The Influence of Hand Dimensions on Finger Flexion during Lower Paleolithic Stone Tool Use in a Comfortable Grip

: Considering the biomechanical and cognitive aspects involved in tool manipulation, hand size emerges as a critical factor. Males, on average, exhibit greater grip strength attributed to larger hand dimensions. Beyond mere physical factors, cognitive components tied to visuospatial abilities also influence stone tool use. However, the intricate relationship between hand size, grip strength, and ergonomic patterns necessitates further exploration. Here, we study the ergonomic pattern of phalanx flexion during the manipulation of Lower Paleolithic stone tools (choppers and handaxes) to understand the nuanced interplay between hand dimensions and grasping behaviors in Lower Paleolithic stone tool use. The static hand posture during the comfortable grasping of each tool is measured using a motion capture hand glove. Flexions are measured at the metacarpophalangeal joint, the proximal interphalangeal joint and the distal interphalangeal joint of each finger. Our investigation into Lower Paleolithic stone tool manipulation reveals gender-based differences in phalanx flexion, with hand dimensions showing correlation only in pooled samples. However, these associations diminish when analyzing males and females separately. This study suggests a minimal link between hand size and grasping behavior within our sample, hinting at the influence of cognitive, behavioral, and motor factors. Exploring lifestyle and psychometric profiles could provide further insights. In the context of early human technology, our results prompt considerations on the evolution of the hand-tool interaction system, linking our tool-dependent culture to our phylogenetic history.


Introduction
Human hand morphology, intricately shaped by evolutionary pressures associated with tool making and usage, serves as a tangible record of our species' adaptive journey [1].Specific characteristics, such as the thumb/index length proportion, have garnered attention for their presumed role in conferring enhanced precision during tool handling [2][3][4][5][6].Napier's seminal classification of grips into "precision" and "power" provides a conceptual foundation for understanding the diverse grasping possibilities exhibited by humans, crucial for both daily activities and tool use [7,8].
While studies in experimental archaeology have shed light on limited grasping patterns associated with Lower Paleolithic stone cutting tools [9], the overarching ergonomic schemes inherent in early and extinct human species remain speculative [9].The handling and manipulation of tools depends significantly on the tool function, and the results of traceological examinations can be helpful in defining the actions for which tools were used.While traceology is commonly applied to tools from later periods, its principles can still be applied to Lower Paleolithic stone tools to infer their use.Despite the challenges, traceological analysis remains a valuable method for gaining insights into the use and functionality of early stone tools [10].
Biomechanical responses during tool use are not only shaped by human physiology but are also intricately linked to the morphology of the stone tools themselves, as evidenced by the work of Patiño et al. (2017) [11].Furthermore, recent studies examining the ergonomic aspects of Lower Paleolithic technology have underscored the influence of tool type and metrics on the intricate patterns of phalanx flexion during comfortable tool grasping [12,13].
Beyond the biomechanical considerations, effective tool manipulation involves a cognitive component tethered to an individual's capacity to perceive object affordances [13].This cognitive aspect, intertwined with sensing capacity, initiates physiological responses associated with attention and arousal [14,15].The role of the parietal cortex in visuospatial integration is particularly pronounced in the context of stone tool use, aligning the cognitive dimensions with the evolutionary trajectory of our visuospatial abilities [16][17][18].
Hand size emerges as a pivotal factor in the nuanced landscape of Lower Paleolithic ergonomic patterns.The applied forces and contact area, intricately linked to grip diameter and hand size, underscore the importance of hand dimensions in shaping the efficiency and comfort of tool grasping [19].Notably, the sexual dimorphism in grip strength, with males generally producing greater force, is a well-established phenomenon [19][20][21][22][23][24][25].The predictive power of hand dimensions, particularly palm width, in estimating grip force has been corroborated by Nicolay and Walker (2005) [25].However, the complex interplay between sociodemographic predictors and grip strength, as explored by Hart (2018) [26], adds layers of nuance to our understanding.
In the context of stone tool use, the biomechanical aspects of the hand have been found to be intimately related to efficiency in cutting tasks [27].Notably, manipulative strength stands out as the primary predictor for efficiency in flake tools, while hand size takes precedence for handaxes [9].Building upon our recent work [13], where we meticulously measured the phalanx flexion of 78 subjects during comfortable stone tool handling for both Oldowan pebble tools and Acheulean handaxes, the present study endeavors to deepen our understanding of the ergonomic hand patterns associated with the grasping of Lower Paleolithic stone tools.Specifically, we aim to assess whether hand dimensions play a role in sexual differences in manipulation patterns, systematically analyzing grasping patterns in males and females under the null hypothesis of no differences and exploring the potential effect of hand dimensions on finger flexion under the null hypothesis of no correlation.This multidimensional approach integrates biomechanical, cognitive, and sociodemographic dimensions, contributing to a more comprehensive understanding of the intricate relationship between human hands and the tools they manipulate.

