SIB RESEMBLANCE IN BONE, MUSCLE AND FAT MEASUREMENTS OF THE HUMAN CALF

BY DAVID HEWITT

Current ideas about the association between hereditary constitution and liability to disease lay considerable stress on the relative amounts of bone, muscle and fat in different individuals. Several standard anthropometric measurements such as cubit and leg length are, in effect, measurements of the long bones. Fat thicknesses can also be measured directly in the living subject by means of skinfold callipers, but amounts of muscle are harder to judge. Fortunately, radiography provides a means by which these three tissues can be differentiated and for this purpose X-rays of the calf are particularly convenient. Tanner (1955) lists a number of studies of sex and age variation in the tissues of the calf. Apart from one paper based on a set of identical triplets (Reynolds & Schoen, 1947) the present study appears to be the first to afford any estimate of the extent to which development of these separate tissues is genetically con- trolled.

The original design of the Oxford Child Health Survey provided for a 5-year follow up of 500 unrelated infants. As the survey progressed opportunities occurred to recruit some younger sibs of the survey children and finally 120 sib pairs came under observation. These 120 pairs may be regarded as reasonably representative of post-war Oxford families except in so far as selection depended on the willingness of a mother to co-operate. Infants weighing less than 4 lb. at birth were not admitted to the survey, and in the later stages of the present study one of the sib pairs was arbitrarily excluded.* At 6-monthly intervals from the age of 6 months to 5 years children were brought to the clinic where routine examination included an X-ray film of the knees and a measurement of the calf girth. Thus, the possible total of knee films, on which the greater part of the calf was ordinarily visible, was 2400. Of this possible total 512 were lost through removals, missed appointments and temporary faults in the X-ray apparatus, and a further 132 films were discarded as unsatisfactoryt, so that only 1756 films have actually been used in the present study. All films were taken with the child supine using a tube-film distance of 36 in. Some distortion was certainly caused by pressure of the calf against the table. Measure- ments were taken on the right leg unless the pose made the left leg more satisfactory. To measure the films the broadest part of the calf was first determined by inspection and a line drawn across it at this point as shown in Fig. 1. (The legs of some infants taper continuously from above the knees-in such cases the line was drawn at the widest part of the muscle shadow.) Distances were then measured along this line between the outer. edges of the calf shadow, between the outer edges of the muscle shadow, and across each bone. All these measurements were made with a transparent rule marked in half-millimetre units. By appropriate addition and subtraction values were obtained for total breadth of bone, of muscle, and of skin plus

* A girl belonging to this pair W&B a sickly child who at one time suffered severe wasting attributed to coeliac disease and later had asymmetrical oedema of the legs persisting over several years.

t For the present purpose it was necessary that the full breadth of the calf should be visible to a level below the widest part, and this was not always the case with 6lma primarily intended for the assessment of maturity changes in the knee-joint. Films were also rejected if the legs appeared to be unduly flexed or rotated. 15 Vo l . 22

214 SIB RESEMBLANCE I N T H E HUMAN CALF

subcutaneous tissue (which will be referred to for simplicity as ‘fat’). Other workers have used a more objective method to locate the transverse line along which to measure, but this was not possible in the present case. In practice it was found that moderate variation in the position and angle of this line added little to the other errors of measurement. In the clinical measurements of calf girth also, the thickest part of the calf was determined by inspection, and all measurements except those on the youngest children were taken with the child standing ‘at ease’. The number of girth measurements available was 2021.

Because of differences in technique no exact comparisons are possible between the calf measurements of Oxford children and those published for other series. However, it can be said that means and standard deviations were in reasonable agreement with those for total calf breadth, fat thickness, and combined muscle and bone breadth of

50

40 ? E E v

30’ 2 i! 2 0 .

d

e

E L

a group of Boston

I I I I I I I I I I Age (months) 6 12 18 24 30 36 41 48 54 60 Total

No. measured 204 213 190 204 167 187 158 167 125 141 1756

Fig. 2. Mean breadth of muscle, bone, and fat tissues measured from calf radiographs of young children.

children (Stuart, Hill & Shaw, 1940). As in the American series total calf breadth was initially greater for boys than for girls, with the position reversed after the first 2 years. In both series the largest proportionate difference between the sexes was the greater fat thickness of the girls. Age trends for each tissue are shown in Fig. 2, where it can be seen that the curves for bone and muscle are of the usual form, but that fat thickness decreased by some 40 % between the ages of 1 and 5 years. (The cross-sectional arm of fat, however, will not have decreased to anything like the same extent, and the total volume of fat in the calf may well increase during these years.)

