The strength of the flexor and extensor muscles of the trunk

THE

STRENGTH OF THE FLEXOR AND MUSCLES OF THE TRUNK*

EXTENSOR

J. D. G. TROUP and A. E. CHAPMAN Biomechanics Laboratory. Department of Anatomy. Royal Free Hospital School of Medicine. London. W.C. 1. Abstract -The maximal pulling and pushing forces exerted by 230 subjects (I 32 females. 98 males) were recorded in standing and sitting postures in which the trunk was kept vertical. the upper limbs horizontal and the position of the pelvis fixed. The turning moments on the trunk were calculated and the results compared with measurements of dynamic strength, the mobility of the lumbar spine and hips. and with the age, weight. stature and lengths of the upper and lower limbs. INTRODUCTION

of the lumbar spine are widespread in industrial communities. and the amount of incapacity they cause is considerable (Troup, 196.5). There is thus a need for objective methods of assessing individual capacity for work involving the spine and trunk. At present when examining a recruit for industry, his physical ability is only judged by the absence of signs of pathological change and by a ‘rule of thumb’. One essential criterion for heavy physical work is strength. but there is little fundamental information concerning the strength of the muscles of the trunk in healthy uninjured people. Even less is known about those with disabilities affecting the lumbar spine. only one study of flexor and extensor strength in subjects with chronic lumbar pain having been made (Alston, Carlson. Feldman. Grimm and Gerontinos, 1966). The muscles of the trunk transmit considerable forces in the course of daily physical activities, but little is known of their magnitude. Groh. Thos and Baumann (1967) analysed the extensor and compressive forces at the lumbosacral level in a male subject. weighing 76 kg and 182 cm in height, in eight postures. omitting the possible effects of raised intraabdominal pressure. The DISORDERS

extensor force was estimated as zero when standing erect. at 350 kg and 1590 kg when holding weights of 25 kg and 200 kg respectively in a stooping posture with knees flexed. Using the results of Morris. Lucas and Bresler (I 961) as a model for the increases in intraabdominal pressure during lifting, Fisher (1967) calculated the forces transmitted by the extensor muscles from photographic records. The maximal estimated forces in the course of lifting 40 lb from the floor with trunk and knees flexed in an average sized man was 451 kg at the lumbosacral level. Lifting a 50 lb weight with knees extended, the maximal calculated forces at this level were 427 kg and 345 kg in an average sized man and woman respectively. Many authors have measured the external forces exerted during static contraction of the muscles of the trunk. the majority recording the strength of the maximal back-lift (pulling vertically on the handles of a dynamometer). A number have studied the effect of change in posture on isometric strength during lifting actions (Vernon. 1924; Bedford and Warner, 1937 Whitney, 1958: Davis, Troup, Whitney and Gear. 1964). But only a few have measured the maximal flexor and extensor forces exerted during static contraction in postures which eliminated the

50

J. D. G. TROUP

effect of gravity (Mayer and Greenberg. 1942; Asmussen and Heebgll-Nielsen. I961 ; Tornvall, 1963). although both Clarke ( 1966) and Laubach and McConville (I 966) measured the forces exerted in recumbent postures and subsequently made corrections for the effect of gravity. The dynamic strength of muscles of the trunk has been given little attention. Kraus and Weber (1945) devised a series of tests of the activity of muscles of the trunk but their interpretation was made qualitatively. Recently, in this laboratory, Grieve (1968) has measured the dynamic strength of the rotator muscles of the trunk in a cinephotographic study of 10 young male adults and the results are now being analysed. This investigation was planned to provide data on the flexor and extensor turning moments on the trunk during maximal static contractions in two postures, standing and sitting. Two tests were devised to measure subjects’ ability to maintain postures at the extremes of flexion and extension while overcoming the effect of gravity on the trunk. In addition the ranges of sagittal movement of the lumbar spine and hips were measured together with subjects’ weight. stature, sitting height, and the lengths of their upper and lower limbs. SUBJECTS

330 healthy, physically active young adults (132 female and 98 male) were investigated. The female subjects (aged 1823 yr, av. 18.9 yr) were students of physical education. Of the male subjects (aged 1839 yr. av. 21.3 yr) 34 were physical education and 64 student-teachers who students regularly engaged in athletic activities. METHODS

