1. Manual Dexterity Activities

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Fight or flight: Experts say human hands evolved for punching and not just dexterity

Published: 23:13 BST, 19 December 2012 Updated: 07:45 BST, 20 December 2012

Human hands evolved for punching - not just dexterity, according to a new study.

Men whacked punching bags for the study that suggests human hands evolved not only for the manual dexterity needed to use tools, play a violin or paint a work of art, but so men could make fists and fight.

Compared with apes, humans have shorter palms and fingers and longer, stronger, flexible thumbs - features that have been long thought to have evolved so our ancestors had the manual dexterity to make and use tools.

The study's senior author biology Professor David Carrier, of the University of Utah in the United States, said: 'The role aggression has played in our evolution has not been adequately appreciated.

'There are people who do not like this idea, but it is clear that compared with other mammals, great apes are a relatively aggressive group, with lots of fighting and violence, and that includes us.

'We're the poster children for violence.'
He said: 'Humans have debated for centuries about whether we are, by nature, aggressive animals.

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'Our anatomy holds clues to that question. If we can understand what our anatomy has evolved to do, we'll have a clearer picture of who we were in the beginning, and whether aggression is part of who we are.'

Prof Carrier agrees that human hands evolved for improved manual dexterity, but added: 'The proportions of our hands also allow us to make a fist, protecting delicate hand bones, muscles and ligaments during hand-to-hand combat.'

He added: 'As our ancestors evolved an individual who could strike with a clenched fist could hit harder without injuring themselves, so they were better able to fight for mates and thus more likely to reproduce.

How we punch: The four fingertip pads touch the pads at the top of the palm, and the thumb wraps in front of the second, third and part of the fourth finger, which are locked in place by the palm at the base of the thumb.

'Fights also were for food, water, land and shelter to support a family, and over pride, reputation and for revenge.'

Prof Carrier and co-author Michael Morgan, a medical student, conducted their study to identify any performance advantages a human fist may provide during fighting.

The first experiment tested the hypothesis that humans can hit harder with a fist.

The researchers had 10 men - aged 22 to 50 and all of them with boxing or martial arts experience - hit a punching bag as hard as they could.

Each subject delivered 18 blows, or three of each for six kinds of hits: overhead hammer fists and slaps, side punches and slaps, and forward punches and palm shoves.

The bag was instrumented to allow calculation of the force of the punches and slaps.

To the researchers' surprise, the peak force was the same, whether the bag was punched with a fist or slapped with an open hand.

However, a fist delivers the same force with one-third of the surface area as the palm and fingers, and 60 per cent of the surface area of the palm alone.

So the peak stress delivered to the punching bag - the force per area - was 1.7 to three times greater with a fist strike compared with a slap.

The second and third experiments tested the theory that a fist provides buttressing to protect the hand during punching.

Compared with a chimpanzee hand, at left, the human hand, at right, has shorter fingers and palms and a longer, stronger more flexible thumb that allows humans to make a clenched fist, which apes cannot.

Manual Dexterity Activities

To do that, the researchers measured the stiffness of the knuckle joint of the first finger, and how force is transferred from the fingers to the thumb.

These experiments found that buttressing provided by the human fist reduced flexing fourfold, and doubled the ability of the fingers to transmit punching force.

Prof Carrier said: 'Because the experiments show the proportions of the human hand provide a performance advantage when striking with a fist, we suggest that the proportions of our hands resulted, in part, from selection to improve fighting performance.

'The standard argument is that once our ancestors came out of the trees, the selection for climbing was gone, so selection for manipulation became dominant, and that's what changed the shape of our ancestors' hands.

'Human-like hand proportions appear in the fossil record at the same time our ancestors started walking upright four million to five million years ago.

'An alternative possible explanation is that we stood up on two legs and evolved these hand proportions to beat each other.'
The findings were published in the Journal of Experimental Biology.

