedto the glove and arm at the following landmarks: theolecranon process (OP), the radial and ulnar styloid proc-esses (RSP and USP, respectively), and metacarpal (MC)heads I, II, and V (see Fig. 1). Each marker consisted ofa base with a short rod projecting upward and a reflec-tive globe. Globes can be screwed and unscrewed fromthe rods to secure or unsecure them from the bases.Markers were affixed to the appropriate position on theglove by pushing the rod through the glove from theinside and screwing the globe back onto the rod suchthat the glove was sandwiched between the base andglobe of each marker. Marker bases were taped to thesubjects using double sided tape to further prevent theirdisplacement. Markers had initially been taped directlyto subjects without the glove; however their frequent dis-placement upon strike necessitated the alternativemethod described above.Data capture occurred in two phases for each subject.

Phase 1 consisted of recording subjects’ excursions at thewrist (extension, flexion, radial deviation, and ulnardeviation). Data were captured from a ‘‘neutral’’ positionin which subjects held their dominant arm flexed at theshoulder joint with the arm, forearm, and palm parallel

Fig. 1. Placement of reflective markers on subjects’ dominant hand (hammer hand). 1, Olecranon process; 2, radial styloid pro-cess; 3, ulnar styloid process; 4, metacarpal I head; 5, metacarpal II head; and 6, metacarpal V head. Markers were held in placewith a tight rayon/cotton/rubber blend fingerless glove and double sided tape.

136 E.M. WILLIAMS ET AL.


to the floor, fully pronated with the palm facing the floorand fingers fully extended. Subjects sequentially movedtheir wrist in each direction to their natural excursionmaximum and held the position for 5 s before releasing,returning to the neutral position, and proceeding to thenext direction. We note that the wrist may be capable ofgreater ranges of movement than these natural excur-sion maxima due to accelerations during rapid move-ments or if subjected to an external force. However,these provide baseline excursion maxima based on eachsubject’s voluntary muscle activity.Phases 2 consisted of data capture during flake pro-

duction. Subjects removed simple flakes from the nodulewithout regard to flake dimensions or mass by forcefullystriking the hammerstone against the nodule, whichsubjects balanced on their left leg. Recording was haltedafter the successful production of each flake and the

flake was retrieved and labeled to allow later associationof each flake with its appropriate knapping trial andlandmark coordinate data. Data for each subject werecaptured at two or more separate knapping sessions toprevent fatigue of the upper limb.

Data analysis

Captured coordinate data were sectioned into individ-ual swings. Those swings with relevant missing datawere removed from the sample set, resulting in 66 flakeproduction knapping cycle swings (Table 1). When sub-jects did not flex or deviate in the ulnar direction pasttheir neutral position reported flexion and ulnar devia-tion means represent degrees above the neutral position(i.e., extension or radial deviation). The position of theradial styloid process was used as a proxy for the wrist’s

Fig. 2. Vertical position (A, mm) and vertical velocity (B, m/s) of the olecranon process, radial styloid process, and the secondmetacarpal head through a typical knapping cycle, illustrating swing initiation (dashed vertical black line), the transition fromupswing to downswing (solid vertical black line), peak wrist extension (black arrow), strike (dashed black arrow), and peak linearvelocity (black stars). Note the scale differences on the left.



position to demarcate swing initiation and termination,vertical and lateral excursions, and the transition fromupswing to downswing. Swing initiation was demarcatedby the lowest vertical position of the wrist immediatelyprior to the initiation of the wrist’s vertical ascent dur-ing upswing. Swing termination was demarcated by thelowest vertical position of the wrist immediately follow-ing the termination of the wrist’s vertical decent duringdownswing. The transition from upswing to downswing(TR) was demarcated by the highest vertical position ofthe wrist during each swing (see Fig. 2). Motion at theshoulder joint was estimated by tracking directionalchanges in the horizontal plane at the OP in relation toits starting position (e.g., the transition from flexion toextension during the knapping cycle).Wrist extension/flexion and radial/ulnar deviation

