emg study of hand muscle recruitment during hard hammer percussion manufacture of oldowan tools

EMG Study of Hand Muscle Recruitment During Hard HammerPercussion Manufacture of Oldowan Tools

MARY W. MARZKE,1* N. TOTH,2 K. SCHICK,2 S. REECE,1B. STEINBERG,1 K. HUNT,2 R.L. LINSCHEID,3 AND K-N. AN3

1Arizona State University, Tempe, Arizona2Indiana University, Bloomington, Indiana3Mayo Clinic, Rochester, Minnesota

KEY WORDS stone tool-making; electromyography; flexor pollicislongus; fossil hominids

ABSTRACT The activity of 17 hand muscles was monitored by electromy-ography (EMG) in three subjects during hard hammer percussion manufac-ture of Oldowan tools. Two of the subjects were archaeologists experienced inthe replication of prehistoric stone tools. Simultaneous videotapes recordedgrips associated with the muscle activities. The purpose of the study was toidentify the muscles most likely to have been strongly and repeatedlyrecruited by early hominids during stone tool-making. This information isfundamental to the identification of skeletal features that may reliablypredict tool-making capabilities in early hominids. The muscles most fre-quently recruited at high force levels for strong precision pinch grips requiredto control the hammerstone and core are the intrinsic muscles of the fifthfinger and the thumb/index finger regions. A productive search for skeletalevidence of habitual Oldowan tool-making behavior will therefore be in theregions of the hand stressed by these intrinsic muscles and in the jointconfigurations affecting the relative lengths of their moment arms. Am J PhysAnthropol 105:315–332, 1998. r 1998 Wiley-Liss, Inc.

Who made Oldowan stone tools? This is arecurring question in paleoanthropology, re-cently raised again with the discovery ofOldowan stone tools in Ethiopia at a leveldating to 2.5–2.6 Ma (Semaw et al., 1997),contemporary with australopithecines andearlier than the most ancient known mem-ber of the genus Homo reported by Kimbel etal. (1996).

The route to the answer should be throughthe fossil hominid hands that were capableof making the tools. Several attempts havebeen made to infer tool-making capabilitiesfrom available skeletal features in fossilhand bones such as muscle attachment ar-eas, bone and joint surface configurations,relative metacarpal robusticity, hand seg-ment proportions, and skeletal topographyaffecting the length of tendon moment arms(Napier, 1962; Ricklan, 1987, 1990; Susman

1988a,b, 1989, 1991, 1994; Susman andCreel, 1979; Susman and Stern, 1979; Sus-man et al., 1984; Marzke, 1997). However,these inferences have been drawn in theabsence of data that indicate 1) whichmuscles are most heavily recruited duringOldowan tool-making, and 2) which skeletalfeatures may be relied upon as predictors ofthe relative size and moment arm lengths ofthese muscles.

In the experiments reported here, we di-rectly sought such data from the perspectiveof tool-making, as a necessary preparationfor detailed examinations of fossil bones.Using electromyography (EMG), we identi-

*Correspondence to: Mary W. Marzke, Department of Anthro-pology, Arizona State University, P.O. Box 872402, Tempe, AZ85287-2402.

Received 30 April 1997; accepted 12 November 1997.

AMERICAN JOURNAL OF PHYSICAL ANTHROPOLOGY 105:315–332 (1998)

r 1998 WILEY-LISS, INC.

fied muscles that are recruited repeatedly athigh levels when a hammerstone held by onehand strikes flakes from a core held in theother hand. Our assumption is that the moststrongly recruited muscles are likely to havegenerated recognizable periosteal reactionsat their attachment sites which could beidentified in hand bones of early hominidtool-makers. This assumption is currentlybeing tested in our laboratory.

In a related kinematics experiment (Mar-zke et al., submitted), we found that eachthumb muscle, with the exception of thetransverse adductor pollicis muscle, has asignificantly larger dynamical moment armin humans than in chimpanzees for at leastone axis of rotation and one thumb joint.Thus, we now have a potential additionalsource of evidence in fossil hominid handsfor stone tool-making capabilities, namely,the orientations and dimensions of skeletalfeatures that determine the relative lengthsof moment arms for the tendons of themuscles that are strongly recruited. A largermoment arm gives a muscle greater me-chanical advantage, maximizing the poten-tial of the muscle to exert torque whileminimizing the number of muscle fibersneeded, thus conserving the energy requiredfor repetitive muscle contraction. Evolution-ary increase in moment arm lengths thuswould have enhanced habitual, effective,hard hammer percussion manufacture ofstone tools, well known by practitioners tomake heavy demands on several groups ofhand muscles. How large these demandsare, and upon which muscle groups, are setforth in the following sections.

MATERIALS AND METHODS

The experiments were conducted in thegait analysis facility of the Mayo ClinicOrthopedic Biomechanics Research Labora-tory in Rochester, Minnesota. The doctorswho inserted the electrodes and the staffwho coordinated data acquisition are allregular participants in the EMG analysis ofclinical disorders of patients who come tothe facility, including patients with prob-lems in the hand.

Subjects

The three authors from Indiana Univer-sity were the subjects of the experiments.

Toth (Subject 1) was selected because he hasbeen manufacturing Oldowan tools by hardhammer percussion regularly for nearly 20years. It can be expected that in the courseof these years he has settled into grips andhand movements that expedite the produc-tion of Oldowan flakes with minimum musclefatigue and pain-producing stress to thejoints. Schick (Subject 2) was selected be-cause she also learned to replicate Oldowantools and has continued the activity overmany years. Because her tool-making activ-ity has been more sporadic, it was antici-pated that she might use some muscle re-cruitment patterns that differ from those ofToth. Hunt (Subject 3) served as the novicetool-maker, whose muscle use we thoughtmight differ still more and might therebyput into relief the patterns in the moreexperienced subjects, and those most likelyto have been arrived at by early hominidsthat habitually made Oldowan tools.

Toth and Schick are right-handed andHunt is left-handed. The terms ‘‘dominant’’and ‘‘nondominant’’ will therefore be appliedto the specific hand used by a subject. Domi-nant hand length and breadth were 18.7 38.6 cm in Subject 1, 17.6 3 8.2 cm in Subject2, and 19.6 3 8.3 cm in Subject 3.

The importance of testing at least threesubjects with differing expertise is thatmuscle recruitment patterns found to becommon to all of them are likely to bepatterns that are most compatible with hu-man hand morphology. The common pat-terns suggest to us the grips and associatedhand musculoskeletal features that shouldbe targeted for analysis in the developmentof biomechanical models for the functionalinterpretation of similar morphology in an-cestral hominids.

Stone tool materials

Stones collected from an Indiana quarrywere used for the experiments. They weremedium-grained, river-worn cobbles withsmall crystal inclusions which can be flakedequally well in any direction, closely similarin composition to stones used by the Old-owan tool-makers.