Introduction

The hand is the most active and interactive part of the upper extremity. Hand dexterity is a term used to explain a range of different hand abilities and performances. These include reaction time; hand preference; wrist flexion speed; finger tapping speed; aiming; hand stability and arm stability (e.g., [1]). From these, four main factors are considered as the most characteristic and reliable for the evaluation of hand dexterity. These include: (i) steadiness; (ii) tracking; (iii) aiming (where typically the participant points to a target object), and (iv) tapping (where the participant taps as fast as possible for a set time period) ([1]; [2]; [3]; [4]; [5]).

The relationship between increased age and reduced hand dexterity has been widely reported in both the clinical and scientific literature. For example, [6] presented the first kinematic assessment that compared the reach-to-grasp movements for gender-matched groups of older aged (60–71 years, n = 12) and younger aged (18–25 years, n = 12) adults. Participants reached to grasp either a small cylinder using a precision grip or a large cylinder using a whole hand prehension. The actions performed by the two groups were similarly coordinated with similar times to peak for wrist velocity and acceleration from movement initiation (i.e. the transport component), and showed no differences in the size of the grip apertures used (i.e. object manipulation). However, the older aged participants made significantly slower movements than the younger aged adults, replicating previous findings (e.g., [7]; [8]; [9]; [10]; [11]). Although movement speed can be encapsulated within the term of hand dexterity, it is worth noting that slower movements with increased age may not necessarily correspond to a reduced performance for the other dexterity factors (e.g., aiming, stability etc.). This point will be investigated in the present study.

Despite numerous studies demonstrating a significant relationship between increased age and reduced hand dexterity, few studies have attempted to investigate the causes of the relationship. Instead, a common explanation is provided in the discussion of these papers stating that the relationship between increased age and reduced hand dexterity is likely caused by a decline in musculoskeletal strength and mass (see for example [12]; [13]; [14]; [15]; [16]; [17]; [18]). Much support can be found for these claims within related literature, with major reduction in muscle mass ranging from 20% to 45% in aging skeletal muscle (described as “sarcopenia of old age”; [19], p477) (see also [20]; [21]; [22]; [23]; [24]; [25]). More precise investigations of hand strength have also demonstrated diminished strength with increased age (e.g., [26]; [27]; [28]; [29]; [30]), with studies reporting that diminished hand strength appears associated to decreasing general muscle mass reduction ([13]; [21]; [25]; [31]). Furthermore, changes in muscle mass with increased age has been linked to changes in peripheral and central nerve conduction ([32]; [33]; [34]), proprioception ([35]) and changes in the human motor unit that relate to a degeneration of the nervous system ([23]; [36]); all of which are likely to impact hand dexterity.

Given the overwhelming evidence for relationships between age and hand dexterity, and between age and grip strength, it is perhaps surprising that there are very few studies that have investigated the relationships between age, grip strength and hand dexterity together in one study. Instead, the few studies considering the three factors in a single study have investigated differences in these factors for different age groups of participants. For example, Marmon et al. [17] tested three groups of adult participants (young 18–36, middle 40–60 and older ≥ 65 years aged adults) on measures of index finger abduction, precision pinch, and hand grip strength and measures of hand dexterity with the Grooved Pegboard test, the game Operation, a scissor task and a tracing task. The results showed significant differences between the young and older age groups for the measures of index finger abduction, precision pinch and handgrip strengths, and further for the measures of hand dexterity. A further analysis showed evidence of significant associations among similar tests (i.e. the strength measures; the steadiness measures and the four function measures) and between different measures. As a final multiple regression analysis showed that the time to complete two functional measures, the Grooved Pegboard and the game Operation were significantly predicted by index finger and grip strength or by pinch steadiness, index finger steadiness and grip strength respectively.

The aim of the current study was to extend these findings and investigate, using regression analyses, the relationships and interactions between age, grip strength and hand dexterity in adults. We hypothesised to replicate previous research by showing negative relations between age and strength, and age and hand dexterity, with increased age being related to reduced strength and hand dexterity. We also hypothesised a positive relationship between strength and dexterity (so that reduced strength was related to reduced hand dexterity). In a second phase of analyses, we then sought to extend the research findings by determining the variance of age and strength on the different components of dexterity (i.e. steadiness; tracking; aiming and tapping). In a final phase of analyses, we explored interactions in age and strength on hand dexterity.