Why invent the handle? Electromyography (EMG) and efficiency of use data investigating the prehistoric origin and selection of hafted stone knives

Abstract

For over 3 million years hominins held stone-cutting tools in the hand, gripping the portion of tool displaying a sharp cutting edge directly. During the late Middle Pleistocene human populations started to produce hafted composite knives, where the stone element displaying a sharp cutting edge was secured in a handle. Prevailing archaeological literature suggests that handles convey benefits to tool users by increasing cutting performance and reducing musculoskeletal stresses, yet to date these hypotheses remain largely untested. Here, we compare the cutting performance of hafted knives, ‘basic’ flake tools, and large bifacial tools during two standardized cutting tasks. Going further, we examine the comparative ergonomics of each tool type through electromyographic (EMG) analysis of nine upper limb muscles. Results suggest that knives (1) recruit muscles responsible for digit flexion (i.e. gripping) and in-hand manipulation relatively less than alternative stone tool types and (2) may convey functional performance benefits relative to unhafted stone tool alternatives when considered as a generalised cutting tool. Furthermore, our data indicate that knives facilitate greater muscle activity in the upper arm and forearm, potentially resulting in the application of greater cutting forces during tool use. Compared to unhafted prehistoric alternatives, hafted stone knives demonstrate increased ergonomic properties and some functional performance benefits. These factors would likely have contributed to the invention and widespread adoption of hafted stone knives during the late Middle Pleistocene.

A biomechanical investigation of the efficiency hypothesis of hafted tool technology

“…The results of this study support previous research highlighting functional efficiency advantages of hafting in a variety of tasks and experimental settings [20,25,27,28]. The adaptive benefits and motivations behind the invention and proliferation of hafted tool technology are likely to be complex and multifaceted [62].…”

“…[25] and Key et al . [28] identify differences in muscle activity and recruitment in scraping and cutting tasks respectively when using hafted and hand-held tools. Similarly, the present study shows that hafted scraping elicits greater activity in muscles around the elbow (figure 9 h ,i) compared with hand-held scraping.…”

“…In this study and that of Key et al . [28], use of a power grip affords a greater use of the upper arm compared with the precision grip, which appears to limit use of these muscles and motions, reducing forces placed through the tool.…”

“…Following this study and the work of Key et al . [28], further experimental research is needed to assess the impact of hafting on other basic activities (drilling and piercing) as well as a variety of haft forms (cleft and juxtaposed, etc.) to help model the spread and amplification of hafted technologies.…”

“…Claud et al [27] found late Middle Palaeolithic flake cleavers functioned more effectively than hand-held cleavers in both tree felling and carcass butchery tasks as measured by number of blows and time required. More recently, Key et al [28] compared the cutting performance of replicated hafted flint knives with effectiveness of non-hafted flint flakes and hand-held large flint bifacial tools across two standardized cutting tasks. They examined ergonomic differences between the tool types using electromyographic (EMG) analysis of nine upper limb muscles involved in gripping (first dorsal interosseous, flexor pollicis longus, abductor digiti minimi, flexor pollicis brevis), and movements of the wrist, elbow and shoulder (flexor pollicis brevis, brachioradialis, flexor carpi radialis, biceps brachii, triceps brachii, anterior deltoid).…”