akiyoshi matsumurai) and morihiko okada2)

J. Anthrop. Soc. Nippon 98(4): 451-470 (1990)

Effects of Erect Bipedal Standing on the

Cross-sectional Geometry of the Rat Femur

Akiyoshi MATSUMURAI) and Morihiko OKADA2)

1) Department of Biology, National Defense Medical College 2) Institute of Health and Sport Sciences, University of Tsukuba

Abstract Effects of erect bipedal standing exercise on the skeletal morphology were

investigated in seventeen growing male rats divided into control and exercise group.

Using the newly devised 'bipedal training box', in which rats achieved a fully upright

stance through positively reinforced operant conditioning, the exercise group was

burdened with the bipedal standing exercise from 64 days to 140 days of age, totally

in 136-138 sessions. At the age of 140 days, the left femur was dissected out, ten

serial cross sections of the femoral diaphysis were cut from proximal to distal and

cross-sectional properties were calculated from the photographs of the sections. The

bipedal standing exercise had the following effects on the femoral diaphysis; an

increase in the cross-sectional area, area moments of inertia in the proximal half of

the shaft, i.e. the strength of the femoral diaphysis increased against axial compressive,

antero-posterior bending and medio-lateral bending, respectively; an increase in the

polar moment of inertia and an external rotation of the principal axis in the vicinity of mid-shaft, i.e. the strength against the torsional load increased and the direction

to resist the maximum bending load more or less approached the antero-posterior

direction. These observations were discussed in comparison with the effects of

quadrupedal running exercise on the femoral cross section previously observed by us.

Keywords Rat, Bipedal standing exercise, Femur, Cross-sectional properties

Introduction

If morphology of a bone is closely related to

the external forces and produced by remodeling

the compact bone under the rule of WOLFF's

Law (WOLFF, 1870, 1892; cited from ROESLER,

1987), existence of a certain bone forms that

correspond to the mode of exercise or locomotion

is expected. To understand how human lower

limb bones adapt to the bipedal walking,

therefore, studies on the relationships between the mechanical environment and the morphology of

hind limb bones is considered to be relevant.

From this viewpoint, analyses have been made

on the relationships between the cross-sectional

geometry of human or primate lower limb bones and external stress factors such as muscle con-

tractions or joint reaction forces induced by their

locomotor behavior (KIMURA,1971; RYBICKI et

a!.,1972; LOVEJOY et al., 1976; MARTIN and

Article No. 9021 Received July 14, 1990

452 A. MATSUMURA and M. OKADA

ATKINSON, 1977; LOVEJOY and TRINKAUS,

1980; PIZIALI et al., 1980; BURR et al., 1981,

1982; TAKAHASHI, 1982a; RUFF and HAYES,

1983). Some investigators have burdened an

actual dynamic exercise on the quadrupedal

animals and attempted to observe the change

in cross-sectional morphology of limb

bones (LANYON et al., 1982; MATSUMURA et

al., 1983; MATSUMURA and OKADA, 1987). To

investigate the effects of bipedalism, however, it

will be useful to burden the experimental animals

with a static as well as dynamic mode of exercise

as occuring in the human bipedal posture, and

to compare the effects of the different modes of

exercises on the bone. When any morphological features are found in the bone corresponding to

certain mode of exercise, relationships will

become clearer between the bone shape and the

external forces acting on the bone, and, in turn,

the locomotor behavior causing them. In the previous studies, as an experimental

animal model for bipedal walking and upright

posture, the rat forelimb-amputated at their early neonatal stage, designated as 'bipedal rat', has

been extensively used (COLTON,1929; GOFF and

LANDMESSER, 1957). The other bipedal model

is the rat forcibly placed in an upright cylinder

and cared for a long time (JINNAKA, 1929;

RIESENFELD, 1966). Many investigators have

used these animal models for examining the

effects of bipedal behavior on the morphology

or strength of hindlimb and vertebral bones

(COLTON, 1929; JINNAKA, 1929; GOFF and LANDMESSER, 1957; SAKAMOTO, 1959;

USHIKUBO, 1959; SAVILL and SMITH, 1966;

KAY and CONDON, 1987; CASSIDY et al., 1988).