Tool Selection and Reproduction
This study focused on two prominent Lower Paleolithic stone tool types-20 choppers and 20 handaxes (Figure 1)-under the null hypothesis that hand size has no effect on their ergonomic grasping patterns, as indicated by finger flexion degrees.To ensure uniformity, an experienced tool maker (MTB) meticulously replicated forty stone tools with standardized dimensions and form, following the methodology outlined in Fedato et al. (2020) [13] (Table 1).The tools were made from Paleozoic quartzite, characterized by a fine grain and consistent structure, and sourced from large, irregular pebbles lacking any major fissures or fractures.Specifically, the tool reproductions were designed to facilitate one-handed manipulation, considering the distinctive grip requirements associated with each tool type.As demonstrated experimentally, handaxes typically involve the whole palm and all five digits, whereas flake tools necessitate controlled pad-to-pad pinching, a feature not considered in our analysis, which is exclusively tailored for a holistic hand tactile experience [9,28].
any major fissures or fractures.Specifically, the tool reproductions were designed to facilitate one-handed manipulation, considering the distinctive grip requirements associated with each tool type.As demonstrated experimentally, handaxes typically involve the whole palm and all five digits, whereas flake tools necessitate controlled pad-to-pad pinching, a feature not considered in our analysis, which is exclusively tailored for a holistic hand tactile experience [9,28].

Participant Recruitment and Characteristics
Enrolled participants comprised 50 females and 28 males, all right-handed adults aged between 23 and 67 years.The recruitment intentionally excluded individuals with previous archaeology experience to maintain this study's focus on the ergonomic relationship between the hand and the tool, specifically emphasizing hand-tool haptic feedback.Elements such as knowledge of tool functions or tasks were deliberately excluded from participant considerations.

Hand Dimension Measurement
Utilizing a Canon CanoScan 8800F 2D scanner (resolution 4800 dpi), hand images were acquired and analyzed using ImageJ 1.46r [29].Hand dimensions measured included hand length (HL), palmar length (PL), and palmar width (PW), following established protocols [30].HL spanned from the distal flexion crease at the wrist to the tip of the third digit, while PL was calculated from the midpoint of the distal transverse crease of the wrist flexures to the most proximal flexion crease of the third finger.PW, representing the distance between the radial aspect of the second metacarpal and the ulnar aspect of the fifth metacarpal, was also measured (Table 2; Figure 2a).

Participant Recruitment and Characteristics
Enrolled participants comprised 50 females and 28 males, all right-handed adults aged between 23 and 67 years.The recruitment intentionally excluded individuals with previous archaeology experience to maintain this study's focus on the ergonomic relationship between the hand and the tool, specifically emphasizing hand-tool haptic feedback.Elements such as knowledge of tool functions or tasks were deliberately excluded from participant considerations.

Hand Dimension Measurement
Utilizing a Canon CanoScan 8800F 2D scanner (resolution 4800 dpi), hand images were acquired and analyzed using ImageJ 1.46r [29].Hand dimensions measured included hand length (HL), palmar length (PL), and palmar width (PW), following established protocols [30].HL spanned from the distal flexion crease at the wrist to the tip of the third digit, while PL was calculated from the midpoint of the distal transverse crease of the wrist flexures to the most proximal flexion crease of the third finger.PW, representing the distance between the radial aspect of the second metacarpal and the ulnar aspect of the fifth metacarpal, was also measured (Table 2; Figure 2a).