A. Meusurernents of static strength The maximal forces exerted while pushing and pulling were measured using the rigid structures illustrated in Figs. I, 2 and 3. It was designed so that subjects applied forces

and A. E. CHAPMAN

manually, their upper limbs horizontal, with their trunks vertical and their pelves held firmly between anterior and posterior padded bars. Forces were measured in two postures, standing and sitting, and the apparatus could be adjusted to suit the dimensions of the subject. The apparatus was constructed of 2 in. tubular steel and timber. and its dimensions are indicated in Fig. 2. The dynamometer and anterior pelvic bar were mounted on the tubular uprights: and the posterior padded bar and the seat were attached to the leading vertical of the structure made of ‘dexion’ which was clamped to the wooden platform of the apparatus. The vertical positions of the dynamometer and of the pelvic bars. and the horizontal positions of the pelvic bars and seat. were all adjustable. The dynamometer consisted of a force transducer mounted at one end of a steel plate, forces being transmitted to the transducer by a steel lever hinged on a block mounted at the other end of the steel plate (see Fig. 3). The force transducer (type UF 2. manufactured by Ether Ltd.) was a bidirectional strain-gauge bridge with a range of 0- IO0 kg. The distance between the centres of the transducer and the hinge was 12 in. Forces were applied to the lever through handles attached at a point 6 in. from the transducer so that the measured force was equal to half the applied force. The handles could also be attached at a point 8 in. from the transducer so that with exceptionally strong subjects the measured force was a third of the applied force and the range of the dynamometer became O-300 kg. The strain gauge bridge had an output resistance of 100012 and a full range output of 4.6 mV/V. It was energised by a 20 V. d.c. source and the output applied via a series resistance to a galvanometer (‘Scalamp’ manufactured by W. G. Pye & Co. Ltd.: 2~ amp f.s.d. at maximal sensitivity. 90fl resistance, 2-set response time). The system was accurate to within 2 kg throughout the

Fig. 1. The apparatus used in measurements of the maximal static strength of muscles of the trunk while pushing and pulling; with subject in the standing position, arms horizontal, trunk vertical and pelvis held between padded bars.

Fig. 3. The dynamometer, showing the bidirectional force transducer mounted on the steel base plate, with the hinged steel lever to which the handles (and tray for calibrating with lead weights) were attached. (facing p. 50)

Fig. 4. Subject performing the-sit and reach test in the inclined position, the table and the subject’s lower limbs being at 60’ to the horizontal. The measuring board is held against the plantar surfaces of the feet.

Fig. 5. Subject performing the prone, trunk raising test, his ankles being held firmly on the examination couch. The horizontal beam, 1 m above the couch, used for measuring the vertical distance raised at the level of the seventh cervical spinous process, is not shown.

Fig. 6. The protractors

used in measurements

of the range of lumbar and hip movement.

THE

FLEXOR

AND

EXTENSOR

MUSCLES

OF THE

TRUNK

Fig. 2. Diagram showing the apparatus used in measurements of the maxim;4 strength of muscles of the trunk. (A) The dynamometer. (9) Anterior pelvic bar. (0 Posterior pelvic bar. (1~) tleel block for use in standing posture. (E) Seat. (F) Wingnut for securing hoizontal position of ‘dexion‘ \Lrucntrc. IG) Leg rest for use with seat(E). The overall dimensions of the apparatus were:(II) height 7’5”; (IV) length 8’3”: (z) width 2’5”: The vertical distance (,\) between t.4) and (B) is the handle/bar height. The dimensicxis of the structure of ‘dexion’, to which the posterior pelvic bar (C) and the seat(E) were attached were:(c) height 3’9”; (.v) length SO”; (19width 1’3”. The dimension\ of the wooden platform were: (.r) length 7’0”: (I) width 2’0”.

range. and was calibrated before and after making observations on a group of subjects using an applied force of up to 160 kg in 10 kg steps. To record the maximal static forces exerted by subjects, the position of the dynamometer was adjusted so that its handles were in the same horizontal plane as the shoulder joints. The vertical positions of the pelvic bars were adjusted so that in the standing position they were at a level midway between the anterior superior iliac spine and the symphysis pubis. In the sitting position. the anterior pelvic bar was relatively higher because of the anteroposterior diameter of the thighs. The horizontal positions of the bars were adjusted so that the shoulder joints were vertically above the hipjoints. and the bars then approximated and locked in position to hold the sub-

ject firmly in the same posture for pushing and pulling. For standing subjects. a wooden block was inserted behind the heels to maintain the malleoli vertically below the hip joints: and when sitting. the lower limbs rested on a board inclined at I IO” to the vertical. no foot rest being provided. The vertical distances between the centres of the pelvic bars and the handles of the dynamometer (the handle/bar height) were recorded to the nearest O-5 cm for each posture. In the sitting position the posterior bar (against which they pushed) was about 2 cm lower than the anterior (against which they pulled). Subjects were asked first to exert a submaximal force to accustom themselves to the apparatus, keeping elbows and knees extended. Then two maximal exertions were made, subjects being asked to hold the