angles were calculated using captured coordinate datafrom the OP, RSP, USP, and the MC II head, and eval-uated through the course of each swing. Using the mid-point between the RSP and USP as the angle vertex(Point A), the following procedure was used to deriveextension/flexion angles (FDV [dorsal/ventral], Fig. 3A)and radial/ulnar deviation angles (FML [medial/lateral],Fig. 3B). Two planes were established: 1) ForearmPlane ML, representing 08 of extension/flexion or theneutral plane, defined by the RSP, USP, and OP,defined as Point B and 2) Forearm Plane DV, the planeperpendicular to Forearm Plane ML which runs alongLine AB, representing 08 radial/ulnar deviation. TheMC II head, defined as Point C, was projected ontoForearm Plane DV (Point CDV) for calculation of FDVor onto Forearm Plane ML (Point CML) for calculationof FML. Extension/flexion angles (FDV) were calculatedas the angle supplementary to BACDV. Radial/ulnardeviation angles (FML) were calculated as the anglesupplementary to BACML (the ulnar deviation anglesreported here, based on the MCII head, are likely to besystematically lower than those reported by studiesthat used the MC III head to calculate radial and ulnardeviation (Youm et al., 1978; Moritomo et al., 2003;Murgia et al., 2004), but ranges of motion would becomparable). All angles were calculated relative to eachsubject’s neutral position as recorded during Phase 1.For statistical analyses, all wrist excursion angles weremeasured in degrees and converted to radians to stand-ardize measurements.

Vertical velocity (v, m/s) and acceleration (a, m/s2)were calculated through each swing at time (t) and posi-tion (x) for each landmark using captured coordinatedata according to:

vðtÞ ¼Xðtþ1Þ � Xðt�1Þðtþ 1Þ � ðt� 1Þ ð1Þ

aðtÞ ¼Vðtþ1Þ � Vðt�1Þðtþ 1Þ � ðt� 1Þ ð2Þ

Angular velocity and angular acceleration at the wristwere calculated through the course of each swing usingFDV. These angles replaced x in Eq. (1). Wrist angles,velocity, acceleration, angular velocity, and angularacceleration were derived using the R-statistical pro-gram language, versions 2.5 and 2.7 (Ihaka and Gentle-man, 1996).Total work per swing (Wtotal, measured in Joules) was

defined as the sum work for the upswing and downswingphases for each swing:

Wtotal ¼ Wup þWdown ð3Þ

The coordinate position of the MC II head was used asa proxy for the hammerstone position for all work pro-duction calculations, unless otherwise stated. Upswingwork production was calculated according to:

Wup ¼ FDdup ð4Þ

where F 5 (9.8 m/s2)(m), m 5 hammerstone mass (kg),and Ddup 5 maximum hammerstone vertical position 2hammerstone vertical position at swing initiation.Downswing work production was defined as the differ-

ence between maximum kinetic energy of the hammer-stone and maximum potential energy of the hammerstone:

Wdown ¼ ð1=2mv2Þ � ð9:8m=s2mDddownÞ ð5Þ

where m 5 hammerstone mass (kg), v 5 maximum ham-merstone velocity (m/s), and Dddown 5 maximum ham-merstone vertical position 2 hammerstone vertical posi-tion at strike.

Fig. 3. Lateral (A) and dorsal (B) views of the forearm with a depiction of extension/flexion (FDV) and radial/ulnar (FML)angles. The midpoint between the radial and ulnar styloid processes is defined as Point A. The olecranon process is defined as PointB. The MC II head is defined as Point C. Forearm Plane ML is defined by the radial and ulnar styloid processes and Point B. Fore-arm Plane DV is perpendicular to Forearm Plane ML and runs along Line AB. Point CDV is Point C as projected onto ForearmPlane DV (A) and Point CML is Point C as projected onto Forearm Plane ML (B). (A) Depiction of the method used to measure theangle of wrist extension/flexion (FDV). Said angle is defined as the supplement to angle BACDV. (B) Depiction of the method used tomeasure the angle of wrist radial/ulnar deviation angles (FML). Said angle is defined as the supplement to angle BACML.