Muscles and activities monitored

Seventeen muscles were monitored withEMG in the course of the study (Table 1), 16

316 M.W. MARZKE ET AL.

functioning on the hand and one (flexorcarpi ulnaris) exclusively on the wrist. Eightare extrinsic and nine are intrinsic muscles.Ten channels of A/D conversion were usedfor receiving muscle signals. One set of tenmuscles in the dominant hand and a slightlydifferent set of ten muscles in the nondomi-nant hand were monitored in the first experi-ment. A year later, a third set of muscles wasmonitored in both hands of Subjects 1 and 2,including four muscles that were not moni-tored in Subject 3 in the first experiment(flexor digiti minimi, adductor pollicis (trans-verse portion) and the first and third palmarinterosseous muscles), and four muscles thatwere monitored for only one hand in Subject3 (the extensor pollicis longus muscle in thedominant hand and the flexor pollicis brevis,first dorsal interosseous, and extensor polli-cis brevis muscles in the nondominant hand).Records were obtained from only one handat a time in each session.

By monitoring a large number of muscles,we were able to identify those muscles thatare most consistently recruited at high lev-els to control stones during hard hammerpercussion. This information allows us toconcentrate in our biomechanical analyseson those regions of the hand skeleton thatare the major foci of stresses associated withstone tool-making and are therefore mostlikely to reveal traces of this activity infossils.

Selection of the muscles was based onseveral criteria. First, muscles known toleave obvious marks of their attachments onbones (including fossils) were preferred (OP,ODM, FPL, D1 (muscle abbreviations aregiven in Table 1); see Napier, 1959, 1962;Day and Scheuer, 1973; Musgrave, 1971;Trinkaus, 1983; Susman, 1988b, 1989). Sec-ond, we targeted those whose moment armsare believed to be enhanced by distinctively

human skeletal configurations (FCU, FPL,FPB, FDM, ODM; see Napier, 1959; Sternand Susman, 1983; Trinkaus, 1983; Ricklan,1987; Sarmiento, 1988). Third, we focusedon muscles of the thumb, index, and fifthfinger that seemed to us most likely to bestrongly recruited in tool-making, based onfrequent grips and hand movements ob-served in our earlier study of tool-making(Marzke and Shackley, 1986). We wereguided in muscle selection (and later in theinterpretation of our data) by predictions ofmuscle interaction in moving and stabilizinglink systems of the hand and wrist, based onthe detailed functional analysis of handmuscle functions by Brand and Hollister(1993) and on EMG studies which recordedmuscle signals during prescribed grips andmovements (Forrest and Basmajian, 1965;Hall and Long, 1968; Close and Kidd, 1969;Long et al., 1970; Long, 1981; Cooney et al.,1985; Hepp-Reymond et al., 1996; Maier andHepp-Reymond, 1995). Fourth, muscles dis-cussed in other functional analyses of handsin connection with grips used in tool-makingwere included (FCU, FPL; see Ricklan, 1987;Susman, 1988a,b).

Each subject sat on a stool and struckflakes from a core held in the nondominanthand with a hammerstone held in the domi-nant hand. Flake removal continued untilthe core was small, allowing for the observa-tion of changes in grip of the core accompany-ing changes in its size.

All subjects used the same hammerstonefor each session and selected cores of roughlythe same size and shape for reduction. In thefirst year’s experiment the hammerstonewas 10 3 8 3 6 cm and weighed 605 g. Thenext year there were two sessions for thedominant hand of both subjects, one with alarge hammerstone (9 3 9 3 7 cm, 780 g)and the second with a small hammerstone

TABLE 1. Muscles monitored in the EMG experiments

Extrinsic muscles Abbreviation Intrinsic muscles Abbreviation

Flexor carpi ulnaris FCU Flexor pollicis brevis FPBFlexor pollicis longus FPL Opponens pollicis OPFlexor digitorum profundus 2 FDP2 Adductor pollicis (transverse) APTFlexor digitorum profundus 5 FDP5 Dorsal interosseus 1 D1Extensor digitorum communis 2 EDC2 Palmar interosseus 1 P1Extensor digitorum communis 5 EDC5 Palmar interosseus 3 P3Extensor pollicis brevis EPB Flexor digiti minimi FDMExtensor pollicis longus EPL Abductor digiti minimi ADM

Opponens digiti minimi ODM

317EMG OF HAND MUSCLES IN OLDOWAN TOOL-MAKING

(7 3 6 3 6 cm, 400 g), but only the smallhammerstone was used when the nondomi-nant hand was monitored.

Procedures for recording musclerecruitment and tool-making activities

Electrode placement and signal trans-mission. Insulated nichrome intramuscu-lar wire electrodes (0.03 mm in diameter)were inserted into the muscles with a 25gauge hypodermic needle. Entry points wereselected that would not interfere with thegrips and hand movements. Electrode place-ment was verified by isolated muscle func-tion testing. The signals were monitoredclosely during the activities since there wasa potential for electrode displacement be-cause of the nature and forcefulness of theactivities. Displacement proved to be rarebut was corrected whenever observed andthe activities were repeated.

Eleven channels of data were collected at728.36 samples per second per channel. TheEMG signals were amplified by a preampli-fier (Motion Lab Systems, Inc., Baton Rouge,LA) worn by the subject in a backpack. Fromthe preamplifier the signals went to aninterface unit (Motion Lab Systems) andthen to an A/D converter operated from anIBM-compatible personal computer. The A/Dconverter was controlled through CODAS(computer-based oscillograph and data acqui-sition system) software. Both the A/D con-verter and software are products of DATAQInstruments Inc., Akron, Ohio.

Hammerstone strike recording. Ten ofthe 11 channels carried signals for themuscles. The eleventh channel carried sig-nals from a microphone, placed near thesubject and connected to the A/D converter.The microphone detected the sounds pro-duced by strikes of the hammerstone on thecore. The pulsed strike waveforms were dis-played on the computer screen above the tenmuscle channels, enabling observation ofmuscle signals relative to the time of strike.

Videotaping. The subject was videotapedduring the activities using a Sony Triniconvideo camera recording at a speed of 60frames/second. The muscle signals and videoimages were simultaneously transmitted toa screen and displayed side by side. Reflec-

tive markers were placed over the radial andulnar styloid processes and over the first,second, third, and fifth metacarpal heads,proximal phalanges, and distal phalanges tofacilitate qualitative observations of thumb,finger, and wrist movements.

Measurement of force potential

Prior to each tool-making session, we mea-sured the maximum force of isometric con-traction for all muscles containing elec-trodes using both the grasp meter and thepinch meter with the NK Hand AssessmentSystem. For each precision grip, the pinchdevice was enclosed in an insulating padand placed between a stone and the thumbor fingers (for a table of human precisiongrips, see Marzke, 1997). The maximumsignal for each muscle provided a baselineagainst which muscle signals from the tool-making activities were compared. When ac-tivity signals exceeded the isometric ones,the maximum activity signal was used asthe baseline.

Data analysis

The signals from muscles recruited duringthe tool-making activities and the isometricsessions were rectified, filtered, and thenanalyzed using WINDAQ software. Theanalysis of muscle signals for each tool-making session was restricted to data col-lected over a range of 16 strike signals. Eachmuscle signal at strike and each peak musclesignal in a strike cycle was compared in sizewith the baseline maximum signal regis-tered for the muscle in the isometric session(or in some cases, reached during the activ-ity) and was assigned to a percentage levelof none (0–5%), low (6–30%), moderate (31–55%), marked (56–80%) or maximum (81–100%). These strike and peak levels werethen entered into a spreadsheet, along withthe time of each signal, for use in the com-parison of patterns and levels of musclerecruitment among the three subjects.