J. D. Ci. TROUP

52

and A. t‘. (‘HAPhlAN

maximum for about I sec. The maximal deflection on the galvanometer held stead), for I set was recorded to the nearest division which represented 2 kg of applied force. The results were recorded to the nearest kg. Taking into account the time of response of the galvanometer. the duration of the exertion was 3-4 sec. The turning moment on the trunk was obtained from the product of the force applied and the appropriate handle/bar height. It was assumed that the resultant of the forces applied to the pelvic bar was through the centre of each bar. The anterior one consisted of a length of 2 in. tubular steel covered in foam rubber; its position midway between the iliac spines and the symphysis pubis ~a4 critical. for direct pressure on the bony points was painful. The posterior bar consisted of a length of foam rubber 2 in. x 2 in. section. being the minimal size consistent with comfort. The errors arising from variation in the vertical position of the resultant of forces applied to the bars were not determined: but taking into account the fact that narrower bars were painful. it was assumed t!tat the forces were evenly distributed. The turning moments on the trunh were recorded to the nearest IO kg cm. In preliminary trials. 24 subjects (I 3 males and I I females. aged 18-28) were tested and retested after an interval of 3 weeks. The coefiicients of correlation between the results of test and retest were as follows:standing. standing. sitting, sitting,

pushing-089: pulling-0.89: pushing-081 pulling-0.93.

:

individual variance was marked but the tests were repeatable for large groups of subjects.

The ability to achieve and hold postures at the extremes of the range of flexion and

extension of the trunk and hip5 MXS measured in t\i o tests. sit curd rctrc~h. incliwd and pr~trc tr/lnX roisinp. ‘1 hey H cre tests of d! namic strength onI!, in as much as movement was required of the subject in performing them. but the measurements were made of postures reached on completing the mo\ ement. In the smrting position4 in \\ hich subjects wcrc at rest. the trunk ~a*; horizontal and thus the etfect of gravity. MXS grcatcr than in the final position. But in the performance of the tests. the acti\,e muscle groups shortened progressively. and the antagonist muscles \t’ere stretched. Subjectively. the effort required increased towards completion of the movement. ( I) Sit rrnti r~w~~/~, incliucd. The posture fol this test is shotin in Fig. 4. The low,er limbs I\ erc suppoi?ed at an inclination of 60” to the horizonta!. and the distance between the finger tips and the plantar surface of the feet was mca?;ured using a ccntimetre scale attached to the measuring board. Some subjects tended to mahe a sudden effort reaching further by use of their momentum. In practice. readings were made of the position reached and held for not less than 3 sec. They were recorded to the nearest centimetre. a negative sign being used for distances short of the plantar surface of the feet. The coefficient of correlation between the results of test and retest of the 24 subjects was OG38. (2) P~CMIC,trunk rcrising. Subjects lay prone. their legs held on the examination couch with their feet over the edge (see Fig. 5). A horizontal metal beam 1 m above the couch was used to provide a base line for measurements of the vertical distance between the position of the spinous process of the seventh cervical vertebra. The vertical distance of C7 was measured first with the subject lying prone relaxed and with the head turned to one side, and then after actively extending the trunk. raising head and shoulders as high as possible. The measurements were made with a metre rule held manually. Theoreti-

1 HE Fl.EXOR

r\ND

EXTENSOR

tally a variety of errors in these measurements could have arisen. In the raised position, the subject’s occiput was generally posterior to the process of C7, and :tccuracy depended on the metre rule remaming vertical; thus skill was required to make the observations. Nonetheless the results were repeatable for groups of subjects, for the coefficient of correlation between the results of test and retest on the 24 subjects was 0.8.;.

The ranges of sagittal movement of the lumbar spine and hips was estimated from observations made on subjects in fully flexed and fully extended postures. Lumbar movement was calculated indirectly by estimating the range of movement between the thoracolumbar region and the femow and subtracting from it the range of movement at the hip joints (Troup, Hood and Chapman. 1968). ( 1) Hip Jlexion/esten~ic)rl. Measurements were made with a protractor (manufactured by the Medical Engineering Development Trust) with 21 in. arms made of laminated plastic (see Fig. 6). The angles between femora were measured on each side with one hip flexed and the other extended in the posture shown in Fig. 7. The average range of sagittal movement of the hip joints was calculated and recorded to the nearest degree. (2) Llrrrrher .Pe.~;o,lic~srensiorl. The angles between a tangent to the spinal contour at TI I/ I2 and the femora were measured using

MIJSC-I ES OF THE

TRUNK

53

a marker held manually at T I I /I 2 and the protractors. They were measured in the flexed and extended postures shown in Figs. 8 and 9. The range of flexionlextension between TI I/ I2 and the femora was calculated and the rnnge of hip flexionlextension subtracted to’ obtain the degrees of lumbar flexion/ extension. This in practice was the range of movement between Tl II I2 and the pelvis rather than of the lumbar region itself.

Fig. 8. The posture of subjects adopted for measurements of the femoral/trunk angle. flexed.

Fig. 9. The posture of subjects adopted for measurements of the femordlltrunk angle. extended.

Fig. 7. The posture of cubjects adopted for measurements of hip Hexiuniextension.

D. Antlrropometric meuswemeats ( I ) Weight was recorded to the nearest kg with subjects wearing socks and underclothes. (2) Stature was recorded to the nearest centimetre using a wooden stadiometer.