To maintain the error rate across multiple (k) compari-sons, a modified Bonferroni adjustment method wasemployed to determine significance (alpha: 0.05) for allanalyses: the k P-values were ordered and the smallestP-value compared was compared to 0.05/k; if that wasfound to be significant, then the next smallest P-valuewas compared to 0.05/(k 2 1), etc. (Holm, 1979).

RESULTS

Upper limb motion patterns

Flake production knapping cycles were divisible intotwo phases with distinct, consistent sets of motion: anupswing and a downswing phase. Upswing was charac-

terized by upward limb motion, flexion of the shoulderand elbow joints, and increasing wrist extension. Down-swing was characterized by downward limb motion,extension at the shoulder joint, continued elbow flexionand wrist extension through peak wrist extension, fol-lowed by rapid elbow extension and wrist flexion (seeFig. 4). Following the termination of downswing, motionpatterns varied widely by subject and trial. Knappingwas often halted between swings to adjust the core orhammerstone. Subjects rarely maintained a fluid knap-ping rhythm, in which they proceeded directly into thenext upswing following downswing termination.Comparison of the timing and magnitude of peak lin-

ear velocities of limb segment endpoints was used toevaluate joint coordination through downswing-the

Fig. 4. Model of the forearm with the radial styloid process (RSP, orange), the second metacarpal head (MC II, black), and theirrespective paths during a typical knapping cycle. A: Upswing initiation. B: Mid-upswing. C: The apex of upswing. D: 0.06 s afterdownswing initiation at peak wrist extension. E: Strike. F: End of downswing. Upswing (A–C) is characterized by: 1) upward limbmotion, 2) flexion at the shoulder and elbow joints, and 3) increasing wrist extension. Downswing (D–F) is characterized by: 1)downward limb motion, 2) extension at the shoulder joint, 3) flexion of the elbow through peak wrist extension, 4) extension of theforearm following peak wrist extension, and 5) increasing wrist flexion (0.05 s after the initiation of downswing).



knapping phase that is responsible for positioning thehammerstone to strike the nodule. All subjects begandownswing with velocity increasing in the negativedirection (i.e., downward) at the OP, RSP, and MC IIhead (see Fig. 2). The OP reached peak linear velocityfirst, then reduced velocity. The RSP and the MCII headcontinued to increase velocity inferiorly, until their veloc-ities peaked just prior to strike. Poststrike, linearvelocities of all segment endpoints quickly decreasedinferiorly, approaching zero velocity (i.e., all segmentendpoints decelerated while continuing to travel down-ward). The temporal onset of upper limb joint peak linearvelocities showed a partial proximal-to-distal relation-ship, with the OP peaking significantly before the RSPand the MCII head (P \ 0.003 and P \ 0.001, respec-tively). All temporal differences between the RSP andMCII head were insignificant (Table 2). Peak linear veloc-ity at each segment endpoint proceeded in a completeproximal-to-distal fashion, with velocity significantlyincreasing from the OP to RSP (P\ 0.0001), and from theRSP to MC II head (P\ 0.05) for each subject (Table 3).

Wrist extension/flexion and radial/ulnardeviation patterns

Subjects’ peak extension range encompassed 30.6–70.18 (total Subject low and high values, respectively).Subjects’ peak knapping extension grand mean was

47.38 (Table 1). Measured relative to their respectiveneutral positions, Subjects C and D utilized 58.7% and56.4% of their muscular-induced extension range, respec-tively, while Subjects A and B utilized 82.5% and 98.4%,respectively (Table 4). Negative values represent theextent to which subjects failed to flex past their neutralposition. Subjects A and B passed their passive muscu-lar-induced extension maxima (as recorded in Phase 1)in 17% and 30.8% of their trials, respectively. All sub-jects did not approach their passive muscular-inducedflexion maxima, flexing on average 12.82–34.148 belowtheir knapping extension peak and all failed to flexbeyond their respective neutral positions except SubjectC in 20% of his trials.Radial deviation was emphasized over ulnar deviation.