Though a few of the cross-sectional properties

have been examined in the femur shaft of the

'bipedal rat' or the rat placed in an upright cylinder (RIESENFELD, 1966; SMITH and

SAVILLE, 1966), the number of studies concerned with the cross-sectional morphology of the

hindlimb bone is scarce. Further, the hindlimb

bone alignment as revealed by X-ray photographs during the bipedal posture (USHIKUBO, 1959;

RIESENFELD, 1966) and measurements of the

duration in which an upright posture was main-

tained (MORAVEC and CLEALL, 1987) suggest

that hindlimbs in these bipedal animals are not

sufficiently loaded by the bipedal standing (KAY and CONDON, 1987). These bipedal animals also

have a handicap because they are physically

injured or constrained, and the stresses given on

the animals are much severer than in the control

animals.

In the present experiment, using the newly

developed 'bipedal training box' (MATSUMURA,

1986), we burdened the normal rats with erect

bipedal standing exercise by means of positively

reinforced operant conditioning. The animals

were loaded under the strictly controlled experi-

mental condition, and morphological changes of

the cross-sections along the femoral diaphysis

were examined.

Materials and Methods

Animals and rearing condition

Seventeen male Sprague-Dawley rats (21 days

old), weighing 45-54 g (Jcl:SD, CLEA JAPAN

Inc.) were divided into control group of 8 and

exercise group of 9 and each animal was housed

separately in a plastic cage of 275 (w) x 165 (d) x 130 (h) mm. The animals were reared under

14-10 LD cycles (the light was on from 5:00 am

to 7:00 pm) and room temperature was controlled

to keep 22 ± 2*. From 21 days till 27 days of

age, the rats were freely fed solid food MF

(ORIENTAL YEAST Co.: Calcium 11.0 g/kg; Phosphorus 8.3 g/kg; Ca:P ratio 1:0.75). From

28 days of age, both groups were routinely fed

the food twice a day (75 min. between

8:00-10:30 am and 75 mini between 7:00-10:00

pm). The total feeding time a day was 150 minutes.

Bipedal Standing and Rat Femur 453

Apparatus for exercise

The apparatus used in this study for exercise

was an experimental chamber, "bipedal training

box" (remodeled type of GT-8010 SKINNER'S

Box, O'HARA & Co. Ltd.), which had been

developed for burdening the small sized animals with erect bipedal standing exercise (MATSU-

MURA, 1986). In the apparatus, a food reward

(100 mg Precision Pellets of O'HARA & Co. Ltd. which is made of the MF, ORIENTAL

YEAST Co.) is given when a bipedal standing

animal pushes a high-positioned lever upward from beneath by the nose (Fig. 1).

Each box is 275(w) x 190(d) x 370(h) mm (300 mm high from the top to the floor) walled with

acrylfiber and alminium panels. One of the 190

mm panels of the box has a food tray approxi-

mately 20 mm above the floor. A stainless steel

lever, 35 mm wide, 12 mm long, and 1.2 mm thick is located on the right side of the panel.

When a rat pushes up the lever certain times

according to the fixed ratio (FR) schedule, a pellet

food is transferred to the food tray from the

pellet dispenser, which is controlled by FR Feed Unit CF-R5 (O'HARA & Co. Ltd.). Height of the

lever can be adjusted at every 10 mm from 40

mm to 290 mm high, to adapt to the animal size.

Weight of the lever can be regulated in a range

between 35 g and over 500 g by a spring in the

loading unit. Five experimental chambers were

connected to the FR Feed Unit for programming

the FR schedules.

Exercise program

After 31 days of age, the rats of the exercise

group were trained erect bipedal standing and

Fig. 1. The 'bipedal training box': A relatively statical load is

given to the hindlimb when a bipedal standing animal pushes

a lever upward from beneath by the nose to obtain a food reward.

Arrow: lever; PD: pellet dispenser; FT: food tray; FU: FR

feeding unit; LU: loading unit.


lever pushing by using the procedure of "shaping" (REYNOLDS, 1975). In the period between 31 days and 57 days of age, the shaping was performed during the night feeding time for

30 minutes a day for a rat, with an interval of 1-3 days. In this period, when a rat pushes up

lever once, one pellet food was obtained (FR 1).

Lever height was set at 100-120% of the tail

length and lever weight was initially set to 35 g.