Before the comparison, a modal level (rep-resenting central tendency) was determinedfor each series of 16 strike signals and foreach series of 16 peak signals for eachmuscle of both hands in all subjects andsessions. Each series of signals was re-garded as a series of 16 trials. Peak signals


for trials varied by two levels in 48% of theseries. Variation was by three levels in 27%,while 20% had a single level. Variation wasover four levels in 5% of the series.

Results of the comparisons were evalu-ated and categorized as follows.

Category 1: Muscles that exhibit markedand maximum modal peak levels in all threesubjects for one or both hands are consid-ered the most important indicators of ha-bitual muscle recruitment during stone tool-making in early hominids. In some cases,the importance of these muscles is empha-sized by their strong recruitment in multiplesessions for Subjects 1 and 2. These we feelare the muscles that direct us to the regionsof the hand and wrist skeleton that are mostlikely to reflect tool-making capabilities inearly hominids.

Category 2: Muscles that exhibit highmodal peak levels in Subjects 1 and 2 alsohave strong implications for early hominidmuscle use during Oldowan tool-making.The category is limited in number, but in-cludes the two experienced subjects.

Category 3: Muscles whose modal peaksare below the marked level in Subject 1, orwhich dropped below marked/maximum lev-els in two of the three sessions for Subject 1,are considered less likely potential stressorsof fossil hominid hand skeletons than thoseexhibiting consistently higher levels.

The reason for our emphasis on data fromSubject 1 is that his many years of tool-knapping are likely to have led him tomuscle recruitment patterns that are mostsimilar to those most effective and economi-cal in early hominid tool-makers who prac-ticed these activities habitually.

The very specific purpose of this studyshould be kept in mind. The relative impor-tance of muscles was judged on the basis oftheir potential contribution to stress on thehand bones, since it is the bones that mustserve as clues to tool-making capabilities infossil species. This weighting of muscles inrelative importance is different from weight-ing that might be applied if we were examin-ing their relative roles as prime movers,antagonists, synergists, and fixators in givenactivities. A muscle that peaks at a low levelmay be an essential contributor to fixation ofa proximal joint during forceful action by

another muscle on a more distal joint, but itsrole is not likely to be recorded by stressmarkers at its attachment points. However,its role could be reflected by relatively largertendon moment arms that may have in-creased in the course of human evolution.The increase would have been advantageousfor keeping energy requirements at the low-est possible level for contraction of the fullcomplement of muscles during tool-makingand other manipulative activities.

RESULTSGrips

Grips were identified at each of the 16strikes for all activities and all subjects.There was consistency among subjects inthe nature and range of grips. To someextent this is to be expected, because Sub-jects 2 and 3 had learned some techniquesfrom observing Subject 1. However, the gripsare very similar to those used independentlyby Shackley in a preceding, unrelated experi-ment (Marzke and Shackley, 1986).

The hammerstone was invariably held bya precision three-jaw chuck grip (Marzke,1997) or by modifications of this grip. Sub-ject 1 tended to place the index finger nearthe third finger, where it avoided directvertical reaction forces of the core. In allsubjects, the side of the fourth finger but-tressed the side of the hammerstone and thefifth finger buttressed the fourth for allhammerstone grips (Fig. 1).

The core was held primarily by precisioncradle grips (Marzke, 1997), involving avariable number of fingers depending on thecore’s size (Fig. 2). The palmar aspects of thefingers were oriented upward, stabilizingthe base of the core against downward blowsby the hammerstone. The flaking edge of thecore rested on the distal finger pads orslightly overhung them. The larger coreswere pressed by the volar aspects of thefingers against the volar aspect of the freethumb and the thenar side of the palm. Asthe cores became smaller, either the volarpads of the fingers and the opposed freethumb maintained the grip or the core wassupported by the volar pads of the fingersagainst an adducted free thumb and thenararea of the palm. Frequently, the wrist wasadducted and flexed, so the flake-removal


surface faced the knapper. The hammer-stone strike of larger cores was usually at aposition on the core above the third-to-fifthfinger distal pads. For smaller cores, thethumb opposed the radial two fingers andflakes were removed above these fingers,while the fourth and fifth fingers were curledout of the way.

The data confirm overwhelmingly an ear-lier finding of Marzke and Shackley (1986)that the precision grip between the thumband volar pad of the index finger simply isnot recruited during Oldowan stone tool-making.

Muscle recruitment

All peak muscle force levels reported be-low are modal levels, shown for all musclesof the dominant hand in Table 2 and for thenondominant hand in Table 3. The musclesare grouped in their order of importance asindicators of prehistoric muscle recruit-ment, based on the criteria in the Materialsand Methods section on data analysis. It isinteresting to note that only three of the ten

Fig. 2. (A) Grips used for cores. Cradle five-jaw gripof large core. (B) Cradle three-jaw grip of small core.

Fig. 1. Grips used for hammerstones. (A) Three-jawchuck full finger grip of small hammerstone. (B) Three-jawchuck full finger grip of large hammerstone, with fourthfinger buttress. Note the somewhat lateral position of theindex finger in (B), typical of the grip by Subject 1.


Article ID # 917 (disk)@xyserv2/disk8/CLS_liss/GRP_phan/JOB_phan105-3/DIV_917z14 angh

muscles that peaked consistently at thesehigh levels were extrinsic muscles (FCU,FDP2, and FDP5).

Muscles with marked/maximum modalpeaks in one or both hands of all sub-jects (Category 1). The FPB musclepeaked at marked/maximum levels in thedominant hand of all subjects, including allthree sessions for Subjects 1 and 2. Allsubjects exhibited marked/maximum levelsof the FCU in the single sessions for bothhands. In the dominant hand, the D1 peakmode was maximum for all three sessions inSubjects 1 and 2 (with the exception of onesession of marked for Subject 2), and wasmarked for Subject 3 in his single session.The FDP2 muscle of the dominant hand wasat marked/maximum levels in two of threesessions for Subject 1, in all three sessionsfor Subject 2, and in the single session forSubject 3. The FDP5 muscle contracted inthe dominant hand of all subjects during onesession each at marked/maximum levels.

Muscles with marked/maximum modalpeaks in one or both hands of the twoexperienced subjects (Category 2). Forthe sessions in which all three subjectsparticipated, two muscles fall into Category2. In the dominant hand of the two experi-enced subjects, the OP peaked at the marked

level while the ADM was recruited at thehighest level (peaks for both muscles weremoderate in Subject 3). For the sessions inwhich only the two experienced subjectsparticipated, four muscles have modal peaksat the marked/maximum levels. In the non-dominant hand, the APT and P1 musclespeaked at maximum level and the FPB atmarked/maximum levels. The FDM musclewas similarly strongly recruited, in this casefor the dominant hand across two sessionsand for the nondominant hand in the onesession for which it was recorded.