All subjects employed at least 35% of their radial devia-tion range, but only one (Subject B) deviated in the ul-nar direction past his neutral position (Tables 1 and 4).All subjects employed significantly more absolute motionin the dorsal-ventral plane compared to the radial-ulnarplane (Bonferroni adjusted P \ 0.02, Fig. 5). However,each subject employed similar percentages of their re-spective total available range of motion in the dorsal-ventral and radial-ulnar planes (Table 4).The timing of peak extension, angular velocity and

angular acceleration were temporally constrained acrossall subjects in relation to both the transition to down-swing and to strike (Table 5).

TABLE 2. Timing of peak linear velocities (m/s) in the upper limb relative to the transition from upswing to downswing and strike

Subject A, n 5 12 Subject A, n 5 12

Landmark OP RSP MC II Landmark OP RSP MC II

Seconds relative to TR 0.088 0.128 0.132 Seconds relative to ST 20.062 20.015 20.012SD 0.031 0.026 0.023 SD 0.025 0.012 0.013OP to RSP P 5 0.003, t 5 3.394 OP to RSP P\ 0.0001, t 5 5.827OP to MC II P 5 0.001, t 5 3.849 OP to MC II P\ 0.0001, t 5 6.147RSP to MC II P 5 0.745, t 5 0.329 RSP to MC II P 5 0.534, t 5 0.632

Subject B, n 5 26 Subject B, n 5 26


Seconds relative to TR 0.034 0.095 0.101 Seconds relative to ST 20.055 20.011 20.006SD 0.032 0.018 0.019 SD 0.061 0.022 0.018OP to RSP P\ 0.0001, t 5 8.435 OP to RSP P 5 0.002, t 5 3.376OP to MC II P\ 0.0001, t 5 9.067 OP to MC II P 5 0.001, t 5 3.876RSP to MC II P 5 0.215, t 5 1.091 RSP to MC II P 5 0.343, t 5 0.998

Subject C, n 5 10 Subject C, n 5 10


Seconds relative to TR 0.032 0.096 0.092 Seconds relative to ST 20.068 20.004 20.008SD 0.010 0.026 0.023 SD 0.033 0.026 0.023OP to RSP P\ 0.0001, t 5 7.155 OP to RSP P\ 0.0002, t 5 4.8OP to MC II P\ 0.0001, t 5 7.398 OP to MC II P\ 0.0003, t 5 4.692RSP to MC II P 5 0.678, t 5 20.359 RSP to MC II P 5 0.798, t 5 20.359

Subject D, n 5 18 Subject D, n 5 18


Seconds relative to TR 0.057 0.103 0.102 Seconds relative to ST 20.048 20.004 20.006SD 0.016 0.016 0.015 SD 0.025 0.009 0.009OP to RSP P\ 0.0001, t 5 8.907 OP to RSP P\ 0.0001, t 5 6.991OP to MC II P\ 0.0001, t 5 8.848 OP to MC II P\ 0.0001, t 5 6.755RSP to MC II P 5 0.863, t 5 20.216 RSP to MC II P 5 0.67, t 5 20.375

Olecranon process (OP), radial styloid process (RSP), second metacarpal head (MC II), the transition from upswing to downswing(TR), strike (ST). Positive values represent seconds after TR or ST, negative values are prior. Bolded results are significant.



Work production

Knapping work production was calculated at the MCIIhead as described above to generate a work productionbaseline. To isolate the contribution to work productiongained through increased velocity at the MCII head,work production was calculated a second time by substi-tuting velocity and vertical displacement at the MCIIhead with the RSP. All subjects produced significantlymore work employing the greater velocities achieved atthe MC II head during both phases of flake production(P � 0.006, Table 6).