The rats took 3-9 times of shaping to learn the

erect bipedal standing and lever pushing up

behavior. After learning the behavior, the rats

were trained for additional 5-19 sessions under

the same FR schedule till 63 days of age. From

64 days to 140 days of age, the rats of the exercise

group were seriously burdened with the erect bipedal standing exercise.

In the bipedal training, burdening condition was determined by four factors, i.e., 1) times of

response to get one pellet food for reinforcement,

2) total standing duration, 3) lever weight and 4)

lever height. The times of lever pushing response

to get one pellet was increased step by step; one

time (FR1) from 31 to 63 days of age, 5 times

(FR5) from 64 to 75 days of age and 10 times

(FR10) from 76 to 140 days of age (Fig. 2A). In FR1, 50-100 lever pushings were performed

within a session of 30-60 minutes. In FR5 and

FR10, 100-200 pushings occured in a session

(Fig. 2A) which took 5-30 minutes. During the exercise, continuous white noise was given to

mask any extraneous sound. After the training,

the rats were returned to the plastic cage and

freely fed till their total feeding time became 75 minutes.

Weight of the lever, 35-50 g during the

shaping, was increased step by step and finally reached 440 g (Fig. 2B). When increasing the

weight, a new weight was selected which all the

rats of the exercise group could push up

continuously.

The lever height was determined in relation to

the height at which the rats were able to touch

their nose from beneath and push up the lever.

During the experiment, the lever height was

increased step by step from 140 to 280 mm

according to the growth of animals. These heights

were 116-125% of the tail length after 64 days

of age (Figs. 2C, 8).

The rats of the exercise group were burdened

totally 136-138 sessions of the erect bipedal

standing exercise during the period from 64-140

days of age. No erect bipedal standing exercise

was imposed on the control rats.

Preservation of experimental materials At the age of 140 days, rats were sacrificed

under ether anesthesia. The left femur was

quickly removed and the fresh weight was weighed to 1/100 mg using a chemical balance

(SHIMADZU Co.) and the maximum length was measured to 1/20 mm with a micrometer. The

femur was air dried for seven days and the dry

weight was weighed and the maximum length was

measured in the same manner as the fresh

samples.

Photographing of femoral cross sections

For the purpose of strengthening the diaphysis

against cutting and of determining the reference

plane with respect to the diaphysis, the left femur was placed on a flat glass board and embedded in bone cement (ZIMMER Inc.) so that the ventral

(posterior) ends of three parts, i, e., medial condyle, lateral condyle and lesser trochanter, are

contained in a horizontal reference plane of the

glass board. Then the cross sections of the femur were cut perpendicularly to the bony long axis

and to the reference plane, along the diaphysis

from proximal to distal at 5% intervals, between

the point of 30% and 75% level of the total length

(Fig. 3). The proximal aspects of the cross section was photographed under an eyepiece micrometer

using a 55 mm lens (Micro Nikkor, NIKON Co.)


Fig. 2. Data of behavioral responses, growth curve, loading condition, and food intake. A: Mean number of lever

pushing for the exercise group per session. Periods of fixed ratio (FR) schedules are indicated. B: Mean growth curve of the body weight in each group and lever weight in the exercise group. C: Mean growth curve of the

tail length in each group and lever height in the exercise group. D: Mean food intake per rat per day in each

group. One g of the solid food MF corresponds to 3.599 Cal.


Fig. 3. Location of the examined cross sections along the femoral diaphysis and typical examples of the cross section (30%, 40%, 50%, 60% and 70% level) in each group.


and printed on the photographic paper

(Hishicopy CH, MITSUBISHI Co.) by 20 hold magnification.

Calculation of cross-sectional properties Geometrical properties of the femur cross

sections (ENDO and TAKAHASHI, 1982; TAKAHASHI, 1982b) were calculated from the

photographs with the aid of the digitizer

(KD4030A, GRAPHTEC Co.; possible reading range is up to 0.1 mm) and microcomputer

(PC-9801F, NEC). The program developed by TAKAHASHI and ADACHI (1982) was rearranged

and used in this study. The calculated properties

are as follows:

1) Area moment of inertia about the x axis:

Here, x and y are the distances measured in

the M-L and in the A-P directions, respectively,

from the centroid to the differentiated area (dA),

2) Area moment of inertia about the y axis:

3) Principal moment of inertia (maximum):

4) Principal moment of inertia (minimum):

5) Polar moment of inertia:

6) Circumference: C

7) Cross-sectional area (area of the compact

bone):

8) Inclination of the principal axis:

9) Section index:

For details about the measurement of the

cross-sectional properties of rat femur, see MATSUMURA, et al. (1983) and MATSUMURA

and OKADA (1987).