Before any correlations were calculated each measurement was transformed to standard units, i.e. its deviation from the appropriate age and sex mean wm divided by the corresponding standard deviation. The objects of this transformation were (i) to permit the use in the same calculation of data referring to boys and to girls, and (ii) to obtain meaningful averages of measurements taken on the same individual at different ages.

Estimates of sib resemblance at ten ages from 6 to 60 months are set out in Table 1 in the form of correlation coefficients, All these coefficients, and those in the following table are of the

Annals of Human Genetics, Vol. 22, Part 3

Hewitt : Sib resemblance in bone, muscle and fat wteasurements of the human calf

Fig. 1. X-ray film of lmeo showing line of measurement of human calf,

(Facing p . 214)

D. H E W I T T 215

- ~ -

0.609 0.500 0.493 0289 0.417 0'393 0.461 0.435

0.390 0.364

-_____ -

intra-class type, thus they tend to be slightly lower than would have been the case if they employed separately estimated means and variances for (say) elder and younger sibs. The coefficients are subject to widely different sampling errors, those for the age 54 months being based on a particularly small number of pairs.

We may consider first the coefficients relating to sib resemblance in respect of bone develop- ment. It is worth bearing in mind that of the three tissues bone is the one which

(a ) has the most sharply defined edges on the film and is therefore easiest to measure accurately ;

( 6 ) is not liable to distortion resulting from pressure of the leg against the table; and ( c ) has the simplest relationship between measurement taken and the actual amount of

tissue present in the calf (both the other tissues occupy annular spaces with an added com- plication in the case of muscle provided by the space between the tibia and fibula).

36-60 0.504 0.508 0.283 0.464 -

0.412

Table 1. Sib correlations based on single measurements of bone, muscle, fat and circumference of calf at ages 6-60 months

Calf measure- ments

No. of pairs Bone breadth Muscle breadth Fat breadth Total breadth

No. of pairs Circumference

6

93 0.354

Correlation at age (in months)

42

45 0'543 0.566 0.335 0.556

73 0.438 -

48

65 0'373 -

54

20 0.25 I

0'595 0.189 0.434

56 0.400 -

Unweighted mean of correlations at ages (in months)

6-60 ~.

- 0'520

0.399

0.450 0.338

- 0.388

These three points may be related to the fact that bone exhibits a higher and more consistent correlation between sibs thaneither of the other tissues. The unweighted mean of the coefficientsfor the first five ages is 0-500, while for the remaining five ages the mean of the coefficients is 0.504.

The sib correlation for muscle is initially much lower than that for bone, being only 0.194 at the age of 6 months, but it increases at each of the four following ages. By the latter half of the period studied it is on a par with the correlation for bone, the average coefficient being three-quarters aa large again as during the first half of the period. The reasons for the rising trend in sib correlation can only be guessed at, but they include the following possibilities :

(a) that the genetically determined potentiat amount of muscle can only be achieved after a period of exercise in walking and running;

(6 ) that as muscle takes up a larger and larger proportion of the total cross-section occupied by bone and muscle together, the measurement taken from the films may become a more reliable index of the amount of muscle actually present.

By contrast with the correlation coefficients for muscle those for fat show the highest values at the youngest ages, the unweighted mean for the first period exceeding that of the second by more than a third. Here again, the remons for the change with age must be conjectural but two suggestions can be made.

216 S I B RESEMBLANCE I N T H E HUMAN CALF

(a) If we think of the fat as forming an annulus with internal and external radii of z1 and z2 then the amount of fat present in a given length of calf will be proportional to

(%-%) (%++l).

