Life In Bone A Look at Skeletal Markers for Activity

Ceilidh Lerwick 12/3/2009

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Activity patterns to bioarchaeologists are a bit like the search for the Holy Grail:

not enough information to figure it all out, and no one is sure that finding the answer is

even possible in the first place. Yet ―(p)hysical activity is a defining characteristic of

human adaptive regimes‖ and ―(w)orkload and activity have enormous implications for

the demographic history of a population.‖ (Larsen 1997:161) So, we continue to delve

into science and information, hoping to find enough clues to lead us to the ―Golden

Cup‖—figuring out the what, how and sometimes even the where and who of the past.

Markers of occupational stress, or MOS, include conditions such as arthritis, robusticity

and articular modifications. (Kennedy 1989, 1998, Wilczak & Kennedy 1998) These

conditions are evaluated for the information they can give us about activity patterns in

the past. But, how is this information being gleaned? What are the questions being

asked, and are we even asking the right questions in the first place?

One of the most well researched markers of activity is osteoarthritis and its

associated counterparts: arthrosis, septic arthritis, rheumatoid arthritis, etc. all to be

understood hereafter in the term: arthritis. Arthritis is a multi-factorial disorder which

is dependent on things such as: genetics, sex, climate, health and weight, metabolism,

and trauma; just to name a few. (Weiss et al 2007, Larsen 1997, Jurmain 1991).

Essentially, arthritis is ―the result of a physiological imbalance between the mechanical

stress in the joint tissue and the ability of the joint tissues to withstand. . . stress.‖

(Radin 1982:20) Even more so, it is the destruction of the joint tissue itself, leading to

porous proliferation, osteophytes, eburnation, etc. Thus arthritis can be a strong

indicator of activity patterns, especially those chronically performed.

For example, Merbs (1983) found a high incidence of arthritis in the left

temporomandibular joint in the Sadlermuit women of northern Canada. He had the

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added benefit of ethnographic information demonstrating that the women in this

population were softening leather using their left side dentition. Stirland (2001) found

that the sailors aboard the 16th century ship Mary Rose had a high prevalence of

thoracic arthritis and from historical and ethnographic accounts was able to deduce this

was from the heavy lifting involved in life at sea. Wells (1982:152) has even referred to

arthritis as ―the most useful of all diseases for reconstructing the lifestyle of early

populations‖; and at the time this statement was made, the sentiment was probably

quite accurate.

Musculo-stress markers, also known as MSMs or enthesopathies, are another

well studied form of activity marker. These are located at the point where muscles

attach to the bone. Bone remodeling theory holds that when muscle insertion sites are

subjected to stress, blood flow is increased, stimulating osteoblasts to form new bone in

the stimulated area. Enthesopathies, or bone proliferation at muscle attachment sites,

is the result. (Larsen 1997, Hawkey & Merbs 1995, Kennedy 1989, 1998, Wilczak &

Kennedy 1998) ―Anthropologists have concluded using bone remodeling theory that

large muscle markers are due to continued muscle use in daily and repetitive tasks

(especially when started at a young age and continued through adulthood), which has

made muscle markers ideal for reconstructing past lifestyles.‖ (Weis 2007:931)

Fur traders are one group in which MSMs have been a significant line of study, in

part because of the documentation available on the physical activities of the occupation,

and also in part because of the vast amount of conspicuous bone modification available

for study in their skeletons. Fur traders are known for having carried immensely heavy

loads and for rowing or mushing for hours on end with only 6-7 hours of rest in a day.

(Lai & Lovell 1992, Lovell & Dublenko 1999). Markers on the fur trader‘s skeletons

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include extremely pronounced muscle attachment sites, especially in the attachment

sites for the trapezius, pronator quadratus, deltoid, brachialis and supinator: all major

muscles used in rowing. Two of the individuals from the Fort Edmonton locale, have

brachialis attachment sites that have progressed to the point of becoming lesions.

(Lovell & Dublenko 1999) Attachments for flexor muscles of the hands are also marked.

Of the two surviving mandibles at the Seafort site, both exhibit pronounced squaring of

the anterior margins of the horizontal ramus as well as considerable gonial eversion.

This is suggestive of a great stress to the masseter muscle. (Lai & Lovell 1992) All

markers are consistent with, even a testament to, the known heavy activities performed

by those involved in the fur trade.