Data Acquisition
This study focused on finger flexion towards the palm, involving the metacarpophalangeal joint (McP), proximal interphalangeal joint (PiP), and distal interphalangeal joint (DiP) (Figure 2b,c).The static hand posture during comfortable tool grasping was recorded using a VMG 30™ motion capture hand glove (Virtual Motion Labs ® ).This device, calibrated before each trial, transformed finger motion into real-time sensory data through joint angle measurements [31].Flexion degrees were measured as the external angle of the phalanx, with labels including McP1-McP5 for the thumb through the little finger and PiP1-PiP5 and DiP2-DiP5 for the proximal and distal interphalangeal joints, respectively (Figure 2).

Experimental Procedure
In each trial, participants manipulated 20 choppers and 20 handaxes, aiming for a comfortable grip with the right hand only (Figure 3).They were instructed that a comfortable grip should ensure secure handling without slipping, free from tension or unpleasant sensations.Participants were encouraged to choose the most suitable grasp type and inform the experimenter when they reached a final comfortable position with the tool.Flexion values were recorded at this position.All subjects gave their informed consent for inclusion before they participated in this study.This study was conducted in accordance

Data Acquisition
This study focused on finger flexion towards the palm, involving the metacarpophalangeal joint (McP), proximal interphalangeal joint (PiP), and distal interphalangeal joint (DiP) (Figure 2b,c).The static hand posture during comfortable tool grasping was recorded using a VMG 30™ motion capture hand glove (Virtual Motion Labs ® ).This device, calibrated before each trial, transformed finger motion into real-time sensory data through joint angle measurements [31].Flexion degrees were measured as the external angle of the phalanx, with labels including McP1-McP5 for the thumb through the little finger and PiP1-PiP5 and DiP2-DiP5 for the proximal and distal interphalangeal joints, respectively (Figure 2).

Experimental Procedure
In each trial, participants manipulated 20 choppers and 20 handaxes, aiming for a comfortable grip with the right hand only (Figure 3).They were instructed that a comfortable grip should ensure secure handling without slipping, free from tension or unpleasant sensations.Participants were encouraged to choose the most suitable grasp type and inform the experimenter when they reached a final comfortable position with the tool.Flexion values were recorded at this position.All subjects gave their informed consent for inclusion before they participated in this study.This study was conducted in accordance with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of the University of Burgos.

Statistical Analysis
For each participant, median values of phalanx flexion were calculated for the forty Lower Paleolithic stone tools.As these values did not follow a normal distribution (Table 3), non-parametric tests were employed for group comparisons and correlation analyses.A Principal Component Analysis (PCA) was conducted on median phalanx flexions to identify the main patterns associated with comfortable grasping.Correlation analyses explored the relationship between these patterns and hand dimensions.Mann-Whitney tests compared phalanx flexion values between males and females.Statistical analyses were conducted using PAST v. 3.18 [32], SPSS 24, and R 3.5.0.

Results
Analyzing the joint angles, the first principal component (PC1) explained 64% of the variance, with PC2 explaining 13%.PC2's eigenvalue, as per the broken stick model, approached the threshold of random variation, urging caution in its interpretation.

Statistical Analysis
For each participant, median values of phalanx flexion were calculated for the forty Lower Paleolithic stone tools.As these values did not follow a normal distribution (Table 3), non-parametric tests were employed for group comparisons and correlation analyses.A Principal Component Analysis (PCA) was conducted on median phalanx flexions to identify the main patterns associated with comfortable grasping.Correlation analyses explored the relationship between these patterns and hand dimensions.Mann-Whitney tests compared phalanx flexion values between males and females.Statistical analyses were conducted using PAST v. 3.18 [32], SPSS 24, and R 3.5.0.

Discussion
In this comprehensive study, we systematically examine comfortable grasping patterns associated with Lower Paleolithic stone tools, particularly choppers and handaxes.Our analysis focuses on quantifying phalanx flexion and considering the potential influence of hand dimensions, including hand length, palmar length, and palmar width, previously correlated with finger length and demonstrating sexual dimorphism [33][34][35].
The primary grasping pattern, represented by the first principal component, shows a strong association with distal phalanges' flexion, particularly evident in the little and ring fingers.An intriguing biomechanical insight arises from the negative correlation between proximal and distal joint flexion, emphasizing the need to consider potential biomechanical or spatial constraints between finger joints when interpreting tool grasping patterns and ergonomics.
The secondary grasping pattern, represented by the second principal component, is linked to the flexion of the distal phalanges, especially in the index and middle fingers.Both grasping patterns modestly correlate with hand dimensions (r ~ 0.39), affirming prior experiments suggesting the influence of specific biometric traits on Lower Paleolithic stone-tool-use efficiency.Grip force and contact area, influenced by handle diameter and hand size, emerge as crucial parameters impacting tool manipulation.
An intriguing revelation emerges when examining the distribution of grasping pat-

