Lateral asymmetry of human long bones

Variability and Evolution, 2001, Vol. 9: 19–32 19

TONKA ÈUK, PETRA LEBEN-SELJAK, MARIJA ŠTEFANÈIÈ University of Ljubljana, Department of Biology, Biotechnical Faculty, Veèna pot 111, SI-1000 Ljubljana, Slovenia

LATERAL ASYMMETRY OF HUMAN LONG BONES ÈUK T., LEBEN-SELJAK P., ŠTEFANÈIÈ M. 2001. Lateral asymmetry of human long bones. Variability and Evolution, Vol. 9: 19–32, Tabs. 2, Figs. 4. Adam Mickiewicz University, Faculty of Biology, Institute of Anthropology, Poznañ.

Abstract: Based on the anthropometric data of long bones of 26 female and 16 male medieval skeletons from the necropolis Središèe by the Drava river in Slovenia an analysis of lateral asymmetry was performed. We determined the degree of direct and standardized asymmetry of specific morphological characteristics in individual long bones and tried to deduct the right- or left-handedness of the individual. Our results indicate directed asymmetry which is more pronounced in the upper extremities. The most asymmetric bone is humerus, reflecting the hand preference. The dominant leg is expressed by the stronger tibia usually on the opposite side of the dominant arm. The stronger development of the left femur as supportive limb is characteristic of both right- and left-handers. Key words: anthropometry, bilateral asymmetry, long bones, medieval skeletons

Introduction During bone dynamic development we discern growth in both length and diameter. Growth in diameter is dependent on periosteal ossification while growth in length depends on endochondral ossification of the epiphyseal plate. Both localized and general factors are present. Of the general ones the best known are genetic factors, the influence of availability of minerals and vitamins, and hormonal

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T. Èuk, P. Leben-Seljak, M. Štefanèiè

regulation. But localized factors, too, play a role: an adequate and uninterrupted blood flow and mechanical stresses on the bone (Ruszowski, Orliè 1977). In humans lateral asymmetry is wellknown. Current studies have confirmed the long held thesis that in the majority of people, in about 90%, the right arm is more developed than the left, while in the legs it is just the reverse, although less marked: 55% to 75% of people have a stronger left leg. This limb dominance only means an advantage in its use as shown by stronger musculature. The long bones are dependent on different degrees of stress and thus morphologically are bilaterally asymmetrical. The degree of asymmetry reflects the degree of force exerted onto the right or left limb, while the particular bone site showing asymmetry indicates the kind of force exerted. Studies of the degree of asymmetry in human long bones began on the nineteenth century. The most frequently used dimensions were the total length and weight measured both in skeletons and living people, and in archeological collections. Although data were gathered in different groups of people living in different circumstances all results agree and demonstrate that bilateral asymmetry is more marked in arm bones than leg bones and that on average right arm bones are longer by 1% to 3% and heavier by 2% to 4% than left limb bones. A few studies also mention that the bones of the left leg, particularly the femur, are on average longer and heavier than on the right but by less than 1%. The greater dimensions of the dominant arm were also confirmed by studies where they tested the link between increased activity and greater muscle mass or an increase in the mineral values of the bones. Thus, it seems that directional asymmetry of the long bones may be caused by local factors, particularly mechanical forces (Ruff, Jones 1981). Detailed statement about skeletal indicators of handedness was published recently (Steele 2000). Steel and Mays (1995) tried to answer the questions of asymmetrical long bone growth. Their measurements of the maximal length of the humerus, radius, and ulna in a series of 271 skeletons from the medieval osteologic collection in Wharram Percy in Yorkshire confirmed the presence of oriented asymmetry in arm bone length. The proportion of people with longer right or left long arm bones in this medieval population agreed with the proportion of right or left handers in modern populations. They determined that right- or left-handedness are the main causes of oriented asymmetry as they determine the use and thus the greater mechanical stress of the dominant arm. This could also be attributed to hidden genetic or hormonal factors, but the authors decry this thesis by the conclusion that asymmetry appears only when the limbs are asymmetrically mechanically loaded and increases with a child’s age. Authors who have studied lateral asymmetry of the legs have confirmed that the left leg is heavier than the right but the results of differences in length are not uniform (Latimer, Lowrance 1965; Singh 1970). Ruff and Hayes studied the asymmetry in the shape of the femur. They determined that the left femur is stronger,