Results

Food intake

The amount of food intake during the experi-

mental period was 8,567 Cal in the control group

and 8,440 Cal in the exercise group. After 60 days

of age, each rat of the control group and the

exercise group ate in average 78.2 Cal and 78.0

Cal per day, respectively (Fig. 2D).

Observation of the bipedal standing posture and

behavior

The photographs in Fig. 4 show a cycle of the

lever pushing up behavior. A sitting rat started

to rise with the hindlimbs plantigrade, then the

hindlimbs gradually became digitigrade and the rat assumed a fully upright posture during the

lever pushing. When pushing up the lever in the

fully upright position, the rat touched the vertical

wall in front of his neck or face to balance

himself (Fig. 4: 4-7). When assuming this

posture, however, forelimbs of the rat appeared not loaded with significant forces.

During the loading period (64-140 days of

age), the number of standing times in each

exercise session in each rat was 17.7 in FR 5 and 12.8 in FR 10 in average (Fig. 2A). In preliminary

experiments using three rats different from those

examined in the present study, we observed that

in the schedule of FR 5 and FR 10, each rat of

the exercise group spent about 3.5 and 6 sec

respectively, in average, holding the erect bipedal

standing to obtain a pellet by lever pushing (Fig.

5). Hence, the total standing duration per exercise

session in the present study might be estimated

to be about 62 sec (FR 5) or 77 sec (FR 10). In

a rat performing 136 exercise sessions, then, total

bipedal standing duration might amount to 171

min through the experimental period (the dura-


Fig. 4. A cycle of bipedal standing and lever pushing behavior of a rat. The photographs flow from left to right.

An arrow head and a small arrow show the lever and food tray, respectively. In a schedule of FR 10, when the

rat has pushed the lever ten times, a pellet is given in the food tray.

tion including half rising becames 203 min).

Body weight and tail length

Growth curve of the body weight and that of

the tail length are shown in Figs. 2B and 2C,

respectively. In either value, there was no

statistically significant difference between the two

groups during the experimental period. The final values are compared in Table 1.

Weight and length of the femur

The weight and the maximum length of the

fresh and dry femur are shown in Table 1. In the

dry weight, the exercise group had a tendency to

show larger values compared to the control

group.

Cross-sectional properties

Results of the calculation of cross-sectional

properties are shown in Tables 2, 3 and Figs. 6, 7. The change of the values along the femoral

diaphysis from proximal to distal and the

differences between the two groups are as

follows.

1. Area moments of inertia (Ix. Iy) and principal

moments of inertia (I1, I2)

The moments of inertia were larger in the

distal and proximal portions. IX and Il marked

the smallest at around the central 50-60% level.

Mean values of the exercise group were higher

than those of the control group at all sections.

This tendency was more pronounced in the

proximal half (30-55% level) of the femur shaft


Fig. 5. Frequency and duration of the bipedal standing behavior recorded in a rat for 30 minutes under the schedule of FR 10. Time passes from left to right. Each row represents 3 minutes and the behavioral record

is serially presented from top to bottom for the 30 minutes. H: half rising position (photographs 2 and 3 in Fig. 4), F: fully upright position (photographs 4-7 in Fig. 4),

R: reinforcement by a pellet food.

in Ix and I2, and in the proximal half (30-55%

level) and distal sections (70-75% level) in I,

and II. The differences of the mean values

between the two groups were significant at 30%,

40% and 50% levels of the shaft for Ix, and at

30% and 45% levels for 12.

2. Polar moment of inertia (Ia)

Because the polar moment of inertia was

calculated as the sum of Ix and Iy or I1 and I2,

this parameter became larger at the distal and

proximal sections and the smallest in the center at 50-65% level. In all the sections, mean values

of the exercise group were larger than those of

the control group and this tendency was con-

spicuous in the proximal portion (30-55% level), difference between the groups being signifi-

cant at 50% level.