Cortical defects have also been classified as MSMs. (Larsen 1997) These defects

are cavities in the cortical surface of bone with a rough or irregular floor and relatively

smooth margins. Like enthesopathies, it has been suggested that cortical defects are

caused by chronic mechanical stress at muscular insertion sites; and cortical defects do

sometimes appear at points near the radial-medial head of the gastrocnemius or the

insertion sites for the pectoralis and teres major. (Larsen 1997) However, this

hypothesis seems a bit deficient in its reasoning. Why would chronic stress produce a

mound (hypertrophic proliferation or enthesopathy) in one instance and then produce a

hole (cortical defect) in another?

In addition, not all cortical defects occur near muscle insertion sites. Stafne‘s

defect, for instance, is a classic cortical defect which occurs on the lingual surface of the

mandible anterior to the pterygoideous internus attachment, inferior to the mylo-

hyoideus attachment, and posterior to the digastric attachment site. In other words,

nowhere near a muscle attachment. It is, however, exactly in the fossa for the

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submaxillary gland. This would appear to indicate a connection to this gland and not a

tendonous articulation. It would at least seem unconnected, or at best tangentially

connected, to any musculo-attachment sites. (Lukacs and Martin 2002, Gray 1918,

Dauzvardis et al 1998, Langlais et al 1976, Kay 1974) In this regard, it is still possible

that cortical defects represent some form of MOS; however, it seems more likely that

they are linked with bursars or some other pathology with merely a tangential

connection to muscle attachments.

A third line of study into skeletal markers of occupational stress is bone

robusticity. In 1892, Julius Wolff made his famous hypothesis that has been come to be

known as Wolff‘s Law. His theory was essentially that in a healthy individual, bone will

adapt to the stress load under which it is placed. If loading on a particular bone

increases, the bone will remodel itself to become stronger; usually involving an increase

in bone density and thickness. This will increase the amount of effort that individual‘s

body needs to put in to maintaining this structure. Therefore, the inverse holds as well:

if loading decreases, the bone will resorb and remodel the bone; decreasing the density

and thickness. (Wolff 1892)

Although modifications have been suggested and made to Wolff‘s original theory,

(Frost 1960, Ruff et al 2006) the basic concept still stands and plays a part in the above

mentioned MSMs. It also plays a significant part in overall bone robusticity. Stock and

Pfeiffer (2001), for example, found a greater lower limb robusticity in African terrestrial

foragers when compared to marine foragers on the Andaman Islands. The marine

foragers also exhibited a greater upper limb robusticity. Their conclusion was that the

terrestrial foragers had a much more mobile lifestyle, at least in terms of using their own

two feet for locomotion; while the islanders‘ bipedal mobility was limited, most travel

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being accomplished while sitting in boats. The upper arm robusticity was explained by

the weight of hauling in fishing nets; an equivalent activity that the Africans did not

have.

―Adults who exercise tend to have higher bone mass and size than adults who are

relatively sedentary‖. (Larsen 1997:196). Thus, studying and quantifying robusticity has

been a key direction in the research into activity patterns and the main method of

quantifying this has been with the use of cross-sectional analysis. Cross-sectional

analysis is a method borrowed from engineering. It applies the basis of beam theory to

evaluate a longbone‘s strength using measurements taken from the cross-section.

Measurements can be taken by cutting the bone perpendicular to the main axis or by use

of modern imaging methods.

Evaluation using the external measurements has been done, but not with the

vigor of the cross-sectional method; the rationale being that external evaluation neglects

the thickness of the cortical bone as well as neglecting the inclusion of all the various

ways bone can strengthen itself. (Larsen 1997, Ruff et al 2006, Demes 2007)

[B]ones are rarely subject to compression alone. During movement, both bone shape and muscle actions create high levels of bending and torsional strains in bones. These forces are resisted not just by the amount of cortical bone, but by how it is distributed in a cross section. Second moments of area are used to estimate the resistance of the cross section to these forces: I, which represents the second moment of area, is used to estimate resistance to bending around a particular axis (for example, I measures resistance around the anteroposterior axis). The polar second moment of area (designated J), which estimates the resistance of the cross section under torsional or twisting forces, is frequently used as a general indicator of bone strength. (Bridges 1996:112)

The benefit of the cross-sectional method is a broader range of information about

how the bone has modified itself, hopefully leading the researcher to a better

understanding of what type of force (torsion, bending, compression, etc.) caused the

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modification. If the type of force is understood, this can lead to the knowledge of what

activities the individual(s) was involved in.