Of the two terms in this product only the first is taken into account by the measurements used, but the second will become increasingly important as the leg grows. Hence, unless genetic control operates specifically and solely on fat thickness, the measurement of fat used in this study will become less appropriate with increasing age. (This is the obverse of the comment made above about the muscle correlations.)

( b ) It seems likely that fat is the tissue most sensitive to diet. Therefore, if dietary differences between sibs are at a minimum during infancy this would provide another reason for expecting the sib correlation to be high at first and lower later.

The sib correlation for total calf breadth appears to be greater than for either fat or muscle yet not so high as for bone. The estimated correlation for total calf size is slightly greater on the basis of radiological measurements than of clinical measurements of circumference.

As was pointed out in a previous paper dealing with Oxford sibs (Hewitt, 1957) correlations based on single measurements are likely to be biased downwards by the errors of observation. In order to counteract this bias further estimates of sib correlation have been made in.which the data used are not single measurements, but averages of all the (standardized) measurements available for a given child. After excluding children who had been measured less than four times the standard deviations of these averaged measurements, expressed in standard units of the original single measurements, were found to be as follows :

Bone breadth Muscle breadth Fat breadth Total breadth Circumference

(based on approximately 8.0 observations per child). i 0.8839 0,7916 0.8’707 0.8512 0.8300 (based on approximately 8.7 observations per child).

It can be seen that this process of averaging has ‘smoothed’ the observations appreciably and it is reasonable to suppose that this is chiefly due to cancelling out of positive and negative errors of observation. At the same time, however, two other kinds of variation may have been partially suppressed. The first of these is the genuine but transient disturbance of growth attributable to environmental stress. In so far as the averaging suppresses this kind of variation coefficients calculated from the averaged data will tend to overstate the importance of genetic factors. Secondly, there is the possibility that some children have an innate tendency to gain or lose ground compared with other children during certain intermediate phases of growth. (This possibility will be discussed below.) In so far as the averaging of successive measurements on the same child suppresses this latter type of variation, coefficients calculated from the averaged data will tend to understate the importance of genetic factors. With these two ten- dencies opposed to one another it may be hoped that coefficients based on averaged data will give a more correct estimate of the genetic correlation between sibs than those based on single measurements.

Such coefficients are presented in Table 2, which also gives estimated correlations for certain subgroups of the sib sample. There were altogether 90 sib pairs for which at least four films of each member were available, and 112 similar pairs for the girth measurement. As was to be

D. K E W I T T 217

expected all the correlations have higher values when estimated on the revised basis, but the rank order of the coefficients for different measurements is unchanged, namely : bone, total breadth, muscle, circumference, fat. Compared with the means in the right-hand column of Table 1 each of these five coefficients is greater by an amount between 28 and 30%. The coefficient for bone breadth is appreciably greater than 0-5, suggesting some degree of assortative mating such as has been directly observed among Oxford parents in respect of height and weight. The lower estimates for muscle and fat are compatible with a ‘true’ value of 0.5. The degree of sib resemblance in bone breadth is very close to that estimated on the same basis for the height and weight of these children, while the lower figure for fat correlation is close to the value previously obtained for skeletal maturity (Hewitt, 1957).

Calf measuroments All pairs All Boy pairs

~

No. of pairs 90 53 25 Bone breadth 0.641 0.622 0.571 Muscle breadth 0 ’ 5 14 0.597 0.562 Fat breadth 0.443 0.434 0.290 Total breadth 0.576 0.576 0- 586

No. of pairs I I 2 64 30 Circumference 0.505 0.498 0.5 I 3

Table 2. Sib correlations based on a minimum of four of each measurement for each child distinguishing sibships of various kinds

Girl pairs

28 0.678 0.628 0.507 0- 568

I Pairs of I 1 I I Pairs of like sex

unlike sex

37 0.632 0.384 0.446 0.567

On looking at the separate coefficients in Table 2 for boy pairs, girl pairs, and pairs of unlike sex it will be found that none of these provide evidence of sex linkage; the only case which can even be regarded as suggestive is that of muscle, where the estimate for like-sexed pairs exceeds that for pairs of unlike sex by just over 50%.