Discussion
In this comprehensive study, we systematically examine comfortable grasping patterns associated with Lower Paleolithic stone tools, particularly choppers and handaxes.Our analysis focuses on quantifying phalanx flexion and considering the potential influence of hand dimensions, including hand length, palmar length, and palmar width, previously correlated with finger length and demonstrating sexual dimorphism [33][34][35].
The primary grasping pattern, represented by the first principal component, shows a strong association with distal phalanges' flexion, particularly evident in the little and ring fingers.An intriguing biomechanical insight arises from the negative correlation between proximal and distal joint flexion, emphasizing the need to consider potential biomechanical or spatial constraints between finger joints when interpreting tool grasping patterns and ergonomics.
The secondary grasping pattern, represented by the second principal component, is linked to the flexion of the distal phalanges, especially in the index and middle fingers.
Both grasping patterns modestly correlate with hand dimensions (r~0.39),affirming prior experiments suggesting the influence of specific biometric traits on Lower Paleolithic stonetool-use efficiency.Grip force and contact area, influenced by handle diameter and hand size, emerge as crucial parameters impacting tool manipulation.
An intriguing revelation emerges when examining the distribution of grasping patterns between males and females.On average, males exhibit more flexion, but the correlation between grasping patterns and hand size vanishes when the sexes are analyzed separately.This suggests that differences in hand size alone do not account for the observed sexual disparities in grasping behavior.Despite a potentially spurious correlation in the pooled analysis, dissecting the groups reveals an absence or extreme weakness in their reciprocal influence, pointing to the involvement of other, yet-to-be-explored factors in sexual differences.
The observed variability in the little and ring fingers, as revealed by the main grasping pattern identified through the first principal component, may be indicative of their distinct functional roles compared to the thumb and index finger.The biomechanical considerations suggest that the thumb and index finger, associated with precision grips [3], are more frequently utilized in fine motor tasks requiring delicate manipulation.In contrast, the little and ring fingers, displaying greater variability in our analysis, may be more involved in tasks requiring power or stability grips, where precision is less critical.This aligns with established biomechanical principles, emphasizing the dominant role of the thumb and index finger in precision grasping activities.While these observations are consistent with known functional anatomy, specific references from reputable anatomy or the biomechanics literature can be consulted for further confirmation and to ensure up-to-date accuracy in the scientific context.
Acknowledging the subjective nature of comfortable grasping, influenced by both physical constraints and individual choices, underscores the complexity of grasping behavior involving sensory, motor, and cognitive systems.The intricate interplay among cognitive abilities, visuospatial strategies, and evolutionary aspects may contribute to sex differences in grasping behavior, suggesting the need for future studies to delve into individual differences in visuospatial skills during stone tool manipulation.
It is crucial to recognize the limitations of the analysis, which focuses specifically on finger flexion during ergonomic and comfortable grasping of tools manipulated with the whole hand.By instructing participants to grasp objects in a way that feels comfortable, we attempt to capture the inherent variability in tool manipulation, considering not only the physical constraints imposed by object shape but also individual choices guided by haptic and ergonomic considerations.This approach aligns with the multifaceted nature of tool use in prehistoric contexts, where individuals may have adapted their grasping techniques based on the unique characteristics of the tools and their intended tasks.It is known that choppers, while often classified as tools, served multiple purposes beyond just tool use, such as being used as cores for producing smaller flakes or as percussive instruments for various activities [36].Additionally, other enigmatic spherical stone objects are present in archaeological Oldowan assemblages and are among the oldest manufactured stone items [37].Even if their functions and manufacture modes are still poorly known, the remarkable geographical and chronological spread of stone balls warrants attention and calls for further investigation into their potential structural and functional roles in the context of human technological advancement [38].
The exclusion of other haptic and cognitive components involved in grasping behavior and the restriction to larger tools warrant future investigations into the ergonomic patterns associated with manipulating smaller objects and tools with different functional demands.
Ongoing analyses, comparing manipulation patterns between expert and non-expert stone tool users, aim to uncover differences in manipulative strategies, while future surveys should consider grasping strategies related to specific functional tasks.
In exploring comfortable grasping patterns for Lower Paleolithic stone tools-specifically choppers and handaxes-our study offers a unique lens into the behaviors of our prehistoric ancestors.By quantifying the degree of phalanx flexion and considering the potential influence of hand dimensions, we delve into the intimate relationship between human hand morphology and tool manipulation efficiency.This endeavor is not merely an analytical exercise but rather an attempt to unravel the ergonomic intricacies that prehistoric humans may have grappled with during their daily interactions with stone tools.
The insights gained from our study carry implications for understanding the daily lives of prehistoric communities.The consideration of grip force, contact area, and the intricate interplay between hand dimensions and grasping behavior speaks to the challenges and adaptations that our ancestors navigated while utilizing stone tools.Future research could expand to include juveniles, offering a more comprehensive understanding of skill acquisition and the transmission of tool-use knowledge.Additionally, further investigation into the manipulation patterns of expert versus non-expert stone tool users would provide valuable insights into the roles of teaching, learning, and mastery within these ancient societies.
Similarly, studies on stone tool use should take into consideration the morphological differences of the hand among early humans, such as Homo habilis, Homo ergaster, and modern humans (Homo sapiens), particularly noting the more robust and curved finger bones in early humans which suggest powerful grips and climbing adaptations, in contrast to the more gracile and dexterous hands of modern humans that are optimized for fine motor skills and complex tool manipulation [28,39].However, even though some differences can be appreciated, the comparison between early and modern humans is challenging due to the scarcity and incompleteness of fossil records, which are often not associated with any particular individual, making it difficult to directly quantify intrinsic hand proportions or provide an overall functional interpretation of the hand in these Homo species.
To enrich our understanding of these ancestral behaviors, future studies could explore individual differences in visuospatial skills during stone tool manipulation, further connecting cognitive abilities and visuospatial strategies to the nuanced ways in which prehistoric humans interacted with their material culture.By expanding our focus beyond finger flexion to encompass other haptic and cognitive components and by considering tools of varying sizes and functional demands, we aim to contribute to a holistic portrayal of the intricate relationship between human hands and the tools that shaped our evolutionary trajectory.