DISCUSSION

The results of this study demonstrate that the wristplays an important role in simple stone tool production,and supports the hypotheses set out in the introduction.First, all knappers displayed broadly similar upper limbmotion patterns. This was evident in the consistentorder of significant events within each subject (i.e., peaksegment endpoint velocity [Table 2]) and among all sub-jects (i.e., peak wrist extension, angular velocity, andangular acceleration [Table 5]) during the downswing

phase. These similarities were expected given the taskuniformity and subjects’ prior exposure to knappingdemonstrations and theory.The second hypothesis that during downswing upper

limb movements will occur in a proximal-to-distalsequence culminating in rapid wrist flexion just prior tostrike was also supported. The shoulder, elbow, andwrist joints moved in a coordinated fashion through theupswing phase. However, subjects replaced a rigid upperlimb with mobile joints that act synergistically—a transi-tion from simple to complex motion patterns (Newell andvan Emmerik, 1989)—following the transition to down-swing. A mobile upper limb offers two advantages overmaintenance of a rigid limb. One, subjects are able toutilize a proximal-to-distal joint sequence in which themost proximal joint begins forward motion before thedistal joints, and the proximal joint reaches peak linearvelocity and begins to slow down prior to the distal jointsreaching their respective peak linear velocities (Fig. 2;Table 2) (Putnam, 1991). This motion sequence canresult in a velocity ‘‘summation effect’’ at the most distaljoint due to torque interactions among the preceding jointssuch that the distal joint experiences greater velocitiesthan could otherwise be achieved (Bunn, 1955; Southard,1989; Putnam, 1991). The results of this summation effectare evident in the knapping kinematics reported here;segment endpoints’ linear velocities significantly increasedfrom the OP to RSP and from the RSP to MC II headduring flake production knapping cycles (Table 3).The second advantage of a mobile upper limb is the ac-

curacy increase afforded by disassociating motion of thedistal joint from the proximal joints (Bernstein, 1967;Arutyunyan et al., 1968; Newell and van Emmerik,1989; Southard, 1989; Anderson and Sidaway, 1994;Hore et al., 1996). Bernstein (1967) hypothesized thatadditional degrees of freedom are liberated through jointdisassociation, thus increasing the potential to coordi-nate a greater number of degrees of freedom andincrease motor control. However, Hore et al. (1996) cau-tioned that the magnitude of the effects of joint kinemat-ics on strike accuracy increases in a proximal-to-distalfashion. Therefore, the wrist joint plays a greater rolethan more proximal joints in determining strike accu-racy by virtue of its distal position. Joint disassociationwas evident in the timing and magnitude of segmentendpoints’ peak linear velocities (Tables 2 and 3). Tempo-ral disassociation between the RSP and MC II head wasnot evident, which may be due to subjects’ relative inex-perience with this behavior, resulting in reduced capabil-ity to achieve joint disassociation (Bernstein, 1967).Given its utility in other accuracy-seeking activities,such as pistol shooting, writing, throwing, and kicking,we suggest that joint disassociation is naturally similarlyemployed during stone tool production to increase strikeaccuracy (Bernstein, 1967; Arutyunyan et al., 1968;Newell and van Emmerik, 1989; Southard, 1989; Ander-

TABLE 3. Peak linear velocities (m/s) in the upper limb duringdownswing

Subject A

Landmark OP RSP MC II

Peak velocity 21.136 23.321 24.080OP to RSP P\ 0.0001, t 5 213.28RSP to MC II P 5 0.004, t 5 23.178

Subject B


Peak velocity 20.124 22.216 23.148OP to RSP P\ 0.0001, t 5 228.796RSP to MC II P\ 0.0001, t 5 28.176

Subject C



Subject D



Olecranon process (OP), radial styloid process (RSP), secondmetacarpal head (MC II). All results are significant.

TABLE 4. Percent of total wrist excursion employed during knapping

Extension Flexion Radial Ulnar Extension-flexion Radial-ulnar

Subject A 82.51 251.89 38.77 24.30 20.88 18.84Subject B 98.40 252.27 50.58 14.45 26.91 23.25Subject C 58.71 29.82 50.77 20.03 23.10 24.41Subject D 56.44 234.39 35.15 23.07 8.94 13.67

The percentage of motion in any one direction represents the motion utilized in that direction relative to each subject’s relevantneutral plane.



