Problems and solutions in entheses studies

Although there are many promising results from population studies of entheses, there are many problems. Other studies, including that by Cardoso and Henderson (2010), have had negative results, where no differences were observable between largely contrasting

populations. A number of reasons could be suggested for this. First, a lack of entheseal pattern in some populations, especially in more modern sedentary populations, may be a result of less intensive and more varied lifestyles, and thus are genuine. Researchers

however, tend to see a lack of a pattern as a failure of the methods. This means negative results are potentially not discussed in the most helpful way. Secondly, inter-observer error plays a large role in determining the possibility of site comparisons. Intra- and inter-observer error can also lead to inconsistent scoring. A lack of clarity in scoring methods renders data un-useable by other researchers. Variation in entheseal expression means that the production of standard recording methods has been problematic, and it remains

uncertain what is the best way of recording entheseal changes. A recent workshop on entheseal change (Santos et al. 2011) highlighted this as a significant problem needing address.

Scoring is also problematic. Some authors (Hawkey and Merbs 1995, Robb 1998b,) suggest a grading scale of 0–3, whereas others (al-Oumaoui et al. 2004, Cashmore 2008) suggest that entheseal changes should either be scored as present or absent. This latter method avoids the problem of trying to assess grade on a change that is so variable, but may lack the sensitivity needed to highlight group differences.

4.2.2.1 Relationship between entheseal change and strength, activity and intensity

The exact nature of the relationship between muscles, enthesis morphology and stress levels is not fully understood. Currently, it is not clear what exactly entheseal changes represent in terms of muscle usage and whether development follows a linear progression equivalent to the graded system of Hawkey and Merbs’ (1995). This problem has been highlighted by many authors (al-Oumaoui et al. 2004, Stirland 1991, Weiss 2003a), who note that many researchers forget that grades 0-3 do not mean that grade 2 is twice as big as grade 1. While on a test of known occupation skeletons from the Coimbra collection, no difference in entheseal scores was found between occupational groups (Cardoso and Henderson 2010), Godde and Wilson-Taylor (2011) have found a correlation between entheseal score and activity in a known population. Significant problems may occur from our definition of ‘occupation’ (Cardoso and Henderson 2013) and also occupational mobility (Henderson et al. 2013).

It is highly likely that there is some genetic component to the development of changes at the sites of entheses; for example, some people are predisposed to forming a lot of bone (Waldron 2009), and it is probable that these people may have elevated entheseal scores.

Hormonal variation has been identified as another key factor in development (Cardoso and Henderson 2010:4) but it is not yet fully understood how this occurs. Finally, muscle development between males and females differs, which may have implications for the identification of gendered division of labour.

4.2.2.2 The impact of age on entheseal change

Age has a cumulative effect on the size and score of entheseal changes (Robb 1998b, Stirland 1998, Weiss 1998 and 2004, Wilczak 1998). Weiss (2004) identified that age was the best indicator of entheseal score in a group of British Columbians and prisoners from Quebec (i.e. older individuals had higher scores). Unsurprisingly, this means that when interpreting entheseal data, consideration of the age of individuals and according controls is required. Despite Weiss (1998) making it clear that age controls were critical when seeking clues to population activities, Weiss (2007) noted 10 years later authors were still not taking age into consideration when presenting data on population activities. Age can be controlled for by selecting the younger members of the population under study, and by carrying out correlation testing to determine whether age is a significant factor in the expression of entheseal scores. These can then be statistically controlled.

4.2.2.3 The relationship between entheseal score, sex and body size

Many authors have explored the differences between male and female patterns of entheseal change both within populations (Eshed et al. 2004, Lovell and Dublenko 1999, Wilczak 1998) and between populations (al-Oumaoui et al. 2004, Churchill and Morris 1998, Munson Chapman 1997, Steen and Lane 1998). The gendered division of labour is often a focus of research, often to establish if it changed at pivotal moments in societies. Many of the studies above have highlighted gender differences, but have been criticised for not considering the impact of sex (Weiss 2007). Sex variation of entheseal scores is largely

related to the body size difference between men and women (Weiss 2007). It has been highlighted that males nearly always have higher entheseal scores than females (Weiss 2007:931), but while some authors use activity differences to explain this, Weiss (2007) argues that when body size is taken into account, these differences disappear. In fact, primate analysis has highlighted that there is a correlation between body size and entheseal scores regardless of locomotor style (Weiss 2004:233). Although this seems discouraging, if females have higher entheseal scores than males, then these are extremely likely to be the result of activity differences. This, however, demonstrates the importance of controlling for sex and body size, and is particularly important in cross-population analyses. Importantly Robb (1998b) has argued that body size cannot be the sole dictator of entheseal scores because we observe asymmetry in entheseal scores and body mass is intrinsically controlled.

4.2.2.4 Entheseal changes and side

General right handedness is a defining characteristic of humans and occurs at a level of nearly 90% in all populations (Cashmore et al. 2008:7). The frequency of left handedness ranges from 0% to 27% globally (Cashmore et al. 2008:8). Dominance of one side therefore has the potential to create side difference in entheseal scores. Research by Weiss (2003a, 2004 and 2007) has shown entheseal scores to be correlated to side, albeit not as strongly as age. While it may be possible to control sidedness through selecting entheses from one side (as in Robb 1998b), this potentially could obscure entheseal patterns, particularly if group differences are related to laterality. In fact, Doyling (2010) found greater asymmetry in white-collar workers over manual labourers in a modern population.

While entheseal asymmetry is usually the result of activity, a differential cause is highlighted by a Neolithic Siberian individual who had pathology which caused severe upper limb asymmetry in entheseal scores (Lieverse et al. 2007).