54

J. D. G. TROUP

and A. E. CHAPMAN

(3) TmcAttrrter heigltt. The length of the lower limb was estimated by measuring the distance between the floor and the surface landmark of the greater trochanter in the erect posture. (4) sitting Iteight from the examination couch to the spinous process of C7 was measured with subjects sitting erect, their thighs fully supported on the couch and their feet hanging unsupported. (5) Rewh. The length of the upper limb was estimated by measuring the distance of the fi;tger tips from the manubrium sterni with arms outstretched and palms appohed either side of the metre rule; this measurement was used to estimate the functional length of the upper limbs relevant to the sit and reach test.

The tests and measurements were made with subjects wearing blouses or shirts, underclothes and socks: they were made in the following order:(1) flexor force, standing; (2) handle/bar height. standing; (3 ) extensor force, standing; (4) flexor force. sitting: (5) handle/bar heights. sitting; (6) extensor force. sittin!? (7) weight; (8) stature: (9) trochanter height; ( 10) sitting height; ( 11) reach; ( 12) prone. trunh rai%inp: (13) sit and reach, horizont:ll: { 14) hip flexionlextension: ( 15 ) trunk/femoral angle. llc xcd; ( 16) trunk/femoral angle. ctlcndccl ( 17) sit and reach. inclincci. The interposition of the anthropomctric tests after measurementh of static pe~lniltcd a pause between the tests ol strengih ObscI~vations could bc made of one subjcc,t in 2025 min; but in practice the) WCIT SCSII iu groups. six taking about I hr. With some of

the female subjects. tests (I J-(6) were performed 48 hr before tests (7)-(17) in order to interfere as little as possible with their programme. RESULTS

The means, standard deviations and ranges of the observations are shown in Table 1.

The forces exerted by the males were significantly greater than those exerted by the females, both in absolute terms and when expressed as a proportion of the body weights of the two sexes. The ratios of mean forces to mean body weights are shown in Table 2. Flexor forces were consistently less in magnitude than extensor forces and flexor turning moments were similarly less than extensor turning moments. The flexor/extensor ratios are given in Table 2. For the standing position, the ratios are the same for both forces and moments, the vertical distances between the pelvic bars and the handles of the dynamometer (the handle/bar heigh:s) being unchanged between pushing and pulling. In the sitting posture, there are small differences between the ratios of forces and moments as the anterior pelvic bar against which they pulled was about 2 cm higher than the posterior bar against which they psshed. The flexor forces exerted were somewhat greater standing than sitting, but with the difference in handle/bar heights in the two postures, the flexor turning moments were greater in the standing position. The converse applied to the extensor forces and moments. the latter being similar in the two postures. The standing/sitting ratios for forces and moments are shown in Table 2. 2.(a) Sit ctrld rem/r. inclirted test Thh mean distances between fingertips and the plantar surface of the feet achieved in the performance of this test were closely similar

THE FLEXOR

AND EXTENSOR

MUSCLES

55

OF THE TRUNK

Table 1.The means. standard deviations and ranges of the flexor and extensor forces exerted while pulling and pushing in the standing and sitting positions. and the resulting turning moments on the trunk; of the dynamic tests of strength; of the estimated ranges of sagittal movement of the hips and lumbar spine: and of the weight. stature. trochanter height, sitting height and reach of I32 female. 98 male. subjects Male subjects

Female subjects Standard deviation mean Flexor force. standing Extensor force, standing Flexor force, sitting Extensor force, sitting Flexor moment. standing Extensor moment. standing Flexor moment. sitting Extensor moment. sitting Sit and reach. inclined Prone, trunk raising Hip flexionlextension Lumbar flexionlextension Weight Stature Trochanter height Sitting height Reach

47 66

kg

kg kg kg kg cm kg cm kgcm kgcm cm zng deg kg cm cm cm cm

2 2180 3050 1660 3040 1 36 I53 81 61 164 86 63 69

IO.4 12.9 8.2 15.6 508 666 331 596 6.9 4.4 8.8 15.7 5.7 5.6 3.3 2.5 3.5

Table 2. Ratios of mean flexor and extensor forces to hody weight. of mean flexor to mean extensor forces and moments. and of mean forces and moments. standing to mean forces and moments, sitting

Flexor force /body weight Extensor force /body weight

Flexor force /extensor force

Flexor moment /extensor moment Force. standing /force. sitting Moment. standing /moment. sitting

Standing Sitting

Females

Males

0.77 0.72

I -04 090

Standing Sitting

I .07 1.40

I.35 I.82

Standing Sitting

0.72 0.52

0.77 0.49

Standing Sitting

0.72 0.55

0.77 0.52

Flexor Extensor

I.07 0.79

I.16 0.74

Flexor Extensor

1.31

I.40 0.94

I +CI

Range min. max. 21 31 25 49 910 1460 900 1700 -17 17 130 38 47 150 77 58 59

74 104 68 120 3400 5460 2650 4500 I8 45 I90 I25 77 178 96 70 78

Standard deviation mean 13.0 l7.R IO.2 22.3 702 952 474 991 7.0 4.4 9.4 14.0 8.6 5.7 4.0 2.8 3.5