Table 1. Physical measurements compared between the groups

wt.: weight B.W.: body weight

3. Circumference (C)

Circumference was larger in the distal and

proximal sections and was the smallest at around the 50-65% level. In all the sections, mean

values of the exercise group were slightly larger

compared to that of the control group although no significant difference existed between the two

groups. 4. Cross-sectional area (A)

Cross-sectional area maximized at 30% level,

gradually decreasing to the minimum at 60-75% level. In all the sections, mean values


the control group and this tendency was con-

spicuous in the proximal half of the femur shaft

with a significant difference at 30%, 40% and

50% levels.

5. Inclination of the principal axis (*p)

Inclination of the principal axis corresponds

to the direction of the maximum area moment

of inertia. Between 30% and 40% level, the in-

clination increased, i.e., the axis rotated clockwise

viewed from distal. Reaching the maximum at

40% level, the inclination reduced to the

minimum at 60% or 65% level, but increased

again from 70% to 75% level. The mean values


the control group from 30% to 60% level, with

a significant difference at the 50% level. The rela-

tion of the two groups reversed between 60% and

65% level, and in the portion between 65% and

75% level; the mean value of the exercise group

became smaller than that of the control group.

6. Section index (SI) Section index minimized at 35% and 70%

levels and maximized at 50% level (Fig. 7). In all

the sections, there was no significant difference

between the exercise and the control group. At

30-35% and 55% levels, however, the exercise

group tended to surpass the control group, sug-

gesting a rounding effect on the cross-sectional

geometry. At 75% level, on the contrary, the exercise group showed a decreasing tendency,

suggesting a flattening effect in the medio-lateral

direction.

Discussion

Forelimb-amputated "bipedal rat" has been

frequently used as an experimental model for the

erect bipedal walking. In these cases, however,

the body weight of the bipedal rats was relatively

small compared to that of the controls

(SAKAMOTO, 1959; SAVILL and SMITH, 1966; MORAVEC and CLEALL, 1987). In the present

study, no statistically significant difference was

found between the bipedal standing group and


Fig. 6. Variation in area moment of inertia about x axis (Ix), area moment of inertia about y axis (Iy,), maximum principal moment of inertia (I 1), minimum principal moment of inertia (I2) and polar moment of inertia (Ip)

along the femoral diaphysis. At each cross section, statistical difference between the two groups is shown with an asterisk (p <0.05). Vertical bars represent standard deviation of means (± SD).


Fig. 7. Variation in circumference (C), cross-sectional area (A), inclination of the principal axis (*p) and section index (SI) along the femoral diaphysis. Other legends are the same as in Fig. 6 except two asterisks showing

the between-group difference at the 1% level of significance.


the control group in the body weight, tail length

and amount of food intake during the experi-

mental period. In view of the growth process, the

subject animals in our experiment are considered

to be suitable for examining the effects of bipedal standing exercise on the bone.

There are only a few studies on the cross-

sectional morphology of the femur in the bipedal

rat. USHIKUBO (1959) indicated that the medio-

lateral diameter in the distal portion of the femur

became larger in the forelimb-amputated rats

than in the controls and the results of our calcula-

tion using the published data of USHIKUBO's

male rats showed a statistically significant

difference between the two groups. SMITH and

SAVILL (1966) found that during the growth of

the rat, cross-sectional area of the femur between

the third trochanter and the condyles, which

seems to be equivalent to 50-75% levels in the

present experiment, increased faster in the forelimb-amputated than in the controls.

RIESENFELD (1966) showed that in rats forelimb-

amputated or reared in an upright cylinder, the index between the medio-lateral and antero-

posterior diameter taken at the mid-point of the femur shaft increased, i.e., the rounding effect

of the cross section occured.

