hypothesise that stone knapping represents a significant selective pressure guiding the evolution of the hominin shoulder girdle and other aspects of the upper ...
Electromyography, Kinematics, and Kinetics of the Upper Limb During Oldowan Stone Tool Manufacture Elen M.
a Feuerriegel ,
Mark
b Halaki ,
Darren
c Reed ,
Colin. P.
d Groves ,
Karen A.
c Ginn
a Department
of Anthropology, University of Washington, Seattle, WA 98195, United States. b Discipline of Exercise and Sport Science, University of Sydney, Lidcombe, NSW 2141, Australia. c Discipline of Biomedical Science, Sydney Medical School, University of Sydney, Lidcombe, NSW 2141, Australia. d School of Archaeology and Anthropology, College of Arts and Social Sciences, Australian National University, Canberra, ACT 2601, Australia.
Introduction The close temporal association of changes to the shape, height, and orientation of the shoulder and structure of its elements with the advent of stone tool technology has led many researchers to hypothesise that stone knapping represents a significant selective pressure guiding the evolution of the hominin shoulder girdle and other aspects of the upper limb (Susman, 1998, Larson, 2007, 2015, Marzke and Marzke, 2000, Tocheri et al., 2008).
Figure 2. The mean ± 95% confidence intervals (grey shaded area) of the time normalised EMG data vs time normalised to cycle duration. Up-swing is 0-70% cycle duration and down-swing is 70100% cycle duration. The vertical dotted lines indicate the end of the up-swing. Hammerstone impact occurred at point 100%.
Few studies have been conducted on the biomechanics and kinematics of the shoulder and elbow during stone knapping and consequently there is little quantitative information available for evaluating hypotheses about the mechanical context of stone tool production at this region. Aim: To identify the normal recruitment pattern of these muscles during knapping to better understand the biomechanical forces present at the upper limb during basic stone tool manufacture.
Results • Significantly greater mean activity levels during the downswing phase (Fig. 2) • Tukey post hoc: average activity levels between the upswing and downswing did not differ in 6 muscles (upper and lower trapezius, infraspinatus, supraspinatus, anterior deltoid and biceps brachii)
Figure 1. Electrode and wireless inertial sensor placement. The small black boxes are the inertial sensors. All electrode cables were taped in place and full range of motion ensured prior to data collection.
Methods Subjects: 16 healthy subjects (9 male, 7 female; age (mean ± standard deviation): 26.8 ± 9.3 (range: 19-54 years); height: 1.70 ± 0.07 m; mass: 71.6 ± 16.9 kg; all right-handed) trained in Oldowan stone knapping until proficient. Instrumentation: Data collected from 15 axioscapular/ humeral, scapulohumeral (incl. rotator cuff), and elbow muscles using surface and intramuscular electrodes. Wireless inertial sensors measured angular orientation of the arm and trunk. A Kistler force plate was used to measure forces (Fig. 1). EMG data normalised to maximum voluntary isometric contraction (MVIC). Procedure: Subject seated cross-legged on force plate. Using their dominant hand, subjects were instructed to knap for 10 minutes, or until 10 appropriate strikes were created for analysis. Analysis: Peak EMG amplitudes were compared across muscles and the up-swing and down-swing phases using a two factor repeated measures ANOVA. Peak EMG timings were compared across muscles during each of the up-swing and downswing phases using a one factor repeated measures ANOVA. Tukey post hoc test was used to explore the results. Statistical significance was set at p < 0.05.
Highest peak EMG in Upswing (% MVIC)
Highest peak EMG in Downswing (%MVIC)
Lower trapezius (26%), Infraspinatus (19%), Wrist extensors (18%)
Lower trapezius (36%), Teres major (33%), Wrist extensors (33%), Pectoralis major (32%), Wrist flexors (29%), Prox. triceps brachii (29%), Latissimus dorsi (28%), Infraspinatus (27%)
• Significant differences in the timing of the muscles in both up-swing (F(12,180)=7.92, p < 0.001) and down-swing (F(14,210)=12.10, p < 0.001). • Muscles producing extension torque at the elbow and shoulder (muscles causing downward acceleration) recruited early in the downswing phase • Elbow and shoulder flexor muscles recruited closer to hammerstone strike Discussion The rotator cuff muscles are important for strike accuracy and glenohumeral stability during Oldowan stone tool manufacture. Infraspinatus, supraspinatus, and subscapularis are recruited in a contrapuntal manner to counterbalance translation of the humeral head caused by flexion and extension torques during knapping tasks (Wattanaprakornkul et al. 2011). Infraspinatus and supraspinatus work to stabilise against potential anterior humeral head displacement caused by flexor muscles, while subscapularis is active in downswing to stabilise against posterior humeral head displacement caused by extensor muscles. Muscle recruitment patterns between subjects for superficial wrist flexors and extensors were highly consistent; however, wrist kinematics are highly inconsistent. These muscles are likely stabilising the elbow for hammerstone impact by producing a “shunting” force to compress the joint during loading. This is contra previous studies which argue that wrist flexors (especially flexor carpi ulnaris) help to accelerate the hammerstone towards the tool stone (Marzke et al. 1998; Williams et al. 2010). Literature Cited Larson, S.G., 2007. Evolutionary transformation of the hominin shoulder. Evol. Anthropol. 16, 172-187. Larson, S.G., 2015. Humeral torsion and throwing proficiency in early human evolution. J. Hum. Evol. 85, 198-205. Marzke, M.W., Marzke, R.F., 2000. Evolution of the human hand: Approaches to acquiring, analysing and interpreting the anatomical evidence. J. Anat. 197, 121-140. Marzke, M.W., Toth, N., Schick, K., Reece, S., Steinberg, B., Hunt, K., Linscheid, R.L., An, K.N., 1998. EMG study of hand muscle recruitment during hard hammer percussion manufacture of Oldowan tools. Am. J. Phys. Anthrop. 105, 315-332. Tocheri, M.W., Orr, C.M., Jacofsky, M.C., Marzke, M.W., 2008. The evolutionary history of the hominin hand since the last common ancestor of Pan and Homo. J. Anat. 212, 544-562. Wattanaprakornkul, D., Cathers, I., Halaki, M., Ginn, K., 2011. The rotator cuff muscles have a direction specific recruitment pattern during shoulder flexion and extension exercises. J. Sci. Med. Sport 14, 376-382. Williams, E.M., Gordon, A.D., Richmond, B.G., 2010. Upper limb kinematics and the role of the wrist during stone tool production. Am. J. Phys. Anthrop. 143, 134-145.