75 98 65 I32 3760 4890 2680 5190 I 36 144 80 73 176 92 67 75

Range max. min. 50 54 37 78 2500 2760 I440 2610 -13 23 I23 50 51 161 83 60 67

II? I43 90 211 5600 7200 4320 7810 I9 47 182 128 96 190 IO? 74 83

in the two sexes. In order to take account of individual differences in the length of upper and lower limbs which might have affected scores, the distances observed were treated by adding trochanter height and subtracting reach. The resulting corrected figures were as follows:-

Females Males

Mean

Standard deviation

l7cm I8 cm

2 6.7 r6.9

Range min . - max. -2 3

33 37

Thus, there was no significant difference between the scores of the two sexes. (b) Prone, trunk raising The mean vertical distances moved at C7 in the performance of prone, trunk raising were 3%6cm for both sexes. When the mean scores were expressed as a proportion of the mean sitting height, the ratios were 0.57 and O-53 for females and males respectively.

_I. D. G. TROUP

56

3. The

relationships

between

and A. E. CHAPMAN

variables

The association between the tests of strength and the other observations has been analysed by calculating the coefficients of correlation between individual variables. The results are shown in Table 3. All the forces and all the turning moments in the static tests of strength correlated significantly with each other for both sexes. Flexor forces correlated as well with each other as with extensor forces, and the same applied to flexor and extensor moments. Forces and moments correlated well with

each other also. particularly those concerning the same manoeuvre. The sit and reach, inclined (S.R.I.) and prone, trunk raising (P.T.R.) tests of strength correlated significantly with each other in the male subjects (p < O-01) but nof in the females. -The results of the sit and reach. inclined tests and of the Flexor moment sitting are plotted on the graph shown in Fig. 10, the relationships being statistically significant (p < O-001 for females. p < O-01 for males). The results of the sit and reach, inclined tests also correlated significantly

females

l

17.132 r =O l 3065 pco~ool

. .

x i 1.56y.15.27~-’

SIT

&I (in& km)

. . 10

. .

. 40

30

29 FLEXOR

MOMENT

SITTING

(lO*kg

Cm)

Fig. 10. Graph showing individual values recorded in the sit and reach, inclined test (in cm) plotted against the flexor moments. sitting (in kg cm). together with the equations of the regression lines. the coeffcients of correlation (r) and the levels of statistical significance (p) for I32 female and 98 male sub.iects. The regression lines are shown together with solid lines indicating one standard deviation on either side of the regression line.

3. Coefficients

ofcorrelation

Levels Males r = 0.20 r = 0.26 r=0*33

r = 0.28

p < 0.001

0.03

0.27

-0.08

0.55 O-74 0.52 0.91

3

0.01

-0.26

0.70 0.76 0.54

4

the variables:

Females r = 0.17 r = 0.22

O,l4

2

0.76 0.67 0.95 0.52

-0. I9 -0.09

0.69 0.95 0.69 0.65

I

between

of significance p < 0.05 p < 0.01

I. Flexor force, standing. 2. Extensor force, standing. 3. Flexor force, sitting. 4. Extensor force, sitting. Flexor moment. standing. 2. Extensor moment, standing. 7. Flexor moment, sitting. 8. Extensor moment, sitting. 9. Sit and reach, inclined. IO. Prone, trunk raising. Il. Hip Hexion extension. 12. Lumbar flexionlextension. 13. Weight. 14. Stature. 15. Trochanter height. 16. Sitting height. 17. Reach.

Table

0.21

-0.20

0.74 0.72

5

above 7

0.43

I4 -0.06

0.11

-0.

0.61

6

the diagonal

0.19

-0.22

8

0.19

0.03

9

for the female II

0.03

0.08

0~26-0~10

IO

subjectsand

0.26’

0.09

12

below

0.23

0.43

13

0.65

046

I4

the diagonal

0.70

0,42

15

0.35‘1

0.42

I6

I7

0.36

for the males

;;’ 5 jr:

J. D. G. TROUP

S8

and A. E. CHAPMAN

with the flexor force, sitting. The results of the prone, trunk raising tests and of the extensor moments, standing are plotted on the graph shown in Fig. I 1, the relationships being statistically significant (p’< 0401 for females, p < 0.01 for males). The results of the test also correlated with the extensor force, standing in both sexes tp < 0401). and with other tests of static strength at lowet levels of significance. The individual measurements of static strength were significantly related to subwith the male jects’ weight, particularly subjects. But there was no significant associa-

tion between weight and either the S.R.I. or the P.T.R. tests. The relationship between the range of sagittal movement of the hips and the static tests of strength was of no significance although there were three exceptions in male subjects in whom there were negative correlations at comparatively low levels of significance. On the other hand the degrees of lumbar flexion/extension correlated significantly in both sexes with the flexor forces and moments in the seated position, and with extensor forces and moments, standing in the females. Hip flexion/extension correlated

n = 1326

female males

r z.00 3656 pt0~0005 X r3*65y_ 106.52 !.“....1 45

40

35

l

TRUNK RAISING (cm) 30 n.906 l

r =+0’3201

25

pt0*005

.