Muscles with modal peaks below themarked level in Subject 1 (Category 3).The FPL of the dominant hand in Subject 1ranged from marked in the first session tolow in the next two sessions. In Subject 2 itranged from marked in the first session tomaximum in the next two sessions. It wasmarked in Subject 3 for the single session inwhich it was monitored. In the nondominanthand, FPL never reached a maximum levelin any subject. It was marked in one sessionfor Subject 2, and otherwise was at a moder-ate level for Subjects 1 (two sessions), 2 (onesession), and 3 (one session).

The EPL, EPB, and P3 muscles were all atmoderate/low levels in both hands of Subject1, compared with marked/maximum levels

TABLE 2. Modal peak levels of all muscles monitored: dominant hand

Subject/session1

Muscle

FPB OP D1 P1 APT FDM ADM ODM P3 FCU FPL EPL EPB FDP2 FDP5

1/1 mrk mrk max max max max mrk mod mrk mrk1/2 mrk max max low mrk low low mod low max1/3 mrk max max low mrk mod low mod mod mod2/1 mrk mrk mrk max low max mrk mrk max max2/2 max max mod max max max max max mrk mrk2/3 max max mod max max max max max max max3/1 mrk mod mrk mod max mrk mrk mod mrk mrk

1 Session 1: 1995; session 2: 1996, small hammer; session 3: 1996, large hammer.

TABLE 3. Modal peak levels of all muscles monitored: nondominant hand

Subject/session1

Muscle

FPB OP D1 P1 APT FDM ADM ODM P3 FCU FPL EPL EPB FDP2 EDC2 FDP5 EDC5

1/1 mod mrk low mrk mod mod low low mod none1/2 mrk low max max mrk low mod none low low2/1 mrk NA2 max mrk mod mrk max max mrk mrk2/2 max mrk max max max mod mrk max max max3/1 max mrk max max mod mod mrk mod mrk mrk

1 Session 1: 1995; session 2: 1996, small hammer.2 Missing data.



in Subject 2 (with the exception of moderatefor P3, nondominant hand) and moderate inSubject 3 for the EPL and EPB sessions inwhich he participated. In the nondominanthand, EDC2 peaked at a low level and EDC5was not recruited at all in Subject 1. Thesemuscles peaked at moderate-to-maximumlevels in Subjects 2 and 3.

Finally, it should be noted that the ODMmuscle does not fall into any of the threecategories. It peaked at the maximum levelin the dominant hand of Subject 1, but sincethe second subject exhibiting a high levelwas #3, rather than #2, the muscle does notmeet the criterion of high level recruitmentin the two experienced subjects.

Patterns of muscle recruitment

There is a consistent tendency in all threesubjects for the peak levels of muscle activ-ity during the strike cycle to exceed thelevels recorded exactly at strike (compareTable 4 with Table 2 and Table 5 with Table3). There is also a consistent tendency in thedominant hand for intrinsic thumb/indexfinger muscles (FPB, OP, D1, P1), FDP2 andFDP5 to peak before strike, and for the fifthfinger intrinsic muscles (FDM, ODM, ADM)to peak shortly after strike, although inseveral cases the latter muscles peak at

strike (Table 6; Fig. 3). The EPB also regu-larly peaks after strike.

In the nondominant hand the FDM, ADM,FPB, APT, and EDC2 consistently peak be-fore strike and FPL, D1, EPB, and FCUafter strike (Table 7).

In all subjects the OP, ODM, and to someextent ADM of both hands continue activityat lower levels throughout the cycle. TheFCU is always active at the same time asADM in the nondominant hand.

It can be seen from Tables 6 and 7 that theremaining muscles in the two hands are lessconsistent in recruitment sequence acrosssubjects and across sessions for single indi-viduals.

DISCUSSION

We anticipated that in this EMG study ofcomplex manipulative activities there wouldbe variability in degrees and patterns ofmuscle recruitment among individuals for agiven activity and even for single individu-als repeating an activity over several trialsand several sessions. It has been shown, forexample, that there is no strictly coherentinterindividual pattern of muscle synergiesin humans for clearly specified manipulativetasks, even when kinematic invariances areobserved in these tasks. Instead, the central

TABLE 4. Modal strike levels of all muscles monitored: dominant hand

Subject/session1

Muscle

FPB OP D1 P1 APT FDM ADM ODM P3 FCU FPL EPL EPB FDP2 FDP5

1/1 mod mod mrk mrk mrk max mod low mod mod1/2 mod mod mrk low mod none low low low mod1/3 mod mrh mrk low mod low low low low low2/1 mrk mod low mrk low mod mod low mod mrk2/2 mod mrk low max mrk mrk mrk max low mod2/3 mrk mrk low max mrk mrk mrk max mrk mod3/1 mod low mod low mrk low low low mod low

1 Session 1: 1995; session 2: 1996, small hammer; session 3: 1996, large hammer.

TABLE 5. Modal strike levels of all muscles monitored: nondominant hand

Subject/session1

Muscle

FPB OP D1 P1 APT FDM ADM ODM P3 FCU FPL EPL EPB FDP2 EDC2 FDP5 EDC5

1/1 mod mrk low mod mod mod low low low none1/2 mrk none mrk mod mrk low low none none none2/1 mod NA2 mod low low mrk low mod low mod2/2 max mod mod mrk max low low max mod mrk3/1 mod low mrk mod low mod mod mod mod mrk

1 Session 1: 1995; session 2: 1996, small hammer.2 Missing data.



nervous system appears to use flexible short-term combinations of muscles for the tasks(Maier and Hepp-Reymond, 1995; Hepp-Reymond et al., 1996). However, it was

predicted that some muscles would be re-cruited consistently at high levels duringhard hammer percussion manufacture ofOldowan tools, indicating that these muscles

TABLE 6. Percentage of trials for which peak activity was pre, post, or on strike: dominant hand1

Muscle

Subject 1 Subject 2 Subject 3

Pre Post On Pre Post On Pre Post On

FPB 71 27 2 94 4 2 94 6 0OP 94 6 0 88 12 0 81 19 0D1 63 31 6 100 0 0 88 6 6P1 63 31 62 100 0 0FDP2 77 19 4 56 44 0 63 37 0FDP5 75 13 12 94 6 0 63 37 0FDM 16 84 0 31 56 13ODM 38 50 12 38 56 6 6 81 13ADM 19 63 18 56 38 6 44 50 6EPB 40 60 0 17 79 4 19 81 0EPL 9 84 7 56 38 6FPL 46 48 6 81 19 0 0 100 0APT 38 56 6 78 22 0P3 3 97 0 47 53 0FCU 88 12 0 81 19 0 19 81 0

1 Percentage calculated for total number of strikes across sessions. The number of sessions varied from 1–3.2 Although the percentages for D1 and P1 are the same across sessions 1 and 2 for both subjects, they are different within sessions,indicating that there was not an identity of signals caused by close proximity of the muscles.