In the above studies, however, the forelimb-

amputated rats usually assumed a half sitting

posture, tucking their hindlimbs under the body to hold a dorsal kyphotic stance (PRATT, 1943;

GOFF and LANDMESSER, 1957; USHIKUBO,

1959; YAMADA, 1962; MORAVEC and CLEALL,

1987). The X-ray photographs of the rat reared

in the upright cylinder also showed hindlimbs

seemingly tucked under the body (RIESENFELD,

1966). When the forelimb-amputated rat stood

with hindlimbs and maintained proper balance,

their posture was very similar to that of a man

lifting a heavy object (USHIKUBO, 1959;

YAMADA, 1962), i, e., they flexed slightly forward in the mid-thoracic region (CASSIDY et

a!.,1988). Also, the feet of the bipedal rats were

evidently farther apart than those of the controls

(COLTON, 1929). KAY and CONDON (1987) concluded that the

bipedal rats did not achieve true bipedalism, or they produced no increased hindlimb loading

because of the following reasons; 1) the result of

comparative analysis of the posture showed no

statistically significant difference in the amount of the time spent in the upright stance between

the bipedal and control groups (MORAVEC and

CLEALL,1987); 2) many past experiments on the

skeletal changes observed in the femur relied on

the indices or the ratios, not on the direct bone

measurements, to determine the differences

between the bipedal and control groups

(COLTON,1929; USHIKUBO,1959; SAKAMOTO, 1959; RIESENFELD, 1966); 3) in the direct bone

measurements, almost no significant difference

was found in the bony parameters of the

hindlimbs.

According to the direct measurements shown

in KAY and CONDON (1987), however, antero-

posterior diameter of the proximal femur is larger in the bipedal rats compared to the controls and

the difference is statistically significant, suggesting that hindlimb loading was actually

increased in the bipeds.

In view of the foregoing observations as well

as the three problems pointed out by KAY and

CONDON (1987), it seems more suitable to con-

clude that cross-sectional changes observed in the

femur of the bipedal rat mainly refrects the

habitual posture and locomotor behavior induced

by the amputation or forcible standing in the

cylinder rather than the effects of the load given

while the rat takes an upright stance. Our rats

burdened with the lever pushing up exercise didn't

show the peculiar postural habit as observed in

the bipedal rats, and showed a more stable and

constant erect bipedal standing posture during the

lever pushing behavior than in the bipedal rats.


Fig. 8. Lateral skeletal view of erect bipedal standing rat in the 'bipedal training box'. The two figures (A and B) were traced from X-ray photographs of the same animal. Lever height is set at 116% of the tail length in A and 125% in B. From 64 days to 140 days of age, the lever height for each rat was set approximately in the

range from 116% to 125%. In both A and B, the vertebral kyphosis is observed around Th 13 portion (an arrow head) and slight lordosis is noticed around L5 portion (double arrow heads). Arrow: femur, L: lever.

When a rat fully stood upright in the bipedal

training box, extension of the hip and knee joints

were remarkable (Fig. 8), and the vertebral

column had a tendency to show a kyphosis in the thoracic portion and a lordosis in the lumbar

portion. The stance of the feet in the erect bipedal standing rat is the same as when they are

crouching and eating food (Fig. 4). In the lever

pushing behavior, the height reached by the nose tip, the weight of the lever, and the times and

duration of the lever pushing could be exactly

recorded (Figs. 2A, 2B, 2C and 5). Since both

exercise and control groups were reared in the

cage 130 mm high which was lower than the

lowest set lever, only the experimental group

regularly practiced a certain amount of bipedal

standing during the exercise period.

In the present experiment, bipedal standing

exercise more or less increased the area moments

of inertia, polar moment of inertia, and cross-


sectional area along the femoral shaft. Con-

spicuous effects, however were noted in the

proximal 30%-50% levels. The increase in the cross-sectional area in this portion is considered to be an adaptation to the axial compressive load.

In the above portion, at the same time, the area

moment of inertia in the antero-posterior direc-

tion became larger in the exercise group than in

the control group. The area moment of inertia

in the medio-lateral direction also showed a

similar tendency. Thus, from the viewpoint of

structural strength, the resistance against both

antero-posterior and medio-lateral bending seems

to have increased in the middle and proximal

shaft of the femur due to the bipedal standing

exercise.

The polar moment of inertia increased in the

exercise group in the proximal half of the femur,

especially in the vicinity of the mid-shaft, sug-

gesting an increase in resistance against the tor-sional load. Further, also near the mid-shaft, the

principal direction of the cross section rotated and became close to the antero-posterior direc-

tion. These changes in the polar moment of

inertia and in the direction of the principal axis

in response to the bipedal standing and lever

pushing exercise suggest the possibility that tor-sions in the external direction were generated in

the middle of the shaft. The rotation of the

principal direction may also indicate an increased registance against the antero-posterior bending

load. Though no significant changes were found,

the section index of the exercise group tended to

become larger in the most proximal portion and

near the center of the shaft, suggesting a rounding

effect.