Conclusions
During comfortable manipulation of Lower Paleolithic stone tools, males and females show different patterns of phalanx flexion.Hand dimensions are related to the patterns of phalanx flexion only when the sample is pooled.However, when males and females are considered separately, the correlation no longer stands.Although the sample size is not particularly large, a relevant association between hand size and grasping behavior is very small, negligible, or even absent, at least with regards to our sample.
Considering that the grasping is related with many aspects beside ergonomics, we may suggest that the differences that we found in our study could be due to certain individuals' cognitive, behavioral, or motor aspects.
For example, experience in large-object manipulation, grip strength, sociodemographic and occupational factors could be responsible for the differences [40][41][42][43][44].If this is the case, information about individuals' lifestyles or psychometric profiles could help us to further understand these results.
To fully understand the Paleolithic techno-complex, it is crucial to consider the broader socio-economic and cultural contexts that influenced lithic production and utilization.Socio-economic factors such as resource distribution, trade networks, and division of labor played significant roles in shaping the production and use of stone tools.Additionally, gender roles in lithic production are essential for understanding past societies, potentially reflecting specific responsibilities and contributions of community members.However, distinguishing gender-specific domains in the archaeological record remains challenging.We must avoid applying preconceived concepts about household structures to archaeological data and critically consider biases in our analogical materials.Addressing these challenges helps us understand the dynamic and fluid nature of gender roles in past societies, avoiding oversimplified interpretations [45].
The continuous use of specific materials for toolmaking over millennia reflects deep familiarity and complex relationships with the environment [46].Thus, tools crafted by early humans were not merely functional objects but embodied cultural conventions and ontological beliefs [47].Recognizing these aspects allows us to appreciate the full spectrum of past human behaviors and interactions with their environment.Integrating these perspectives provides a more comprehensive and nuanced interpretation of the prehistoric techno-complex, acknowledging that functional, economic, and spiritual-symbolic meanings are all integral parts of human life.
The results presented here should be interpreted in two different contexts.Firstly, they add to the general study of haptic exploration behavior, bridging perception and cognition associated with hand-tool interaction [17,[48][49][50][51]. Secondly, taking into consideration that these large-sized tools represent the earliest record of a consistent human technology, we should consider whether or not these behaviors can also provide information regarding the evolution of the hand-tool interaction system.We humans are obligatory tool users [52] and have a tool-dependent culture [53] and technology-based cognition [54]; we should hence consider that our haptic capacities are deeply rooted in our phylogenetic history.