Fig. 3. WINDAQ screen display. Hammerstone strike signals are on the top line, followed below bysignals for D1 (line 2), OP (3), FPB (4), ODM (5), and ADM (6). The vertical line runs through thebeginning of a strike signal. Note that the peaks of the three intrinsic thumb muscles (D1, OP, FPB)precede the strike line and the peaks of the intrinsic fifth finger muscles are after strike (ODM) and atstrike (ADM).



are likely to have become important compo-nents of flexible muscle synergies as earlyhominids became increasingly dependent onhard hammer percussion manufacture ofOldowan tools. It was also hoped that pat-terns might emerge in the recruitment ofmuscles associated with distinctive patternsof skeletal morphology in humans, whichmight lead to hypotheses explaining theorigins of the distinctive features.

It turned out that, indeed, five muscleswere consistently strongly recruited (at lev-els of marked to maximum) in at least onehand of all three subjects (FPB and FCU inboth hands; D1, FDP2, FDP5 in the domi-nant hand). These form our top category ofimportance as evidence for likely high-levelmuscle recruitment in fossil hominid tool-making species.

Two additional muscles (OP and ADM ofthe dominant hand) were at marked/maxi-mum levels in the two experienced subjects,compared with a moderate level in the thirdsubject. Although by our criteria this resultplaces the two muscles in Category 2, it isnoteworthy that these are the only two trialseries in which Subject 3 differed whenSubjects 1 and 2 exhibited marked/maxi-mum levels, and the difference was by onlyone level.

Three additional muscles peaked at highlevels for the two experienced subjects in thesessions that did not include Subject 3 (alsoCategory 2): FDM in both hands and APT

and P1 at maximum level in the nondomi-nant hand. The FPB muscle, which peakedat marked/maximum levels in the dominanthand of all subjects, also peaked at thesehigh levels in the nondominant hand of thetwo subjects for whom it was monitored.

Of this total group of ten muscles (one inboth categories, four in Category 1, and fivein Category 2), four are in the region of thefifth finger and six are in the thumb/indexfinger region. Only three (FCU, FDP2, andFDP5) are extrinsic muscles.

Interesting patterns in the recruitment ofthese muscles were discerned, in addition toconsistencies in the levels of activity. Thepossible relevance of these findings to thefunctional interpretation of hand structurein fossil hominids is discussed below.

Distribution and sequence patternsof strongly recruited muscles

In the nondominant hand holding thecore, the fifth finger plays critical roles in 1)maneuvering the core into position for re-moving each flake, and then 2) stabilizingthe core against impact by the hammer-stone. Frequently, the hammerstone strikesdownward right over the fifth finger pad,which is directed upward and cradles thebase of the core, whose edge lies eitherdirectly above the finger pad or slightlyoverhanging it. The fifth finger is flexed andstabilized at the metacarpophalangeal andcarpometacarpal joints by the FDM and

TABLE 7. Percentage of trials for which peak activity was pre, post, or on strike: nondominant hand1

Muscle

Subject 1 Subject 2 Subject 3

Pre Post On Pre Post On Pre Post On

FDM 94 6 0 75 25 0ADM 63 31 6 100 0 0 50 50 0FPB 56 31 13 56 44 0APT 56 44 0 75 25 0EDC2 69 25 6 100 0 0 88 6 6FPL 31 69 0 38 62 0 25 75 0D1 0 94 6 25 75 0EPB 6 94 0 31 69 0FCU 25 75 0 0 100 0 19 81 0EPL 38 62 0 59 41 0 50 50 0FDP2 6 94 0 47 53 0 50 50 0FDP5 63 37 0 12 88 0 44 56 0EDC5 6 88 6 88 12 0 38 62 0OP 94 6 0 19 81 0 19 81 0P1 75 25 0 0 100 0P3 19 81 0 75 25 0ODM 31 69 0 0 100 0 50 44 6

1 Percentages calculated for total number of strikes across sessions. The number of sessions varied from 1–2.



ADM and is held extended at the more distaljoints, providing maximum contact of thevolar skin with the core. (Note that proximaland distal interphalangeal extension is anatural accompaniment to contraction of theADM because this muscle sends a tendon tothe extensor expansion.) As the core is re-duced in size, flake removal moves towardthe levels of the fourth and third fingers andthe fifth finger curls out of the way.

In the dominant hand, the fifth fingerbuttresses the fourth finger, which in turnsupports the lateral side of the hammer-stone. Flexion of the distal phalanx of thefifth finger by the FDP5 keeps it clear of thestriking surface, while contraction of theintrinsic muscles draws the fifth fingeragainst the fourth finger and stabilizes thefifth carpometacarpal and metacarpophalan-geal joints.

The FCU appears to play an importantrole in functions of the ADM muscle. It isalways contracting in the nondominant handwhen the ADM is in action. Since bothmuscles attach to the pisiform bone, whosejoint with the triquetrum allows approxi-mately 1 cm of movement, contraction of theFCU is necessary to stabilize the pisiformfor effective ADM action (Brand and Hollis-ter, 1993). The ADM abducts and stabilizesthe fifth finger carpometacarpal and meta-carpophalangeal joints of the nondominanthand when the finger positions and thenstabilizes the core against displacement byhammerstone strike. The FCU usually peaksafter strike, flexing and adducting againstdownward displacement of the core by thehammerstone.

The FCU function of wrist flexion/adduc-tion was also important in the dominanthand of Subjects 1 and 2. The muscle peakedwhen the wrist flicked, accelerating the stoneimmediately prior to strike. However thewrist was maintained more in line with theforearm in Subject 3 and FCU peaked afterstrike when the subject flexed and adductedthe wrist and rotated the hammerstone 90degrees before beginning the upward swing.

All but one of the thumb/index fingermuscles with marked to maximum peaks(FPB, D1, OP, P1, APT, and FDP2) areintrinsic muscles, acting on the trapezio-metacarpal and both metacarpophalangeal

joints. Since nearly half of D1 originatesalong the proximal ulnar shaft of the firstmetacarpal (Linscheid et al., 1991), it isprobably functioning not only to flex andabduct the index finger but also (when itcontracts with FPB) to stabilize the thumbmetacarpal against the pull of FPB, as pre-dicted by Brand and Hollister (1993). Thesetwo muscles, together with OP, tend to peakprior to strike in the dominant hand, whichis wielding the hammerstone with a three-jaw chuck precision pinch grip. The FDP2muscle functions at the same time relativeto strike, securing the index finger compo-nent of the grip. In the nondominant hand,the APT tends to peak at the same time asFPB relative to strike, in keeping with thefunctional pattern predicted by Brand andHollister (1993). Although P1 is stronglyrecruited in the nondominant hand, therelative timing of its strongest contraction isnot clearly predictable.

The data on muscle peak sequence indi-cate that the muscles at the radial side of thedominant hand prepare the hammerstonefor a forceful strike and then relax the gripright at strike, minimizing the upward reac-tion force of the core on this part of thehammerstone hand. Following strike (or atstrike), muscles on the ulnar side of thedominant hand help to resist displacementof the hammerstone by the core reactionforce and maintain the grip as the arm israised for the next cycle.