As already mentioned there is a possibility that the averaging process which is the basis of the revised coefficients in Table 2 may have suppressed some genetic as well as non-genetic elements in the variation of the individual measurements. This possibility was tested as follows. The 5-year period was halved and for each child who had been measured at least three times in both the first and second half two separate averages were calculated. If the average in the second half exceeded that in the first the child was classified as ‘plus’, otherwise as ‘minus’. A count was then made of the number of pairs in which both children had been classified as plus or both as minus and those in which they were classified as of opposite sign. The numbers of pluses and minuses and of agreements and disagreements of sign are shown in Table 3 for forty-eight pairs of sibs. Also shown in this table are the number of agreements and disagree- ments expected on the basis of the null hypothesis that differences in standard measurement between the first- and second-half periods are due to chance. As regards bone and fat the null hypothesis appears adequate, but in the case of muscle it does not seem to be satisfactory. The phenomenon may be looked a t in another way. The difference between the estimates of sib resemblance given in Tables 1 and 2 may be separated into two components-that due to revision of the estimated sib covariance (affecting the numerator of the ratio from which r is calculated), and that due to revision of the estimated population variance (affecting the

218 S I B RESEMBLANCE I N THE HUMAN CALF

denominator). As implied by the figures previousJy quoted for standard deviations of the averaged measurements it is the reduced estimate of t,he population variance which is responsible for the higher coefficients in Table 2. But there was also some reduction in the estimates of covariance, and it is interesting to note the relative magnitude of this reduction for the separtlte tissues. Taking the mean of ten estimates of covariances based on single measuremenbs as 100 yo these reductions were as follows: bone 2.3 %, muscle, 16.6 yo, fat, 1.8 yo. The figure for muscle is obviously much greater than for the other two tissues. This corroborates the finding presenteil in Table 3 and shows that in the case of muscle the sib correlation based on averaged data does not express the whole of the sib resemblance. That is to say, besides the tendency for sibs to resemble one another in their deviation from the group mean a t a particular age, we are led to postulate a tendency for them to exhibit a common time-trend relative to the grotty niem. Such a phenomenon, though never observable in 'cross-sectional ' studies, is not altogether

Table 3. Comparison oj trends relative to group mean in members of tlw sume sib pctii for three calf measurements

Calf measuremente

Bone breadth Muscle breadth Fat breadth

No. of children for whoni the relative

trend was

Positive 1 Negative

No. (and expected no.) of pairs in which signs of the trends

Agreed I Disagreed

xz with one U.P. for muscle broadth 3.510. P-0.06.

unexpected and has already been reported for skeletal maturation in the Oxford series (Hewitt, 1957). It was also clearly exhibited by the triplets studied by Reynolds & Schoen (1947).

It follows that an unbiased estimate of the sib correlation for breadth of muscle at a particular age would exceed the 0.514 shown in Table 2, possibly equalling the coefficient actually obtained for bone breadth. These values are quite high in relation to those customarily obtained from human data. Even for a character upon which environmental variations exert no effect one would not in theory expect sib correlations to exceed 0-5, except as a result of assortative mating or of sampling fluctuations. In practice, however, correlations as high as those recorded here for bone and muscle breadths cannot be taken to indicate a complete absence of environ- mentally caused variation, since in human data variations of this kind are quite as likely to accentuate sib resemblance as to reduce it. Nevertheless, the present estimates do suggest that with the possible exception of fat, the truly genetic correlations are near their theoretical maximum in this sample of post-war Oxford children. This interpretation is consistent with the difficulty previously experienced in attempting to demonstrate any substantial differences in growth which could be confidently attributed to environment (Hewitt & Stewart, 1952; Acheson & Hewitt, 1964; Hewitt, Westropp & Acheson, 1955). Even in the case of the fat measurements which, on the evidence of the present study, seem to be the most sensitive to environment, the numbers available in the sib sample are too small to demonstrate any certain