The authors previously examined the cross-

sectional properties of the femur shaft when the

rats were loaded with forced running exercise

(MATSUMURA and OKADA, 1987). The prop-erties varied with characteristic patterns along the femoral diaphysis from proximal to distal,

which are basically in agreement with those observed in the present experiment. Comparing

the effects of quadrupedal running exercise with

those of the bipedal standing, it becomes evident

in both experiments that the possible effects of

loading are recognized in the cross-sectional prop-

erties of the femur proximal to the mid-shaft.

Especially, an increase in the cross-sectional area

from mid-shaft to the proximal 30% level and

in the area moment of inertia in antero-posterior

direction in the vicinity of the lesser trochanter

is in common to both exercises.

On the other hand, the flattening of the cross-

sections along the femoral diaphysis proximal to

the mid-shaft induced by running exercise was not

observed in the bipedal standing exercise. Possible

reason for such commoness and difference could

be that in the running exercise, the femoral

cross sections adapts exclusively to the medio-

lateral bending and axial compressive load in the

proximal diaphysis, while in the bipedal standing exercise, they adapt to both medio-lateral and

antero-posterior bending or axial compressive

load in the proximal diaphysis and, at the same

time, also to torsional load at the mid-shaft.

Such a difference in adaptation of the cross-

sectional properties. between the quadrupedal running and bipedal standing exercises appears to

provide an experimental evidence for the different morphological change related to different mode

of loading. Our observation adds support to the

points of RUFF (1989) that intra- and interspecific differences in locomotor behavior can be identi-fied through the cross-sectional characteristics.

Comparing the cross-sectional geometry of the

femur in three species of macaques, BURR et al.

(1989) concluded that 'barrel-shaped' femur may be associated with leaping behavior and that the

structural rigidity of the femur per unit body weight is larger in the species spending longer

time in terrestrial environments.

When the mode of exercise is different,


participating muscles, timing of muscle contrac-tions, gravitational force, and joint reaction

forces would be different, which may yield a

different mechanical environment for the bone.

In the quadrupedal running exercise, those

muscles which insert or arise from the third

trochanter, i.e., m. pectineus, mm. adductor

longus et brevis, m. gluteus maximus, and m.

vastus lateralis are very instrumental

(NICOLOPOULOS-STOURNARAS and ILES,1984), and thus are thought to be responsible for the

cross-sectional geometry (MATSUMURA and

OKADA, 1987). On the other hand, in the rats loaded with the bipedal standing exercise, the

relative weight of individual muscle groups of the

hindlimb and hip tended to increase excluding the

intra-pelvic muscles. Among the thigh muscles,

relative weight of m. biceps femoris (anterior head) increased and that of m. gracilis posticus

also tended to increase (MATSUMURA and

OKADA, 1990). In the bipedal standing, therefore, muscle groups or timing of contrac-

tions different from those functioning or

occurring in the running exercise are supposed to

be related to the femur cross-sectional geometry.

To identify the relations, however, closer

examinations of the muscle tissue as well as

electromyographic observation will be necessary.

A femur with flattened and pilastered cross

section has been well-documented for the femur of the prehistoric and modern hunter-

gatherers, and one reason for such morphology has been claimed to be vigorous muscular

actions in the lower extremity (BUXTON, 1938;

YAMAGUCHI, 1982; KIMURA and TAKAHASHI,

1982). Identification of the muscular contribu-

tions in different mode of exercise, using

hereditary homogenous rats, may throw light on the above discussion of the human femur.

Since the cross-sectional area in the proximal

shaft of the femur increased in the bipedal

standing exercise, suggesting an increase in the

axial compressive strength, it appears that

gravitational effects as well as muscular forces

play an important role in the morphological

adaptation of the femur. A closer examination

from this viewpoint is also needed.

Acknowledgments

The authors would like to express their

gratitude to Prof. S. INOKUCHI of Showa University for providing experimental facilities

and encouragements. They are also grateful to

Prof. B. ENDO of the University of Tokyo for

helpful suggestions. Thanks are also due to Mr.