Figure 1 .
Figure 1.Replicas of Lower Paleolithic stone tools used in the experiment.

Figure 1 .
Figure 1.Replicas of Lower Paleolithic stone tools used in the experiment.

Figure 2 .
Figure 2. (a) The hand dimensions measured in this study are hand length (HL), palmar length (PL) and palmar width (PW).(b) Phalanx joints are situated at the metacarpophalangeal joint (MCPred dots), proximal interphalangeal joint (PIP-yellow dots), and distal interphalangeal joint (DIPgreen dots).(c) Static hand posture was measured by recording the angular position of the 14 joint angles (b) of the fingers and of the thumb, during the comfortable grasping of each tool.

Figure 2 .
Figure 2. (a) The hand dimensions measured in this study are hand length (HL), palmar length (PL) and palmar width (PW).(b) Phalanx joints are situated at the metacarpophalangeal joint (MCP-red dots), proximal interphalangeal joint (PIP-yellow dots), and distal interphalangeal joint (DIP-green dots).(c) Static hand posture was measured by recording the angular position of the 14 joint angles (b) of the fingers and of the thumb, during the comfortable grasping of each tool.
Quaternary 2024, 7, x FOR PEER REVIEW 5 of 15with the Declaration of Helsinki, and the protocol was approved by the Ethics Committee of the University of Burgos.

Figure 3 .
Figure 3. Experimental procedure.Using a cyber glove, finger flexion during stone tool manipulation was recorded.

Figure 3 .
Figure 3. Experimental procedure.Using a cyber glove, finger flexion during stone tool manipulation was recorded.

Figure 4 .
Figure 4. PC1 and PC2 of phalanx flexions: the loading values of the 14 joint angles are shown as colored dots.Colors indicate which range of values the loadings belong to.Positive loadings are increasingly dark red, and negative loadings are increasingly green.PC1 and PC2 scores of males and females are plotted in a violin plot.

Figure 5 .
Figure 5. Correlation between hand measures and principal components of phalanx flexion.Hand measures are expressed in centimeters.

Figure 6 .
Figure 6.Distribution of the values of degree of phalanx flexion in males and females (median, interquartile, and range).Higher values mean more flexion.Significant differences between males and females are marked with an asterisk.

Figure 6 .
Figure 6.Distribution of the values of degree of phalanx flexion in males and females (median, interquartile, and range).Higher values mean more flexion.Significant differences between males and females are marked with an asterisk.

Table 1 .
Median values of stone tool measurements (in mm).

Table 1 .
Median values of stone tool measurements (in mm).

Table 2 .
Summary statistics of palmar length (PL), palmar width (PW), and hand length (HL) for females (F) and males (M).Values are expressed in cm.

Table 2 .
Summary statistics of palmar length (PL), palmar width (PW), and hand length (HL) for females (F) and males (M).Values are expressed in cm.

Table 3 .
Shapiro-Wilk (W) test for normal distribution of the 14 variables of phalanx flexion.With p values less than 0.05, the data tested are not normally distributed.

Table 3 .
Shapiro-Wilk (W) test for normal distribution of the 14 variables of phalanx flexion.With p values less than 0.05, the data tested are not normally distributed.

Table 5 .
Correlation between the first two components of phalanx flexion and hand dimensions, for males and females.

Table 6 .
Spearman rank correlation test between the 14 variables of phalanx flexion and hand dimensions, for males and females.

Table 7 .
Mann-Whitney U test between males and females.The Holm-Bonferroni sequential method for multiple hypothesis tests is used to correct Type I errors.Statistically significant differences between males and females are displayed in bold.