D. H E W I T T 219

effect. An extension of the measurements to the whole Oxford series, however, would probably confirm some such effect, a t least in relation to diet.*

Many anthropometric measurements, apart from those taken with fat callipers, include a contribution from each of the three tissues considered in this study. It has been claimed that the special value of the X-ray measurements is that they permit separate consideration of each tissue, It is, therefore, of some interest to see how far growth in any of these tissues is deter- mined independently of growth in the other two. This question can be explored by means of ' within-child' correlations, but the errors of the individual measurements could have a more serious effect on the estimated correlation than in the ' between-child' case. For, owing to the fact that the muscle and fat measurements were obtained by subtraction, an error in the record for bone will be matched by an error of opposite sign for muscle, and the same applies as between muscle and fat. However, this bias is not likely to be important if the averaged measure- ments are used and it is these which form the basis of the coefficients presented in Table 4.

Table 4. Correlations between diflerent tissues in the same child. Each assessment based on a minimum of four measurements

I Hand measurements 1 210 children I IOO boys I IIO girls I Bone x muscle + 0.085 +o.orr +o.r6o Bone x fat Muscle x fat + 0.008 + 0.050 - 0'033

1 -0'037 1 -0.083 1 4-0.008 1 I n the sample as a whole none of the inter-tissue correlations exceeds 0.1, and with one

exception (bone x muscle in girls) none of the coefficients even approach the conventional level of significance. Thus, there is no evidence of any important overlap between the genetic factors controlling the three tissue breadths. The large intertissue correlations reported by Stuart et al. (1940) are in no way comparable with those in Table 4, since they relate to areas of the calf film, that is, both variables in each correlation are largely determined by leg length. I n a sample of fifty-seven children aged 7 4 Reynolds (1944) also found no significant relation between the breadths of bone and muscle or of muscle and fat. However, Reynolds considered that there was a significant correlation between the breadths of bone and fat, + 0.30 lying well above the 99 yo confidence range for the estimate based on Oxford children. The difference between these two findings seems unlikely to be due to chance, but i t may be accounted €or by the difference in age between the two samples of children.

Nearly all the children referred to in Table 4 also had repeated assessments of skeletal maturity of the hand by the method of Acheson (1954). Skeletal maturity has a substantial correlation with the more familiar composite indices of growth, height and weight, and it was of interest to see how far this correlation extended to the individual tissues measured in this study. The relevant coeficients are set out in Table 5 . These within-child correlations are con- siderably smaller than the between-sib correlations for the same character and they are of reasonable relative magnitudes. The highest is that for bone ; although bone size is deliberately

* As a trial, 100 infants (50 of each sex) who received little or no breast milk have been cornparod with 100 others who were completely breast fed for at least 5 months. During the second 6 months of life 58 of the group with prolonged breast feeding showed an increase in fat thickness of the calf. In the recently wecmed group the corresponding number was only 30, the difference being significant at the 0.1 yo level.

220 SIB RESEMBLANCE I N THE HUMAN C A L F

excluded from consideration in the maturity ratings it is evidently the case that among children of a given age those with the largest bones have reached a slightly more advanced stage of skeletal maturation. The correlation with muscle breadth is estimated to be only half as great as with bone breadth, and that with fat is quite negligible-as could be expected in an age range when, other things being equal, the more mature child is the one with the thinner covering of fat.

Table 5. Correlation of each tissue with skeletal maturity of the hand in the same child. Each assessment bused on a minimum of four measurements

I Hand measurements I 200 children I 96 boys I 104 girls I Skeletal maturity x bone +0-278 4- 0.273 +0281 Skeletal maturity x muscle I + 0 * 1 3 I 1 +0‘190 1 +0’071 I Skeletal maturity x fat + 0.004 + 0’047 - 0’039

Estimates were also made of the within-child correlation for the other two calf measures- total breadth and circumference-though there is obviously no possibility of these two being independently determined. Here the average of ten coefficients based on single measurements was found to be 0.789, and that based on the average of four or more measurements was 0.882. Thus, less than a quarter of the variance in calf circumference is independent of variation in the transverse diameter. I n a smaller sample of 7- and 10-year-old children Reynolds (1944) obtained an even higher correlation between breadth and circumference.