The problem with cross-sectional analysis is wrapped-up in the very reason it was

created. Engineers use it to test the strength of beams (or tubes) for use in construction.

A beam is a long stretch of material, say iron or wood, in which the sides are parallel to

one another. In the case of a tube the sides are parallel to the central axis. In the case of

an I-beam, the sides are connected by a wall that runs perpendicular to the sides. In

each case, the beam is a geometric object with no alteration of its structure from one end

to another.

A bone, on the other hand, is not a geometric object. At least not in the man

made sense. A long bone is essentially a tube, but the sides rarely, if ever, run parallel

to the central axis. In fact the sides rarely, if ever, even run in a straight line. There are

dips and twists and undulations. In addition to this, a beam is essentially a static being.

It is what it is and, except for some changes that do occur with time and environment

(iron rusts, wood decomposes, etc.) continue to be what it is until its use has finished.

Bone, on the other hand, is not static. While bone will always remain bone, it is a

dynamic substance: increasing density here, shifting shape there, until the original

shape is augmented and morphed to withstand the stress it is under.

Cross-sectional analysis has yet to account for this dynamic nature of bone beam

analysis. And so, researchers continue to make statements such as this one by Knusel

(2007): ―Although there is a growing understanding of how bone responds to stress, it is

rarely possible to identify the occupation even of living people from physical changes to

their bodies.‖ Jurmain (1991) also cautioned that the connection of specific physical

stressors with skeletal characteristics is virtually impossible for prehistoric groups and

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may not be possible even for people for which historical records and ethnographic

analogy are available.

Yet there is, if not disagreement with these sentiments; there is hope. Ronchese

published his 1948 book Occupational Marks and Other Physical Signs, much of which

is soft tissue related, but some skeletal information is included. Rogers et al (1987) at

least attempt to list arthropathies with most probable cause. And Kennedy et al (1999)

published a list of skeletal markers and their suggested causation.

So, will bioarchaeology be able to find the Holy Grail? While physical context and

material culture give clues to past behavior, analysis of the skeletons themselves is the

most direct way to reconstruct individual conduct and to explore intra- and

interpopulational differences in behavior. Research has only scratched the surface of

the enormous amount of information available from the skeleton. Living beings are

dynamic. Perhaps our techniques need to be as well. Can we compile our information

based on more than one evaluation? Start seeing the body as material culture like any

other artifact we would study. (Soafer 2006) Taking their cue from Lawrence Angel

Perhaps the Sauls (1989) have it right in looking at the story the skeleton tells you and

telling it in an osteobiography.

Take, for example, this lower right leg from a Fort Edmonton fur trader (Fig 1).

There is a pronounced hypertrophic MSM for the soleus on the posterior tibia. In

addition, there is an arthritic lesion at the proximal tibio-fibular articulation in the form

of remodeled bony proliferation on the tibia and resorption on the fibula. There is no

mention of this from Lovell & Dublenko (1999), and while it is difficult to say from this

photo, there seems to be a septic nature to this pathology. This condition is actually

apparent in two of the fur trader skeletons. There is a similar resorptive lesion on the

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medio-proximal tibial surface at the

articulation of the semimembranosus

ligament. There is no arthritis or

eburnation on the superior surface of the

tibial proximal condyles.

Lovell & Dublenko (1999:254)

surmise that these lesions ―could be due

to the locomotor stresses of portaging

although an alternative explanation, and

perhaps a better one in this culture-

historical context given that the lesion

appears unilaterally, on the right leg, in

both individuals, is that the lesion results

from habitual ‗kicking‘ of the leg when

driving dog sleds.‖ No further investigation was made within this paper.

Is this then the end of the information these bones can tell us? Is this a complete

osteobiography? Is there more that these bones can say if we but knew how to listen?