2c

t l

20

30

40

50

60

70

EXTENSORMOMENT STANDING t102 kg cm) Fig. 1I. Graph showing individual values recorded in the prone, trunk raising test (in cm) plotted against the extensor moment. standing tin kg cm), together with the equations of the regression lines. the coefficients of correlation (r) and the levels of statistical significance (p) for 132 female and 98 male subjects. The regression lines are shown together with solid lines indicating one standard deviation on either side of the regression line.

I

l

.

THF

FLEXOR

AND

EXTENSOR

significantly with both the S.R.l. and P.T.R. tests particularly with the males; in both sexes, lumbar flexionlextension significantly associated (p < OGOl) with S.R. I. and P.T.R. tests of strength.

the

and was the

DJSCliSSJON

The measurements of maximal static strength were made in postures in which the effect of gravity. was, for all practical purposes, eliminated. Thus the results are comparable with those of Mayer and Greenberg ( 1942) who measured the strength of schoolchildren in recumbent postures, Asmussen and HeebolI-Nielsen (196 1) who studied subjects of both sexes aged between 14 and 65 yr and Tornvall ( 1963) who investigated young adult males; and with those of Clarke t 1966) and Laubach and McConville ( 1966) who measured the strength of male students in recumbent postures and subsequently made corrections for the effect of gravity on the trunk. The ratios of flexor to extensor forces in the standing posture were of the order of 3 :4 which is in accord with the results of Mayer and Greenberg (1942) and Asmussen and Heeboll-Nielsen (1961). Tornvall (1963). Clarke ( 1966) and Laubach and McConville t 1966) reported flexor strength more or less equal to extensor. their subjects applying forces to the dynamometer. not manually, but through pads on the chest. This may account for the differences as chest pads might be difficult to locate, the posterior pad tending to rise and so reduce the force applied. and the anterior pad tending to fall. In the sitting posture, the flexor muscles of the trunk are in a relatively shortened position, and lengthened when standing. The flexor forces exerted and the flexor turning moments were both greater standing than sitting. This follows the expected pattern that muscles can transmit greater forces when in a lengthened position (Mayer and Greenberg, 1942; Williams and Stutzman, 1959). But it does not necessarily follow that

MUSCLES

OF THE TRUNK

59

the flexor muscles of the trunk were in fact transmitting greater forces because their mechanical advantage differed in the two postures, particularly the advantage of the muscles of the anterior abdominal wall which is improved on flexion on account of the greater antero-posterior diameter of the trunk in the lumbar region. The difference in mechanical advantage of the intraabdominal pressure in the two postures has not been measured. The extensor turning moments on the trunk in the two postures showed no significant difference although the extensor forces were markedly greater when sitting. The reason was the reduction in the length of the effective lever, the handle/bar height, compared with the standing posture. Again there is a difference in the mechanical advantage of the extensor muscles in the two postures. When flexed. the tips of the spinous processes are prominent posteriorly. and in the extended posture. the lumbar erectores spinae form two bulky ridges which are posterior to the spinous processes. Thus the centre through which the extensor forces are transmitted is situated more posteriorly and with a longer lever for action than when sitting, but the difference has not been measured. A further unknown factor, concerning the significance of the similarity of extensor turning moments in the two postures. is the effect of increases in intraabdominal pressure which were not measured in this investigation. Such increases are believed to represent an additional extensor mechanism which reduces both extensor and intervertebral compression forces needed to produce a given moment (Davis, 1956; Bartelink. 1957; Davis, 1959a, 1959b; Morris, Lucas and Bresler, 1961: Davis et ~1.. 1964; Davis and Troup, 1964, 1966). With a given increase in intraabdominal pressure its mechanical advantage as an extensor mechanism is theoretically greater in the flexed position (Davis and Troup, 1965). The comparative disadvantage of the lumbar erector spinae in the flexed