Medical interest in physique tends to focus on the relative rather than the absolute amounts of bone, muscle and fat, and a t least one attempt has been made to study the inheritance of somatotype as a whole (Parnell, 1957). However, a separate consideration of the three main elements which make up the somatotype seems to provide a simpler and possibly a more realistic basis for genetic studies. No attempt has been made in the present investigation to compare sibs in respect of any general characterization of physique, but in order to illustrate what might be expected some calculations were made using two of the tissue measurements a t a time. For this purpose the averaged data which form the basis of Table 2 were used, but only the crude relative magnitudes were taken into account, i.e. whether the standardized assess- ment of a particular child was greater or less for (say) bone than for muscle. Ninety sibships were then distributed in the cells of 2 x 2 tables according to the relative magnitudes observed in the first and second members of each pair. From these tables the following tetrachoric correlation coefficients were calculated :

relative amounts of bone and muscle relative amounts of muscle and fat relative amounts of fat and bone

+ 0.189; + 0.414; + 0.356.

The coefficients are distinctly lower than those in Table 2 and it must be concluded (particularly in view of the tendency for tetrachoric r to take larger values than those obtained in product- moment correlations) that measurable sib resemblance is less on this basis than when the tissues are considered one a t a time.

D. H E W I T T 22 1

SUMMARY Separate measurements of bone, muscle and fat at ages up to 5 have been made from calf X-rays of 120 pairs of sibs attending the Oxford Child Health Survey. The degree of sib re- semblance in these measurements and in external calf circumference is found to be high, particularly when allowance is made for unavoidable errors of measurement.

Sib resemblance in respect of muscle breadth increases with age, while for fat thickness it appears to decrease. In the case of muscle there is some evidence that fluctuations in growth as well as the average rate of growth are genetically controlled.

There is no evidence of sex-linkage. Correlation between different tissues .in the same child is negligible.

Assessments of skeletal maturity show a moderate correlation with bone diameter, but little association with muscle and none with fat. The amount of measurable sib resemblance is greater when these tissues are examined separately than when they are considered in relation to one another.

The Oxford Child Health Survey was launched by the late Prof. John Ryle with assistance from the Medical Research Council ; in recent years it has been directed by Dr A. M. Stewart. I am greatly indebted to Dr C. K. Westropp and Dr J. Parfit who were responsible for the clinical work of the survey, to Miss E. Roberts and Miss M. Barnes who took all the X-rays, to Drs R. M. Acheson, L. Mynors and E. Jefferson who made the assessments of skeletal maturity and to Dr J . M. Tanner for criticisms of the draft.

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HEWITT, D. (1957). Some familial correlations in height, weight and skeletal maturity. Ann. Hum. Genet.,

HEWITT, D. & STEWART, A. M. (1952). A study of the influence of social and genetic factors on infant weight.

HEWITT, D., WESTROPP, C. K. & ACHESON, R. M. (1955). Effect of childish ailments on skeletal develop- ment. Brit. J. Prev. SOC. Med. 9, 179-86.

PARNELL, R. W. (1957). Personal communication. REYNOLDS, E. L. (1944). Differential growth in the leg during childhood. Child Developm. 15, 181-205. REYNOLDS, E. L. & SCHOEN, G. (1947). Growth patterns of identical triplets from 8 through 18 years.

ChiEd ffevelopm. 18, 13@ 51. STUART, H. C., HILL. P. & SHAW, C. (1940). The growth of bone, muscle and overlying tissue as revealed

by studies of roentgenograms of the leg area. Monogr. SOC. Res. Child Develop. no. 26. N.R.C. Wash- ington.

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Hum. Biol. 24, 309-19.

TANNER, J. M. (1955). Growth at Adoleecence. Oxford: Blackwell.