There is no suggestion explaining the hypertrophy of the popliteal line. No cross-

sectional analysis was done, or if it was, the results were not published. What can the

cortical bone density and distribution tell about this individual‘s activities? Why is there

no eburnation on the condylar articular surface, but so much arthritis around the

proximal metaphysis area? Activity patterns in youth affect the joints more than the

diaphyses of bones; thus joint changes can be a clue to the identity of activities begun in

youth, while shaft changes can be a clue to activities continued into or simple taken on

Figure 1 from Lovell and Dublenko (1999)

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in adulthood. (Knusel 2007, Ruff et al 1994) Could this knowledge be applied to the fur

trader skeletons? What can this concept tell us about the strength of the cortical bone of

this lower leg and what can it tell us about the obvious arthritic degeneration?

In addition, what about the condition of the fibula?

Fibulae are bones that are often regaled into the category of

―highly variable‖ without any significance being assigned to

that variability. It should not be forgotten that osteology is

covered by pathology, and there are actually quite a few

muscles that attach to (and therefore act upon) the fibula.

The fibula also acts as a strut or buttress, giving support to

the lateral condyle of the tibia and acting to stabilize the

ankle. (Lambert 1971) This function is highly suggestive of a

bone with strong forces acting upon it; and where there are

forces, there is Wolff‘s Law.

Notice that the Fort Edmonton fibula (fig 1) has a very

thin tubular shaft with a relatively large proximal head.

Compare this to a ―normal‖ fibula (fig 2) which has a more

triangular shaft with rounded edges. Could there be certain

muscle forces, certain mechanical loadings, and therefore

certain activities that can be surmised from this fibular

morphology?

This fibula (fig 1) also has what is usually referred to as a hypertrophic

interosseous crest. Notice the difference in the hypertrophy of the broadly rounded

popliteal line compared to the thin, wide, and even sharp hypertrophy of the

Figure 2 from Gray (1918)

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interosseous crest. Should not only the question ―Why is the crest hypertrophic?‖ be

asked; but also ―Why is the crest hypertrophic in this manner?‖ Is there something in

the way the interosseous membrane functions that would produce such hypertrophy?

(Currently the function of interosseous membranes is poorly understood. See Skahen et

al 1997a & b and Wallace et al 1997).

What is to be gleaned from the apparent robusticity

of this tibia and the overall robusticity of the fur trader

skeletons in general? Take, for example, these two ulnae,

also from the Fort Edmonton site. The ulna on the right is

from a male. The one on the left is from a female. Both

ulnae scored an R3 using the Hawkey & Merbs (1995)

criteria. (Lovell & Dublenko 1999) Weiss (2007) cautions

that both size of the element and age of the individual need

to be taken in to consideration when assessing trait

robusticity. Even taking the minimal size differences into

account, the two ulnae do appear homogeneous, at least

where robusticity is concerned. This would seem to suggest

the same amount and similar type of work between males

and females. And yet, from ethnographic accounts, we know that women in the fur

trade did not perform the same tasks as the men. Perhaps robusticity is an erroneous

assessment and morphology the better evaluation. Could more information be gleaned

from not only the robustness, but also the apparent angle difference in the necks of the

two ulnae; or the shape difference in the interosseous crest?

Figure 3 from Lovell and Dublenko (1999)

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Researchers are looking at shape as an indicator of activity. Yet this is still playing

only a small part in their investigations. Rhodes and Knusel (2005), spend only 17 lines

in a 10 page paper addressing the shape of the humeri under investigation. Shaw and

Stock (2009) give it more discussion: 49 lines in the same number of pages. And, a few

researchers are showing shape its due deference. Galtes et al (2009) use the function of

shape to better understand curvature in the radius, and Bridges (1996) tries to include

shape and morphology to aid in her investigation of the shift over time from robusticity

to a more gracile skeleton among the genus Homo. Perhaps this line of research will aid

in the continued search for the Holy Grail.

Bones experience forces throughout an individual‘s life. Wolff‘s law applies and

bone modifies itself to enable itself to withstand the force or forces in question, whether

torsion, bending, shear, compression, tension or all of the above. In attempting to

discover the Holy Grail—markers of occupational stress and ultimately knowledge of

past activities—bioarchaeologists use arthritis, MSMs, and robusticity as primary lines

of investigation. There are problems with each method, but continued use of

methodology will hopefully lead to a refinement and improvement in techniques. And

perhaps trying to read the whole story the bones are telling us, rather than just a section

of the script will lead us to better techniques and even better questions to ask. Perhaps

thinking outside the box would benefit us all?

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