In the nondominant hand, the FDM andADM consistently peak before strike, whenthe finger is flexed at the metacarpophalan-geal joint with the phalanges extended un-der the core, performing a crucial function ofstabilizing the core against the force of thehammerstone blow. Peak contraction by theFPB and APT muscles at the same timerelative to strike apparently maintains pres-sure of the thenar region against pressureby the hypothenar region.

The distribution and sequence patterns ofthe strongly recruited muscles lead to theconclusion that an important source of thedistinctively human ability to control coresand hammerstones by one hand lies in theintrinsic muscles at the margins of the hand.Paleoanthropologists therefore need to ex-tend their focus beyond the extrinsic flexor



pollicis longus muscle to skeletal featuresthat reflect the size and moment arms ofthese intrinsic muscles. In particular, fea-tures enhancing torque of the fifth fingermuscles and tolerance of load associatedwith their strong contraction were probablyas important to early tool-making hominidsas features relating to the high recruitmentof intrinsic muscles in the thumb/index re-gion. The probable importance of the fifthfinger in the power (‘‘squeeze’’) grip has beendiscussed previously (Marzke et al., 1992),but our findings here suggest that use of theprecision grips associated with forceful ma-nipulation of stone tools in hard hammerflake removal may also have been a signifi-cant factor early in the evolution of thehominid hand.

Flexor pollicis longus / extensor pollicisbrevis functions

It is interesting that the FPL muscle wasnot consistently strongly recruited at highlevels in any subject for the nondominanthand, or in our most experienced subject (#I)for either hand during strong precision pinchgrips in the tool-making experiments. Maierand Hepp-Reymond (1995) also found intheir EMG analysis that the FPL muscle isnot a primary contributor to pinch grips.There are two possible, related explanationsfor this finding, suggested by Subject 1, whoactually exhibited the lowest overall EMGactivity for the FPL muscle in our sample(Fig. 4). First, he has found that mainte-nance of an extended distal interphalangealjoint of the thumb enhances his grip byproviding contact between the full surface ofthe free thumb and the stone. (Note thatdistal interphalangeal extension of thethumb is a natural accompaniment to meta-carpophalangeal flexion when the thumbmoves into opposition to the fingers, becauseboth the abductor pollicis brevis and adduc-tor pollicis muscles send tendons to theextensor expansion.) Second, he has alsofound that his thumb muscles soon becomefatigued when he uses a firm pinch gripinvolving only the distal thumb pad. Fatiguemay be attributed in part to the smallmoment arm of the FPL, compared with thelarger intrinsic muscle moment arms at themore proximal thumb joints reported by

Smutz et al. (submitted). It has been notedby Brand and Hollister (1993) that use of theFPL muscle for pinch requires much largermoments at the proximal joints than at thedistal because of the need to maintain thumbstability. Thus, it seems likely that Oldowantool-makers would have avoided pinch gripsthat required repeated high-level contrac-tion of the FPL because these grips wouldhave elicited greater thumb muscle contractionoverall, leading to earlier onset of fatigue.

However, in tool-using activities moni-tored immediately after tool-making in ourfirst experiment, we found that FPL wasrecruited by the dominant hand at consis-tently high levels in two of the three subjects(#1 and #2). The muscle peaked at themarked level when a power squeeze gripwas used to wield a cylindrical digging stick,and at the maximum level when they clubbedwith a cylindrical tool. This finding is consis-tent with the observation by Ohman et al.(1995) about the probable role of powergripping in the evolution of human thumbmorphology.

It is also possible that manipulative activi-ties other than tool-making and tool use mayhave been factors in the evolution of thehominid FPL apparatus. In this connection,we are examining thumb use among nonhu-man primates, looking for possible corre-lates among their varied patterns of FPLmusculoskeletal morphology and demands

Fig. 4. Peak level modes for FPL activity in each offive experimental sessions for Subject 1. The dominanthand is shown in sessions 1–3 and the nondominanthand in sessions 4–5. Level 1: low; level 2: moderate;level 3: marked; level 4: maximum.



of their manipulative and locomotor behav-iors.

Like the FPL, the EPB muscle is morefrequently present in humans than in greatapes. It extends the metacarpophalangealjoint independently of the interphalangealjoint, and is thus able to maintain the meta-carpophalangeal joint in extension while thedistal phalanx is flexed by the FPL muscle(Ou, 1979, p. 17, indicates co-contraction ofthe FPL and EPB muscles during thumbinterphalangeal joint flexion). This patternraises the question of whether the twomuscles might have increased in frequencytogether, with the evolution of early hominidmanipulative behaviors requiring this com-bination of thumb joint positions. Threeactivities requiring the combination can becited in support of an affirmative answer.First, the control of large cores by the non-dominant hand requires metacarpopha-langeal extension by EPB while the distalinterphalangeal joint flexes and extends in-dependently in precision maneuvering ofthe core. Second, activities eliciting strongcontraction of the FPL require stabilizationby strong contraction of muscles at all posi-tions around the metacarpophalangeal joint,including the EPB. Since hard hammer per-cussion manufacture of Oldowan tools doesnot seem to require this latter combinationat high levels, other possible tool-making,tool-using, and foraging activities are beingexamined. (It is interesting to note that FPLand EPB both consistently peaked beforestrike during use of the cylindrical tool indigging, using the power squeeze grip.)Third, if the earliest stone tool-makers lackedmodern human relative thumb proportionsand/or large intrinsic thumb muscle torquepotential, an FPL with relatively high torquepotential, together with an EPB to helpstabilize the metacarpophalangeal joint,might have contributed the torques neededto control stones by one hand against exter-nal forces, albeit with fatigue.

Opponens pollicis/opponens digiti minimifunctions

The OP and ODM muscles were distinc-tive during the experiments in their ten-dency to contract in both hands throughoutthe strike cycle. These muscles both origi-

nate from the flexor retinaculum over thewrist, and contraction of one tends to drawthe other into contraction (Forrest and Bas-majian, 1965). They are important position-ing muscles which cup the palm, accommo-dating the palm and fingers to varyingshapes of the stones. They peak in activitywhen strong pinch grips require not onlypositioning but also stabilization of the car-pometacarpal joints of the thumb and fifthfinger during flexion of the metacarpophalan-geal joints.

Possible skeletal correlates of musclefunction

The results of the EMG experiments di-rect our attention to the fifth finger andthumb/index finger metacarpal regions asprimary sources of evidence for habitualhard hammer percussion capabilities in earlytool-makers. The intrinsic muscles, in par-ticular, position the thenar and hypothenarregions of the palm, together with theirphalangeal links, in accommodation to theshapes and sizes of the hammerstones andcores and they generate the force needed tomaintain the grips as the hammerstone isaccelerated and strikes the core. These arethe regions that appear to sustain the great-est stress associated with repeated, strongintrinsic muscle contraction. Following arefour types of skeletal morphology that mightreveal evidence for strong recruitment ofthese muscles.

