Biomechanical analysis of lithic tools production: impact on the evolution of early hominin upper limb?

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Keywords:

• Percussion
• electromyography
• kinematics
• task complexity
• paleontology
• evolution

1. Introduction

The percussion movements, as supposedly encountered during stone tool production, are likely to have promoted or benefited from specific anatomical changes throughout human evolution. Specifically, the wrist extension capability, the robust thumb observed in the genus Homo, could have facilitated early stone tool production (Williams et al. 2014; Rolian et al. 2011). Numerous other anatomical variations have been identified among early hominins (i.e., Australopithecus and early representatives of the genus Homo). For instance, some early hominins would exhibit a more developed rotator cuff, a longer biceps and a specific glenohumeral joint when compared with extant humans, which could have caused a more pronounced humerus antepulsion (Arias-Martorell 2018). Consequently, not only the evolution of the end-off effector, i.e., the hand, but also the whole upper limb biomechanics may be linked with stone tool production. Therefore, analysing joints range of motion and muscles demand associated with percussion movement is needed. Lastly, Biryukova et al. (2008) measured a high and constant kinetic energy at the point of impact of a percussor used by expert subjects while they reported that the movement pattern was variable. These observations agreed with the uncontrolled manifolds (UCM) theory (Scholz and Schoner 1999) which differentiates the performance variable supposed similar across the trials from the elementary variables supposed differ. This theory is linked with the motor control and furthermore, with the task complexity.

Besides morphological considerations, human evolutionary history was associated with cognitive changes, and it has been demonstrated that studying the task complexity may be more insightful (Bardo et al. 2015).

The aim of this study was to compare three techniques of percussion identified from the archaeological record, in terms of physical and motor control demand in order to assess to what extent such requirements was compatible with early hominin upper limb anatomy.

2. Methods

One expert subject performed several trials using three different techniques of stone tool production. We distinguished the free hand (FH) technique in which the core is placed on the non-dominant hand and struck with a hammerstone, the passive hammer (PH) technique in which the core is struck against a percussor held with both hands, and the bipolar technique (BP) in which the core is placed on an anvil and struck with a hammerstone. The purpose of the task was to detach flakes until it was no longer possible. Raw material from West Turkana (Kenya) were used. Three-dimensional kinematic data were collected at 300 Hz frequency via a 19-camera optoelectronic system (Oqus, Qualisys) and seven joint angles of the upper-limbs were analysed. Electromyographic data were collected from 14 muscles of upper limb using bipolar surface electrodes with a 1000 Hz frequency. We had the Pectoral major (PM_L/R), Deltoid (D_L/R), Extensor ulnar carpi (EUC_L/R), Flexor ulnar carpi (FUC_L/R), Triceps brachii (TB_L/R), Biceps brachii (BB_L/R), Infraspinatus (I_L/R). EMG data were normalised to maximum voluntary isometric contraction (MVIC). Both systems were synchronized. Kinematic data were low-pass (20 Hz) filtered with a two-order Butterworth filter whereas the EMG data were pass-band (20 Hz-500Hz) filtered with a fourth order. Data processing was conducted using custom Matlab (Matworks Inc, Novi, USA) routines and Opensim (Delp et al. 2007). The model of Rajagopal et al. (2016) was scaled to match subject’s anthropometry based on experimental markers placed on anatomical landmarks.

Each movement was separated into three phases. The preparation phase during which the subject positioned the stone, the up-swing phase during which he raised his arm and the down-swing phase during which he executed the strike.

In order to analyze the movement pattern variability, the joint-angles contribution over time were collected using the principal-component analysis method (PCA) and analyzed through the uncontrolled manifold (UCM) theory. The task saving between the techniques was compared with the integrated electromyography (iEMG). The task efficiency was measured by the ratio: $$Rf=\frac{\mathit{Number}of\mathit{validated}\mathit{cycles}}{\mathit{Number}of\mathit{invalidated}\mathit{cycles}}*100$$

iEMG of fourteen muscles were compared across techniques during the up-swing and down-swing phases using a one factor repeated measures ANOVA just like Rf parameter.

3. Results and discussion

Preliminary results tended to show a preference for the carpi muscles for the whole techniques. BP and FH technique had a similarly pattern and a tightening force of the non-dominant arm that seemed not negligible.

The UCM variability of movement pattern and muscular pattern of each task will be analysed in order to determine if the latest techniques are more complex. The task saving and task efficiency will be measured. The different results obtained will be compared with those of Biryukova and Bril (2008) for FH technique, and Liu et al. (2009) for PH technique.

4. Conclusions

This transversal study between biomechanics and paleoanthropology provides original information on hominin technological behaviours. Also, this study encourages the analysis of the motor control in the context of anatomical evolution.

Figure 1. Comparison of iEMG (averages accrossall cycles and all trials) between PH, BP and FH techniques.

Additional information

Funding

This research is conducted within the frame of the CNRS-INEE International Research Network IRNGDRI0870, ANR-12-CULT-0006 and ANR-17CE27-0005-02 HOMTECH.