60

J. D. G. TROUP

and A. E. CHAPMAN

position may therefore be cancelled to some extent by the comparative advantage of increases in intraabdominal pressure. The magnitude of the extensor turning moments gives some indication of the intervertebral compressive forces and the extensor forces transmitted by postvertebral muscles and ligaments. In the male subjects the mean extensor moments were of the order of 5000 kg cm. Assuming that intervertebral disc between the fourth and fifth lumbar vertebrae is vertically below the shoulder joint, and that the distance between them is x 0.80 of the handle/bar height. the mechanical couple at L4/5 would equal 4000 kg cm. If the moment of the couple, in this instance the distance between the line of action of the extensor force and the intervertebral compressive force is 6 cm, then the extensor force equals 667 kg: and the compressive is 667 kg plus say 36 kg. about half the mean body weight, a total of 703 kg. However. the effect of increased intraabdominal pressure has been left out of account. Davis (1965) calculated that it may minimise the extensor and compressive forces induced in a given situation by as much as 25 per cent, thus the theoretical forces considered here would be reduced to 500 kg and 527 kg respectively. Although the physical properties of the intervertebral disc and the vertebral bodies in relation to their ability to resist compressive forces has received a good deal of attention (Hirsch and Nachemson, 1954; Hirsch, 1955: Perey. 1957; Rolander, 1966; Galante, 1967), very little work has been done concerning the tissues which transmit extensor forces. Etemadi ( 1963) weighed individual slips of muscle composing the erector spinae and presented the results in terms of the weights of muscle acting at each vertebral segment. But there has been no morphological study of the fibrous tissues which transmit extensor forces postvertebrally. This is of particular relevance when considering the extensor forces in postures in which ‘flexion-relaxation’ of the erector

spinae obtains. For instance, for a posture stooping and holding a weight of 25 kg above the floor with extended knees, Groh er al. ( 1967) calculated the extensor force in a man (stature 182 cm. weight 76 kg) to be 468 kg. In this posture, the lumbar erector spinae muscles are likely to be relatively inactive (Floyd and Silver, 195 1, 1955; Morris, Benner and Lucas, 1962; Yoshinaga, Hasegawa, Yonemoto, Koda, Takata, Sakurai, Serizawa, Yagi, Miura and lzeki, 1965: Pauly, 1966). However, the ligamentous structures are probably inadequate for the transmission of such a force. Jonck (1961) has shown that the supraspinous ligaments are seldom present in the lumbar region, and that the direction of the fibres of the interspinous ligaments is more or less normal to the line of action of extensor forces; and the ligamenta flava are elastic. Stripped of muscles, the lumbar spine has little intrinsic stability (Lucas and Bresler, 1960) and the flexor moments which can be resisted in osteoligamentous preparations without rupture are less than those which are induced in the living in the stooping posture (Eie, 1966). Extensor forces must therefore be transmitted to some extent by the lumbar fascia and also by the tendons of the erector spinae muscles. Jonck ( 1961) demonstrated the presence of small bundles of elastic fibres between the tendons of the muscle, and it is possible that these tendons can serve a ligamentous function when the muscles are stretched. Individual values for the static tests of strength correlated significantly with each other, but flexor forces and moments were not associated any more significantly with each other in the two postures than they were with extensor forces and moments. The relationships between the individual results of the sit and reach, inclined test and the flexor moment sitting, and between those of the prone, trunk raising test and the extensor moment, standing were statistically significant. Nevertheless the degree of scatter (see Figs. 10 and 11) is such that individual

THE

FLEXOR

AND

EXTENSOR

predictions of static strength based on the performance in these tests would be unreliable. The greater the mobility of the subject the less the effect of gravity on the trunk at the extremes of the ranges of movement at which measurements were made. In fact the individual results in these two tests correlate as significantly with the tests of mobility as they do with the appropriate measurement of static strength. The relationships between static tests of strength and the ranges of flexionlextension of the lumbar spine and hips were generally not of statistical significance, confirming the results of Laubach and McConville (1966). However, there were two exceptions. First, the relationship between lumbar mobility and flexor forces and moments in the sitting position. This may be attributed to a reduced ability of subjects to exert strength if the sitting posture was close to the extreme of their ranges of flexion, the flexor muscles being already in the shortest position. The other exception concerns the negative correlation between hip mobility and extensor strength in males in the sitting position. No obvious expianation emerges but it is noteworthy that there is a negative correlation also between hip flexion/extension and body weight, while the correlation between body weight and strength is strongly positive. The absence of a significant cot-relationship between lumbar and hip flexionlextension is discussed elsewhere (Troup et al., 1968). This investigation has provided a certain amount of information concerning the strength of the muscles of the trunk and of the mobility of the lumbar spine and hips which is relevant to the problem of determining individual capacity for heavy work. However it is dificult to obtain a measure of the strength actually needed for a particular heavy manual task because of the nature of the skills used when applying forces to external objects. One obvious method by which a force can be applied to overcome the inertia of a load is by the transfer of momentum from the body.