Large tendon and muscle moment arms.Data have recently been obtained on kine-matic moment arms of thumb tendons inchimpanzees and humans (Marzke, 1997;Marzke et al., 1997a, submitted; Smutz etal., submitted), using methods developed atMayo Clinic (Chao et al., 1989; An et al.,1991). The results show larger moment armsof all thumb muscles in humans except thetransverse adductor pollicis muscle for atleast one axis of rotation at one thumb joint.The differences are significant at the P 50.02 level and below. These findings justify3-D quantitative comparison of contempo-rary and fossil hominid and great ape skel-etal configurations that determine the rela-tive sizes of these moment arms. Thiscomparison is now in progress. Since the



experimental study showed in many cases agreater contribution of moment arms thanmuscle physiological cross-sectional areas todifferences between the species in potentialtorque, reliable skeletal moment arm mea-surements will strongly improve our abilityto recognize human-like intrinsic muscletorque capabilities (and, therefore, stonetool-making capabilities) in fossil hominids.

The larger moment arms in humans pro-vide greater mechanical advantage to thesemuscles. Facility for economical, constantuse of thumb and fifth finger intrinsicmuscles in forceful pinch grips by each handseparately is essential for the one-handedcontrol of stones required by habitual hardhammer percussion tool manufacture. In-trinsic muscle size is limited to some extentby the small surface areas available formuscle attachment in the hand bones. Exten-sion of origins to adjacent muscle fascia isnot a significant option, since it compro-mises the independent actions of musclesrequired for dexterous manipulative behav-ior.1 Intrinsic muscle size is also restrictedby limits to the size of the joint surfaces thatmust accommodate the high loads by thecontracting muscles. It is therefore advanta-geous to have skeletal features that maxi-mize the moment arms of these stronglyrecruited hand muscles, thus enhancingmuscle torque without requiring further in-crease of muscle size.

Tool use involving the power squeeze gripmay explain an interesting feature of thumbskeletal morphology in early hominids thatenhances the FPL moment arm. The largevolar fossa on the distal phalanx of fossilhominid species, which is reported to havebeen the insertion area of tendon fibers foran FPL muscle (Napier, 1962; Trinkaus,1983; Susman, 1988b, 1989; Ricklan, 1990;Marzke, 1997), actually has been found toaccommodate a sesamoid bone, which is

regularly present in the distal interphalan-geal joint capsule of humans (Joseph, 1951)and raises the tendon above the center ofjoint rotation, providing leverage for FPLflexion of the distal phalanx (Marzke et al.,1997b).2 Our recent dissections of the jointhave shown that the sesamoid articulateswith the proximal end of the distal phalanxat its anterior midpoint in humans and inother, nonhuman species. When the distalphalanx flexes, the sesamoid runs into theshaft of the proximal phalanx and the fossaof the distal phalanx abuts against the distalend of the sesamoid. The fossa is filled withareolar tissue, and in some cases with a fatpad. The FPL tendon inserts primarily intoa ridge of bone that forms a horseshoearound the sides and distal end of the fossa.Thus, the size of the fossa may not necessar-ily reflect the size of the muscle in fossils; itprobably does reflect the presence of a sesa-moid, which would have aided in giving themuscle its reasonably large-sized momentarm.

Relatively high first and fifth metacar-pal robusticity. A comparison of the Ha-dar and Sterkfontein australopithecines withchimpanzees and humans shows that theHadar australopithecines resemble chimpan-zees in having a relatively gracile fifth meta-carpal, whereas the Sterkfontein australopi-thecines resemble modern humans, in whichthe fifth metacarpal is the most robust ofmetacarpals 2–5 (Marzke et al., 1992). Asystematic comparison of large samples ofgreat apes and humans is needed to furtherexplore these differences and to determinewhether all apes have relatively gracile fifthmetacarpals. An additional comparative ca-daver study is needed to determine whethervariations in fifth finger robusticity corre-lates with variations in fifth finger musclesize. The same complementary skeletal and

1In fact, dexterity in humans is enhanced by extensive differen-tiation of the thumb muscles. For example, a separate slip of theoblique adductor muscle, the first palmar interosseus, insertsinto the extensor expansion of the thumb rather than into boneand capsule, and another probable offshoot of the oblique adduc-tor muscle, the deep head of the FPB muscle, usually insertsexclusively into the radial sesamoid and the metacarpophalan-geal joint capsule, rather than into the extensor expansion likethe superficial FPB head (Linscheid et al., 1991). These muscleoffshoots are seen in some monkey species, as well as in humans(Lewis, 1989).

2We have found a large sesamoid bone in the distal pollicalinterphalangeal joint capsule of Papio cynocephalus, togetherwith a human-like attachment of the FPL tendon on the distalphalanx. A similar, proportionately smaller sesamoid bone wasfound in a specimen of Pan troglodytes, which has an FPL tendonwith only tenosynovial attachment to radial fibers of the flexorpollicis longus muscle. In both a gibbon and a siamang thumb,the FPL tendon was observed to be raised slightly away from thejoint by a thickening of the fibrous capsule. We suspect thatadditional nonhuman primate specimens, with and without afunctioning flexor pollicis longus muscle, will exhibit a similarpattern.



cadaver studies are needed with regard tofirst metacarpal robusticity. If correlationsare found among relative intrinsic musclesize and relative metacarpal robusticity, thisskeletal feature could be an important indi-cator of tool-making capability in fossilhominids.

Relatively large muscle stress markingson hand bones. It has long been assumedthat variations in the size of muscle mark-ings on hand bones correlate with variationsin hand muscle size in living species (see, forexample, Susman’s 1988b discussion of theopponens pollicis muscle, pp. 152 and 169).This assumption is currently being tested inour laboratory. Among the markings thatare most interesting in connection with tool-making capabilities, and which should bepossible to measure, are the D1 origin areaon the thumb metacarpal and the OP andODM insertion areas along the first and fifthmetacarpal shafts.

The D1 origin area is important because ofthe apparent role these first metacarpalfibers play in stabilizing the metacarpal ofthe dominant hand during strong contrac-tion of the FPB. Presence in a fossil hominidthumb metacarpal of both a large momentarm for the FPB muscle and a large originarea for D1 on the first metacarpal wouldimply habitual use of the hand in activitieslike hard hammer percussion manufactureof stone tools, since this activity elicits con-traction of these muscles at high levels.

Evidence for asymmetries in musclerecruitment

There is evidence in the data for substan-tial differences between the dominant andnondominant hands in muscle recruitment.Differences of three levels were recorded inSubject 1 for D1, ODM, and APT (Fig. 5),and a difference of three levels (ODM) wasfound in Subject 2. Note that the higherlevels were not restricted to the dominanthand. This finding indicates that clues toright- or left-hand dominance in samples offossil hominid hands will need to be soughtin skeletal features associated with specificmuscle groups. The finding is consistentwith our observations that 1) both hands aresubjected to considerable stress when a stone

in one hand is struck by a stone in the otherhand, and 2) both hands tire during hardhammer percussion manufacture of stonetools.3 It is also interesting to note that therewas a greater range of variability of peaklevels and timing in the nondominant hand.Muscles of the nondominant hand accountfor five of the six trial series that exhibited arange of variation across four levels, andonly half the muscles in the nondominanthand exhibited consistent peak timing rela-tive to strike, compared with two-thirds inthe dominant hand.