Lateral asymmetry of human long bones

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particularly in women but they did not find a difference in length (Ruff 1992). Macho (1991) was interested in how much bilateral asymmetry we can notice by measuring whole femurs. He chose those measurements most likely to be influenced by different forces. After statistical analysis on 166 South African Negroes thigh bones, he came to the conclusion that on average the left leg was stronger and in most people, regardless of right- or left-handedness, was the supporting leg while the right was used for other functions. In this study, performed by anthropometric methods in a long bone series of medieval skeletons from Središèe by the Drava river, we tried to determine: 1. the degree of asymmetry of specific morphologic characteristics in individual long bones, 2. whether may we deduct the right- or left-handedness of the individual on the basis of this material. Materials and methods The sample The sample consists of 248 more or less preserved skeletons from the tenth to the fifteenth centuries exhumed in 1993 and 1994 in Središèe by the Drava river, NE Slovenia. For the analysis we chose adult skeletons with preserved upper arm bones and at least one pair of another long bones. In all, we included 42 skeletons, 26 female and 16 male. Skeletal sex was determined by standard anthropological methods (Recommendations 1980). Methods To gain as much metric data as possible and to ensure that all parts of bones were represented, we chose those measurements according to Martin and Saller (1957) as well as 3 of our own. The maximum circumference of the humeral shaft (H-MCD) was measured with a measuring tape at the point of greatest circumference, approximately in the middle of the shaft or on the deltoid tuberosity. The bitubercular width of the humerus (H-BTW) was measured with a sliding caliper between the highest points of the major and minor tubercles. The sagittal diameter of the distal epiphysis of the ulna (U-SDD) was measured with a sliding caliper at the widest part of the epiphysis in the saggital plane. All measurements are given in centimeters with a precision of one millimeter as allowed by the instruments. The degree of direct asymmetry A for every measurement in individual skeletons was expressed as the difference between values in paired bones. Direct asymmetry is given in the same values as the measurements, that is in cm. It also told us how

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many units one bone differed from the other and thus gave us the direction of asymmetry: A=D–L (A, the degree of direct asymmetry; D, value of the right bone; L, value of the left bone). The degree of asymmetry was standardized by the mean values of measurements on the right and left sides. This standardized degree of asymmetry SA represented a ratio and thus had no units but made possible comparisons of any size regardless of the specific bone or the size of the skeleton. Just as direct asymmetry it also gave the direction of the asymmetry.

SA =

(D – L)

 D+ L    2 

⋅ 1000

The statistical analysis was performed for individual data. The statistical results were checked by the two-way Student’s t-test. All analyses were done in Excel 7.0 for Windows 97. Because of the small number of paired bones, we did not separate them by sexes, following Steel and Mays (1995) who had found no differences in the degree of lateral asymmetry in the sexes, at least for lengths of the upper extremities. Results Tables 1 and 2 show the results of the analysis of metric data of the long bones and the standardized degree of asymmetry The results confirm the presence of oriented asymmetry more prominent in the arms than the legs. The average lateral asymmetry in the arms was to the right, in the legs to the left. By far the most asymmetric bone was the humerus where almost all the parameters were highly significant but particularly the minimal circumference of the shaft, the bitubercular width, and the maximal length. The total humeral length, the projection of the upper arm, only showed a smaller degree of asymmetry, yet it was still statistically significant. In the lower extremities only 2 parameters were significant: the length and the central circumference of the femur. These results are the average of a series where all preserved paired bones were studied. It is known that left-handers have a contrary direction of their asymmetry to right-handers in their arm bones, particularly the humerus, and this lowers average values. Because of this, we excluded „probable left-handers” and analysed them separately. Our criterion for exclusion was the direction of asymmetry in the most asymmetric bone, the humerus. This decision, of course, was somewhat hazardous as it was based only on the expression of anthropometric characteristics but the difference in the 2 groups was so large that it seemed reasonable. We add that the

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Lateral asymmetry of human long bones

percentage of left handers does reflect the actual numbers in the population, 15%, but in such a small series as ours was represented only by 5 subjects. We determined that the graphs of right-handers were very similar to the entire average population. In the arm bones the degree of right directed asymmetry was Table 1 The direction of lateral asymmetry of long bones Measure

Direction of lateral asymmetry N*

R>L

[%]

R=L

[%]

R