**1. Introduction**

The hand is essential to how modern humans interact with their environment, as it was for our extinct relatives. The enhanced dexterity of the human hand is unique among living primates and is generally considered to have evolved through both (1) adaptation to bipedalism and a relaxation of locomotor selective pressures on the hands and (2) increasingly more complex tool production and use in hominins (i.e., group consisting of modern humans and our closely related extinct relatives) [1,2]. The use of stone tools would have allowed early hominins to access different and potentially higher-quality foods (e.g., marrow) [3,4]. The manufacture and use of even relatively simple stone tools, such as Oldowan technology (2.6–1.7 million years ago) [5,6], would have required both

**Citation:** Bardo, A.; Kivell, T.L.; Town, K.; Donati, G.; Ballieux, H.; Stamate, C.; Edginton, T.; Forrester, G.S. Get a Grip: Variation in Human Hand Grip Strength and Implications for Human Evolution. *Symmetry* **2021**, *13*, 1142. https://doi.org/10.3390/ sym13071142

Academic Editor: Antoine Balzeau

Received: 14 May 2021 Accepted: 8 June 2021 Published: 26 June 2021

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increased cognitive function (e.g., learning, working memory/future thinking, planning and decision-making etc.) [7,8] and particular biomechanical demands on the anatomy of the hands [9–11]. Thus, it is likely that tool production and use played a critical role in shaping both cognitive development (e.g., with the crucial role of social learning) [12] and hand morphology. For example, a long, powerful thumb and relatively short fingers facilitate the forceful precision and power-squeeze gripping that are considered to be unique human abilities [1,13]. Although modern humans are also adept as using their hands for locomotion [14,15], the upper limbs are predominantly used for manipulation.

Humans are unique among primates in the strength of population-level hand directional asymmetry (DA) or laterality (i.e., preference for one hand, called the dominant hand, over the other, non-dominant hand), with 85–90% of humans being right-handed regardless of geographical region and ethnicity [16–19]. Non-human primates also show population laterality for object manipulation, but not with the same strength as that found in humans ([20–22], see [23]) and their laterality can also vary depending on the complexity of the manual task (e.g., bimanual manipulative action versus tool use) [24]. Moreover, motor skill biases for tool use in chimpanzees may be supported by anatomically asymmetric, left-biased brain regions analogous to Broca's and Wernicke's area in humans [25], brain regions that are both implicated in the perception and production of speech. Handedness (i.e., side preference for the right or the left hand) in humans is thought to have played an important role in the lateralisation of the human brain for language [26] and the emergence of other complex cognitive functions, including tool use [27,28], manual gestures [29,30], and throwing [31]. Greenfield [32] proposed that it was the motor sequencing for tool use—requiring dexterous hierarchical motor activities—that paved a way for the emergence of language that likely emerged first in the form of hand gestures [33,34]. Thus, more dexterous hands may have increased object manipulation capabilities that, in turn, increased hemispheric specialisation and DA, suggesting that the capacities for tool use and language evolved together [32]. However, when population-level handedness first evolved within the hominin clade remains unclear [35,36].

Hand size, shape [37–40], and bone morphology are also highly variable among recent human populations [41,42]. How this variability potentially affects hand function may provide insights into the evolution of the human hand. For example, ergonomic studies have shown that handle design is important for hand grip performance (e.g., time to complete the task and strength use) [43] and that hand size strongly affects performance [44], indicating the importance of designing tools in accordance with current anthropometric data. Moreover, individuals with relatively longer fingers and therefore larger joint surfaces require less force during stone tool production than those with shorter fingers [45]. Key and Lycett [46] found that through experimental stone tool use, grip strength was the primary contributing biometric factor for stone cutting efficiency. Therefore, both hand shape and hand strength were likely important factors in the efficient stone tool production and use, and would have played an important role in the evolution of hominin cultural technology.

Hand grip strength is commonly measured in a clinical or sports medicine context as an indicator of overall muscle strength [47–51]. Grip strength reflects the gross power of the hand and has been found to be strongly associated with physical activity [52–55], as well as anthropometric traits, such as age and sex [56–58], hand length and shape [59,60], handedness [61], and body mass index [62–64]. For example, males typically have a stronger average grip strength than females [56,65]. In both sexes, hand asymmetry in grip strength was found, with the dominant hand (defined as the hand used most within the context of object manipulation) is approximatively 10% stronger than the non-dominant hand [61], although this difference is more pronounced, and is therefore more of a DA, for right-handed individuals than left-handed individuals [66]. Furthermore, hand size has been shown to be positively correlated with grip strength for both sexes [67–70]. It was also found that hand shape influences grip strength [59,71], such that, for both sexes, people with bigger hands (i.e., large hand length and width) were significantly stronger than people with smaller hands. Moreover, Carlson [72] proposed that, although variation

in hand grip strength primarily reflected differences in soft tissue and skeletal morphology, changes in grip strength across the lifespan were also significantly influenced by neural mechanisms (e.g., central nervous system recruiting motoneurons to mediate the control of coordinated movement). Thus, grip strength can be used as a marker of brain health [72]. Indeed, maximum grip strength provides a discriminating measure of cognitive function, such as how central nervous system disorders (e.g., vascular disorders, structural disorders or degeneration) affect the quality of motor coordination [72]. The rate of decline in cognitive function (e.g., motor and perceptual speed, memory, and spatial functioning) has also been shown to correlate with a decline in grip strength, especially towards the end of life [73].

However, most previous studies of grip strength have focused on specific populations [57,74–76], occupations and activities [51,52,54], or sex and age [77–79] with the aim to better understand health, but these same methods may also be useful for understanding the broader scope of form and function from an evolutionary perspective. Although informative, these studies do not fully capture the potential variability in the key factors that can affect grip strength, particularly hand size, shape, and daily use. To broaden our understanding of the link between hand form and function, this study aims to evaluate the variation of grip strength in a heterogeneous and international group of human adults across the lifespan. We test the potential influence of age, sex, asymmetry in hand dominance and preference (i.e., right- vs. left-hand), hand shape, and lifestyle factors (i.e., occupation, practice of sport and music) on grip strength. In this context, we explore hand asymmetry in grip strength as an indicator of brain/manual lateralisation, with hand dominance (i.e., significant difference between the dominant and the non-dominant hand, without taking into account the left-right direction) and DA (i.e., pattern of bilateral variation observed when one side, right or left, is significantly stronger than the other). Based on previous studies, we predict that (1) males will be significantly stronger than females; (2) younger participants will be stronger than the oldest participants; (3) hand asymmetry will be found, with the dominant hand significantly stronger than the non-dominant hand and that this effect will be stronger for right-handed compared with left-handed individuals indicating DA; (4) participants with wider hands (i.e., a hand wider than it is long) will be significantly stronger than those with smaller or longer hands (a hand longer than it is wide); and (5) participants that regularly practice sport, music, or occupational activities that engage their hands will be stronger than those who do not.

## **2. Materials and Methods**