son and Sidaway, 1994; Hore et al., 1996). Ongoing kine-matic studies are being conducted with skilled knappersto directly measure strike accuracy as it relates to jointdisassociation.The consistent proximal-to-distal sequence supports

the hypothesis that the wrist plays an important role instone tool production, through greater mechanical workand likely greater accuracy. The wrist reaches peakextension 0.05 s after the transition to downswing, set-ting up the wrist for flexion prior to strike (Table 5). Thedegree of wrist extension (individual means of 36–668 forsubjects in this study) used is likely a function of themanner in which the moment arms and tensile strengthsof the forearm’s flexor muscles change with the degree ofwrist extension/flexion. The relationship between themoment arms of two of the forearm’s flexors (FCU andflexor carpi radialis) and wrist posture approximate posi-tive second order polynomial relationships (Pigeon et al.,1996), meaning that their mechanical advantageincreases as wrist extension increases. Thus, by utilizinggreater degrees of wrist extension knappers are able tomore fully exploit the forearm’s flexors.The rapid release of peak wrist extension (i.e., burst of

wrist flexion), illustrated by angular velocities and accel-erations recorded at the wrist, peaked at 0.019 s and0.032 s prior to strike, respectively (Table 5). In theirEMG study of muscle recruitment during stone tool pro-duction, Marzke et al. (1998) reported that FCU muscleactivity in the hammer hand peaked immediately priorto strike, inducing a wrist ‘‘flick’’ which further acceler-ated the hammerstone toward the nodule. The peakangular velocities and angular accelerations recordedhere are likely due in part to strong FCU recruitmentfollowing peak wrist extension.The relationship between muscles’ mechanical advant-

age and the position of the wrist also appears to influ-ence the degree of flexion subjects utilized. All subjects

employed greater than 50% of their respective extensionrange, but subjects flexed minimally out of peak exten-sion, and typically avoided wrist flexion past their neu-tral position (Tables 1 and 4). Maintaining the wrist inan extended position may reflect a strategy to maintainhammerstone control against the strong reaction forcesproduced at strike. According to Pigeon et al. (1996), thedigital flexors are weaker when the wrist is held in aflexed position compared to an extended position, whichmay render the hammerstone more susceptible to dis-placement when the wrist is strongly flexed. By avoidingexcessive wrist flexion the knapper may be better able tomaintain a tighter grip on the hammerstone. The avoid-ance of strong flexion, particularly past Forearm PlaneML, has also been reported in pitching activities(Debicki et al., 2004), during which flexion-inducinginteractive torques produced by the more proximal upperlimb segments and experienced at the distal joints weredampened by heightened forearm extensor activity. Thedampening effect on the part of the extensors allowedpitchers to better control wrist flexion and ball release,thereby increasing target accuracy.The results of this study also support the final hypoth-

esis that rapid wrist flexion from an extended positionsignificantly contributes to the production of mechanical

Fig. 5. Total extension-flexion (grey) and radial-ulnar deviation excursions (white), measured in degrees. Extreme outliers (datapoints greater than three times the box height from the 25th and 75th percentiles) are not shown. Individual subject (left) and com-posite results (right) are reported.

TABLE 5. Timing of peak extension, angular velocity (m/s) andangular acceleration (m/s2) at the wrist

Subject total, n 5 66 Time 695% CI

Peak extension TR 1 0.053 60.006Peak extension ST 2 0.055 60.008Angular velocity TR 1 0.095 60.008Angular velocity ST 2 0.019 60.006Angular acceleration TR 1 0.078 60.01Angular acceleration ST 2 0.032 60.007

Transition from up-swing to down-swing (TR), strike (ST).



work (and by extension to strike force given F ¼ dðmvÞdt ;

Table 6). This contribution is evident in comparing thesignificantly greater linear velocities attained at theMCII head and the RSP. Achieving significantly greaterlinear velocities and rapid angular velocities and acceler-ations as the wrist flicks the hammerstone toward thenodule may be dependent on reaching a minimumdegree of wrist extension due to the mechanical relation-ship between the FCU and FCR and the degree of wristextension (Pigeon et al., 1996). The observation that thewrist and particularly wrist extension plays an impor-tant role in the biomechanics of stone tool-making isconsistent with hypotheses suggesting that stone tool-making, as well as other upper limb dependent activ-ities, may have influenced the evolution of the homininwrist.Kanzi, a male bonobo chimpanzee that has been prac-