MUSCLES

OF THE

TRUNK

61

Another method appears to be that of coordinating muscular activity so that at critical points of time the prime movers are contracting eccentrically in order to develop the gteatest’ tension. For instance, in dynamic tests of strength in which the torque transmitted by the muscles of the trunk in rotator activities has been measured using a cinephotographic technique in this laboratory. Grieve (1968) has found that for short periods of the order of O-10 set the torque developed is far in excess of the maximal isometric torque. With similar methods, together with the use of accelerometry and measurements of intraabdominal pressure further experiments are now being planned. It is hoped to learn more of the relationship between static and dynamic measurements of strength and of the means by which external forces are exerted by the muscles of the trunk. and thus of physical capacity for heavy work involving the spine and trunk. Ackno,c./edXemrnIs-This work was supported principally by The Nuffield Foundation. and also by the National Coal Board and the Council of the Royal Free Hospital School of Medicine. It could not have been done without the active cooperation of many volunteers, and it is with great pleasure that we acknowledge our debt to the Principal. Chelsea College of Physical Education and the Warden. University of London Goldsmith’s College, to their staff and students. and to the staff and students of the medical school. We are particularly grateful to Professor R. E. M. Bowden in whose department the work was done, to Professor P. R. Davis, and Dr. D. W. Grieve for their criticism and encouragement. We also thank Mrs. M. E. Monk-Jones for advice on statistical problems: Mr. P. C. Dunton for writing the computer- programme: Miss F. M. Ellis. Mr. B. Clark and Mr. R. J. Skvrme for their photography: and Mr. F. W. Hart and Mr.-J. Adams for technical assistance.

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62

J. D. G. TROUP

Bedford, T. and Warner, C. G. (19.:7) Strength tegt: observations on the effect of posture on the ctrrngth cJf pull. I.uncer 1937.2, 1328-1329. Clarke, H. H. I 1966) Muscular Srren~th trnd En&r.,~r~t !’ in Man. Prentice-Hall, New Jersey. Davis, P. R. (1956) Variations of the human in:rclabdominal pressure during weight-lifting in variou\ postures../. Anut. 90.601 (PI. Davis, P. R. (1Y59a) The posture of the trunk during the lifting of weights. Br. med. J. 1.87-89. Davis, P. R. ( 1Y59b) The causation of herniae by weightlifting. Lancer 1959.2, 155-157. Davis. P. R. (I 965) Personal communication. Davis, P. R. and Troup, J. D. G. (1964) Pressures in the trunk cavities when pulling, pushing and lifting. Ergonomics 7.465-474.

Davis. P. R. and Troup, J. I). G. (1965) Effects on rhe trunk of h,mdling heavy loads in ditferent postures. Proc. 2nd Ittf. Ergonomics Congr., t!Iarfttrtrtfd, I YhJ. pp. 313-327. Davis. P. R. and Troup, J. D. G. (I 966) Effects on the trunk of erecting pit-props at different working heights. Er.gwromicr

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Davis. I’. R.. Whitney. R. J.. Troup. J. D. G. and Gear. R. J. ( 1964) Llnpublished data. Eie. N. ( 1966) Load capacity of the low back. J. Oslo Cv HI>\/‘.F. 16. :.:-98. Etemadi. ‘4. A. ( 196.3) Observations on the musculature and innervation uf the human back. M.Sc. Thesis, University of London. Fisher. B. ( 1967) .,I biomechanicul model for the analysis of dynamic activities. Part III of A biomechanical analyGs of materials handling activities. Ind. Engng Dept. Rep.. University of Michigan. l-loyd, W. F. and Silver. P. H. S. (I 95 I) Function of the erectores spinae in Hevion of the trunk. I.trnc’ct 1951.1. 133-33-1. Floyd. W. P. and Silver. P. H. S. (I 955) The function of the c’rectores spinae in certain movements and postures in man. ./. !‘/t.vsiol. 129. 184-103. Galante. J. 0. ( I Y67) Tensile properties of the human lumbar dnnulu\ fibrosus. .Jcro orthop. wand., Snppl. 100.

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Troup. J. D. G. (1965) The relation of lumbar spine disorders to heavy manual work and lifting. Lancel. 1965. 1,857-86 I. Troup, J. D. G., Hood, C. A. and Chapman. A. E. ( 1968) Measurements of the sagittal mobility of the lumbar spine and hips. Ann. phys. Med. In press. Vernon. H. M. ( 1924) The effects of posture and rest in muscular work. Part B. The influence of rest pauses and changes of posture on the capacity for muscular wrh Ind. Fntigue Res. Board. Rept. 29. X-55. Whitney, K. I. ( 1958) The strength of the lifting action in man. Oeottottfic.5I. IO I - I ZX. Williams. ‘il. and Stutrman, 1.. I 19.(Y) Strength variation through the range of joint motion. Physio//rer. Rev. 39. 145-152. Yoshinaga. H., Hasegawa, Y.. Yonemoto, K., Koda, M., Takata. W., Sakurai. T.. Serizawa. Y., Yagi, A., Miura, 51. and Izeki. nl. (1965) Electromyogram of trunk muscles in view of their protecting function of spinal column. (in Japanese) J. Jrrp. nrrhop. .4ssn. 39. 2 I-28.