Evidence in muscle recruitmentfor economy in tool-making

We are impressed by the difference be-tween Subject 1 and the other subjects inmuscle recruitment levels. His activity sig-nals were consistently lower relative to hismaximum possible signals, often by two orthree levels. His data account for the twocases in which muscle levels did not exceedthe level of ‘‘none,’’ 13 of the 14 occurrencesof the low modal level, and 11 of the 21moderate level occurrences, whereas Sub-

3It is interesting to note that the radiologist who read a CTscan of the hands of Subject 1 (our most experienced tool-maker),without any information about the background and occupation ofthe subject, inferred from the relative muscle developmentapparent in the scan that the subject was left-handed. In fact, thesubject is right-handed, but apparently the many years ofprehistoric tool replication have generated relatively greaterdevelopment of at least some muscles in the nondominant handthat holds the core.

Fig. 5. Asymmetries in muscle recruitment. Peaklevel modes for three muscles are compared for thedominant and nondominant hands of Subject 1. Level 1:low; level 2: moderate; level 3: marked; level 4: maxi-mum.



ject 2 exhibits the two highest levels for allmuscles except P1 and ODM (dominanthand), and P3 and FPL (nondominant hand).Subject 1 also exhibited substantially lessdiscernable muscle activity between strikes,except for the OP and ODM muscles. InSubject 3, almost all the muscles were con-tinually active. Subject 2 was intermediate,with several muscles continually active. Thedifference probably reflects the many yearsof practice for Subject 1 (compared with lesspractice in Subject 2 and no practice inSubject 3), involving adjustments in thesensorimotor control of grip patterns andhand movements which minimize prolongedstress on the musculoskeletal system andexploit the larger moment arms of intrinsicmuscles. We suspect that the grips andmuscle recruitment patterns he has arrivedat are quite likely to approach those ofOldowan tool makers, to the extent thatthese would have been facilitated by earlyhominid hand morphology.

The higher level of overall muscle recruit-ment by Subject 2 also may be explained inpart by the smaller size of her hand, com-pared with that of the two male subjects.

CONCLUSIONS

Based on the results of the experimentsreported here, our conclusions are:

1. The primary muscles involved in hardhammer percussion manufacture of Old-owan tools are likely to have been the FPB,FDM, and FCU of both hands, the OP, D1,ADM, FDP2, and FDP5 of the dominanthand, and the APT and P1 of the nondomi-nant hand. Five of these muscles (FPB, D1,FDP2, FDP5 of the dominant hand and FCUof both hands) had marked/maximum peaklevels in all three subjects, constituting thefirst, most compelling category of evidencefor their importance in Oldowan tool-mak-ing. The other five muscles had marked/maximum peak levels in the two experi-enced subjects, constituting our secondcategory of strong implications for impor-tance in tool-making. (This second group offive muscles includes OP, ADM, and FDM ofthe dominant hand and APT and P1 in thenondominant hand.) These ten muscles areprobably the ones that would have securedthe one-handed precision pinch grips of ham-

merstones and cores against the large exter-nal forces in stone tool-making, and wouldhave stressed the joints with their strongcontraction.

2. Skeletal features associated with thehuman FPL muscle are not likely to haveevolved exclusively in association with useof precision pinch grips in tool-making activi-ties. The muscle in the dominant hand of ourmost experienced tool-maker peaked at ahigh level during only one of three sessions,and its recruitment for all subjects was atrelatively low levels in the nondominanthand. However, in the two experienced sub-jects it did peak at the two highest levelsduring use of the power squeeze grip ofcylindrical objects for two tool-using activi-ties, digging and clubbing.

Distal interphalangeal thumb joint mor-phology reflects the relative size of the FPLtendon moment arm, perhaps more clearlythan the size (and relative force potential) ofthe FPL muscle. This morphology, togetherwith thumb use in non-tool-making activi-ties, should be reexamined comparatively incatarrhines before further inferences abouttool-making and tool-using capabilities aredrawn from the specific morphology of thisregion in early hominid thumb bones.

3. The high level of intrinsic muscle re-cruitment in Oldowan core reduction is likelyto have been accommodated by adjustmentsin robusticity and in bone and joint configu-rations of the first, second, and fifth metacar-pals relating to muscle force production andtendon moment arms (some during lifetimeand others in the course of hominid evolu-tion). A productive search for skeletal evi-dence of habitual Oldowan tool-making be-havior would therefore be in the first, second,and fifth metacarpal regions.

4. Comparative data from subjects withdifferent levels of experience in Oldowantool-making clearly demonstrate the advan-tage of flexible muscle synergies, which wereinferred by Hepp-Reymond et al. (1996)from the results of their controlled studies ofhuman precision gripping. The ability toachieve economy through flexible, selectivemuscle recruitment in repetitive and stress-ful manipulative behaviors such as tool-making, illustrated by Subject 1, would havebeen advantageous to early hominids, just



as increased cerebral control of the forelimbpostulated by Vilensky and Larson (1989)for early primates would have enhancedtheir increased forelimb use in feeding andother manipulative behaviors.

5. Finally, it is possible that clues to handpreference in fossil hominids may be soughtin the future through skeletal features asso-ciated with specific and different sets ofmuscle groups in the right and left hands offossil hominid samples. Both hands weresubmitted to large external and internalforces in hard hammer percussion manufac-ture of stone tools, but the stresses weredifferent for each hand.

It should be kept in mind that the Old-owan tool-making activity analyzed in thisreport is only one in a wide range of manipu-lative activities that are likely to have beenfacilitated and enhanced by the distinctivepatterns of hand morphology that emergedin the course of early hominid evolution. Infact, as noted in our discussion of FPL, weincluded in the experiments several tool-using activities (e.g., digging and clubbing),whose associated grips and muscle recruit-ment will be reported on more fully in futurecommunications. It is also possible that otherfood-gathering and food-processing activi-ties without tools, for example, breakingnuts or breaking bones for marrow, mayhave been important components of earlyhominid foraging behavior and may havedepended on strong, repeated contraction ofsome of the same muscles heavily recruitedfor tool behaviors. This hypothesis remainsto be tested by tooth-wear analysis and byEMG analysis of hand muscle activities inthese behaviors.

We would also like to stress that theresults of our experiments illuminate mor-phological features that we think needed tobe in place in early hominids for habitual,effective Oldowan tool-making. But tool-making did not necessarily lead to the evolu-tion of all these features. If fossil hominidhands lack these features, they probablywere not used habitually for making stonetools. If they have these features, we canreasonably predict that they could havebeen the hands of tool-makers.

ACKNOWLEDGMENTS

We thank the following at the Mayo ClinicOrthopedic Biomechanics Laboratory, whoseabundant expertise, computer support, andingenuity were crucial to the study: W.Litchy, M.D. (now with Riverside Neurology,Riverside Clinic, Minneapolis, MN), M.Harper, M.D. (electrode implantation andremoval), D. Hanson (data collection), andE. Growney (bioengineering support). S.Brown and J. Roeth (Arizona State Univer-sity) provided valuable assistance with dataanalysis. R. Marzke’s contributions as con-sultant at all stages of the project are grate-fully acknowledged.

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emg study of hand muscle recruitment during hard hammer percussion manufacture of oldowan tools

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