ticing stone tool production since 1990, provides a readycontrast to the knapping strategies reported here and anillustration of the effects on stone tool production ofupper limb anatomy that is primitive in some respects.Toth et al. (1993) reported that Kanzi preferred to knapusing a ‘‘hard, rapid thrust’’ to throw cores against hardobjects, rather than employing a hand-held hard ham-mer method. The authors stated that with a hard ham-mer, Kanzi’s applied force was often insufficient for frac-turing rocks in a manner producing useful cores, flakes,or edges. Resulting cores were described as ‘‘simple,’’with ‘‘noninvasive flake scars and steep, battered edges,’’(Toth et al., 1993), markedly different from descriptionsof the earliest Oldowan cores from Gona and Lokalalei(Roche et al., 1999; Semaw, 2000; Semaw et al., 2003).The difference in preferred method may have been aneffect of Kanzi’s inability to employ effectively the hardhammer method due in part to Pan’s limited wrist exten-sion, as well as more limited opposability between thethumb and ulnar fingers (Marzke, 1983; Guthrie, 1991),differences in thumb musculature and robusticity(Susman, 1994; Marzke et al., 1999; Diogo and Wood,2009) and carpal arrangement (Lewis, 1977; Marzke,1983; Tocheri et al., 2005, 2008; Marzke et al., 2010).However, Kanzi’s knapping ability and stone tool

assemblages associated with multiple hominoid speciesprovide strong evidence that more than one hominoidspecies practiced stone tool behaviors (Susman, 1988;Marzke, 1997; Morwood et al., 2004; Mercader et al.,2007). Thus rather than adhering to what increasinglyappears to be a false dichotomy between the ability orinability to fashion and use stone tools, it may prove

more beneficial to investigate the factors involved ineffective and efficient tool use and production, such asmechanical efficiency and strike accuracy.

CONCLUSIONS

Our analysis of upper limb motion during simple flakeproduction shows that during downswing the upper limbmovements occur in a proximal-to-distal sequence, cul-minating in rapid wrist flexion just prior to strike, andthat gross limb motion patterns are consistent (e.g., inorder and timing of key events) within and among sub-jects. In this manner, knapping is similar to other com-plex upper limb activities involving striking or throwing,such as hammering, pitching, and dart-throwing. Theresults also show that during knapping use of a mobileversus rigid wrist significantly increases mechanicalwork, and is also thought to be critical for strike accu-racy. Wrist extension plays an important role in thisincreased mechanical work by positioning the hand foreffective flexor muscle recruitment and rapid flexion im-mediately prior to strike. These findings support the hy-pothesis that knapping, as well as other complex upperlimb activities, was an important factor in the evolution-ary reorganization of the human wrist. Further, the useof the higher degrees of wrist extension afforded to mod-ern humans has a demonstrated impact on economic andeffective stone tool production, providing a method toincrease joint linear velocity, strike force, and potentiallystrike accuracy. Future studies would benefit from anapproach investigating the factors involved in effectiveand efficient tool use and production, such as mechanicalefficiency and strike accuracy.

ACKNOWLEDGMENTS

The authors would like to thank the subjects who par-ticipated in this study and Can Kirmizibayrak for hisinvaluable assistance in the MOCA laboratory. We alsothank Chris Ruff and two anonymous reviewers for theirhelpful comments on the manuscript.

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TABLE 6. Work production (J) at the second metacarpal head and the radial styloid process

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n 12 26 10 18

UpswingLandmark MCII RSP MCII RSP MCII RSP MCII RSPMean 3.552 2.909 1.776 1.386 1.800 1.383 2.322 1.861SD 0.796 0.655 0.239 0.206 0.347 0.299 0.264 0.238P 0.004 \0.0001 0.006 \0.0001t 22.198 26.293 23.094 25.581

DownswingLandmark MCII RSP MCII RSP MCII RSP MCII RSPMean 3.077 1.500 1.336 0.051 1.975 0.720 2.986 1.948SD 1.432 1.024 0.772 0.424 1.129 0.712 0.906 0.945P 0.005 \0.0001 0.005 0.002t 23.144 27.440 23.214 23.414

All results are significant.



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