H. ISONO, Mr. M. MORIMOTO, and Mr. K.

NAKAI for their technical assistance.

Part of this paper was presented at the 12th

International Congress of Anthropological and

Ethnological Sciences in Zagreb, Yugoslavia.

抄録

二足起立運動負荷がラット大腿骨の断面形態に及ぼ

す影響

松村秋芳・岡田守彦

ヒトの下肢が二足行動にどのように適応している

かを理解するために,力学的な環境と骨形態の関連

性を研究することは重要である.この観点からこれ

までヒトやサル類の下肢骨を材料として,骨体横断

面の力学的特性値や骨に生ずる応力と骨に作用する

筋収縮力との関連性,あるいはロコモーション様式

との関係が調べられてきた.特定の様式の運動負荷

が骨の形態に及ぼす影響を詳しく調べるためには,

同一の遺伝的背景をもち,生活環境と負荷の条件を

コントロールした実験動物を用いることが望ましい.

すでに著者らはラットを用いて強制走行運動負荷が

大腿骨の骨幹に沿った断面特性値に及ぼす影響につ

いて報告した.

今回は Sprague-Dawley 系の雄ラット17匹を実験

群と対照群に分け,二足起立運動負荷が成長期の

ラット大腿骨骨幹に沿った部位ごとの断面形態に及

ぼす影響をしらべた.装置は,オペラント条件付けに

よるレバー押しを利用して小型実験動物に二足起立


運動を負荷できる"バイペダル・トレーニングボッ

クス"を用いた.負荷は1日2セッション,朝と夜の

摂食時間帯に定率給餌の条件下でセッションあたり

のレバー押し反応数にして平均125回与えた.動物

の体重には実験期間を通して両群間で有意差がな

かった.大腿骨は近位側から最大長の30%-75%の

区間を5%おきに10箇所切断し,拡大写真から断面

特性値を計算した.その結果,二足起立運動負荷は大

腿骨骨幹に沿った部位ごとの断面形態に対して次の

ような効果を示した.

1) 全般的に断面2次モーメント,主断面2次

モーメント,断面2次極モーメントおよび断面積を

大きくする傾向がある.

2) とくに骨幹の中央部ないし近位部において,

前後方向,左右方向の断面2次モーメントおよび断

面積を大きくする.すなわち,これらの部位では前後

方向,左右方向の曲げ強度と骨幹の長軸方向の圧縮

強度が増加する.

3) とくに骨幹中央部近辺において,断面2次極

モーメントを大きくし,主軸を外旋させる.すなわ

ち,これらの部位では擬りに対する抵抗が増加する

が,このときの振りの加わる方向は外旋方向と考え

られる.

4) これらの結果と,さきに報告した四足走行負

荷ラット(MATSUMURA and OKADA,1987)の大腿骨

断面形態の比較から,以下のような共通点および相

違点が認められた.

a) 両負荷様式ともに,中央部周辺ないし近位部

で断面積と左右方向断面2次モーメント,また近位

部で前後方向断面2次モーメントが増加する.

b) 二足起立負荷ラットでは,前後方向断面2次

モーメントの増加が,中央部周辺からやや近位部に

かけても生ずるが,この部位における同様の変化は

走行負荷ラットには認められない.

c)二足起立負荷ラットは,中央部周辺において,

走行負荷ラットでは認められない断面2次極モーメ

ントの増加と主軸方向の外旋を示す.

d) 走行負荷ラットでは,中央部周辺から近位側

で断面形態の主軸方向への扁平化が観察されたが,

二足起立負荷ラットではこのような変化はみられな

い.

以上の結果より,運動負荷はその様式にかかわら

ず,ラット大腿骨の中央部周辺から近位側において

断面形態の変化をもたらす一方,特定の部位におい

て,運動様式の違いに対応する変化を生じさぜるこ

とが明らかとなった.

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松村秋芳

AkiyoShi MATSUMURA

防衛医科大学校生物学科

〒359所沢市並木3-2

Department of Biology, National Defense Medical College

3-2 Namiki, Tokorozawa 359, Japan

akiyoshi matsumurai) and morihiko okada2)

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