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Spinal shrinkage in females: circadian variation and theeffects of circuit weight-trainingJANE WILBY a , K. LINGE b , T. REILLY a & J. D. G. TROUP ba Department of Sport and Recreation Studies , Liverpool Polytechnic , Byrom Street,Liverpool, L3 3AF, Englandb Department of Orthopaedic and Accident Surgery , Royal Liverpool Hospital, University ofLiverpool , PO Box 147, Liverpool, L69 3BX, EnglandPublished online: 27 Jul 2007.

To cite this article: JANE WILBY , K. LINGE , T. REILLY & J. D. G. TROUP (1987) Spinal shrinkage in females: circadianvariation and the effects of circuit weight-training, Ergonomics, 30:1, 47-54, DOI: 10.1080/00140138708969676

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Spinal shrinkage in females: circadian variation and the - effects of circuit weight-training

By JANE WILBY*, K. LINGE~, T. REILLY* and J. D. G. TROUP~ Department of Sport and Recreation Studies, Liverpool Polytechnic,

Byrom Street, Liverpool L3 3AF, England t Department of Orthopaedic and Accident Surgery, Royal Liverpool Hospital,

University of Liverpool, PO Box 147, Liverpool L69 3BX, England

Keywords: Shrinkage; Circadian variation; Weight-training; Strength.

Compressive loading of the spine leads to spinal shrinkage and a loss of stature. The aim of this study was to measure the circadian variation in the stature of females and to examine differential effects of spinal loading with the time of day. Ten females aged 20-30 years were studied in nine measurements on a 24-hour period. Between

. ' - rising and retiring to bed, the mean peak-to-trough variation was 15.4 mm (0.92% of stature). Subjects underwent two sequences of 20 min circuit-training with weights, followed by 20 min reclining in Fowler's position and 20 min standing erect, these being first thing in the morning after 7 hours of sleep and before retiring at midnight. Mean loss of height from circuit-training was 5.4 mm in the morning and 4.3 mm in the evening (P <0.001): mean height regained in Fowler's position was 4.5 mm and 3.4 mm respectively (P < 005). There were significant height losses in the subsequent standing posture but no time of day effect. Height losses with exercise were related to the isometric strength of the back muscles and to the perceived exertion. Results support the hypothesis that height losses from a given spinal load will be of smaller amplitude when body height is in the trough of circadian variation.

1. Introduction The intervertebral disc loses height due to axial loading such as induced by weight-

training: whenever the compressive load exceeds the osmotic pressure of discal tissues, fluid is expelled through the disc wall. The result is reflected in a loss of stature (Troup et al. 1985). This is known as shrinkage and can be used to indicate the load on the spine (Eklund and Corlett 1984). Apart from acute and dynamic loading, the disc also loses height during the day, depending on the nature, level and intennittency of activity. Consequently, investigation of how the time of day influences the shrinkage caused by a set weigh t-training circuit was of interest. . The problem until recently has been how to measure stature sufficiently accurately

to monitor this loss of disc height. This was overcome by procedures developed jointly at Nottingham and at Liverpool (Eklund and Corlett 1983, Reilly et al. 19841, partly by training but mainly by measuring stature in an erect relaxed posture with the subject tilted within the measurement apparatus. Essential features of the apparatus have been described elsewhere (Tyrrell et al. 1 985). The posture is standardized for each individual and the angle of the head is also controlled by means of the alignment of spectacles worn by the subject. Inter-individual differences in spinal contour can be ac- commodated and precise reproduction of t he spinal posture attained. Measurement is standardized for the stage of the respiratory cycle, observations being made at the end of a normal expiration. Subjects are considered to be trained when ten consecutive measures have a standard deviation of less than 0.5 mm.

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48 J. Wilby et al.

In previous studies using male subjects, we have shown that statun varies with time -ofday about 1% of its total body length (Reilly et al. 1984, Leatt et al. 1986). No data are available for females who may have different rates of shrinkage from those of males. Consequently the first aim of this study was to measure the circadian variation in the shrinkage of females.

As loss of disc height results in changes in the dynamic response characteristics of the disc, a further aim was to determine whether there is a time of day effect on the shrinkage due to fixed exercise regime. Finally, we wished to observe the rates of recovery with a standard orthopaedic procedure followed by a period of standing upright.

2. Metbods Ten females aged 20-30 (mean f s.d. =22.2 & 3.1) years acted as subjects. Mean

(& s.d.) values for height were 166 ( f 5) cm, corresponding values for body weight being 61-4 (f 6*7).kg. The equipment used was a modified version of that used earlier by Leatt et at (1 986), weighing scales being employed to ensure consistent distribution of weight between forefeet and heels. The subject was inclined 1 3", and four microswitches were used to control spinal curvature. A circular plastic disc with a travel of 50 mrn was connected to a Mercer dial gauge with a precision of 0-01 mm. Two strain gauges on a spring riding on the upper surface of the disc transducea any vertical movement of the disc to a chart recorder. The microswitches for the cervical and lumbar curvature were adjustable in the horizontal and vertical planes, the other two, for the head and sacrum, being adjustable in the vertical plane only. An infrared emitter in the centre of the spectacles in conjunction with an infrared receiver positioned directly in front of the subject controlled the posture of the head.

All subjects had a minimum of three 20min sessions entailing 90 measurements or more for training in the measurement procedure. Prior to any recorded experimental work, the standard deviation of ten consecutive measurements was less than 0.5 mm in each subject. In the final pre-experimental test, individual standard deviations ranged from 0-49 to 0.19 mm. subjects refrained from any sporting activity and otherwise kept to a normal daily routine on the day of testing.

The circadian variation was measured at nine times during the day. The first measurement was immediately before retiring for sleep in the laboratory at midnight. Sleep was interrupted for measurements at 03.45 hours and heigbt was measured again at 07.30 hours immediately after rising. Six other times of day at which measurements were taken were 08.1 5, 09.00, 10.00, 12.00, 16.00 and 20.00 hours. These times were chosen so that the results could be compared with previous observations on males.

Subjects then underwent two sequences of 20 min circuit-training, each sequence consisting of eight exercises until completion of the 20 min period (figure 1). The regime was conducted first thing in the morning on rising and after 22.00 hours. The circuit is typical of that used for circulatory training rather than for specific muscle strength training. At the end of the exercises, height was measured: subjects' breathing had normalized sufficiently within 4 rnin to pennit a recording of stature. Perceived exertion (RPE) was rated at the end of the exercises using Borg's (1970) scale. The subjects were allowed 20 min in Fowler's recovery posture (Leatt et a/. 1985), with5 min interruptions for measurement of height. Then the subjects remained in the standing position for a further 20 rnin, height again being measured every 5 min.

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Spinal shrinkage in females.

Figure 1. The sequence of eight exercises which formed the circuit of weight-training.

Before any exercise took pIace back strength was measured with a Takei Kigi Kyogo dynamometer. The best of three trials, after a single warm-up effort, was recorded. The subjects then did rou'tine stretching exercises for 2 min as a safety precaution before starting on the circuit-training regime.

3. Results and discussion Between rising from and retiring to bed, the mean peak-to-trough variation was

15-4 mm or 0.92% of body height. Results illustrated in figure 2 show that the cosine

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J. Wilby et al.

TIME OF DAY (hours) Figure 2. Changes in height during a 24-hour period from a midnight baseline set at 7-5 mrn.

The unbroken line indicates observed mean values with 95% confidence limits represented by the bars: the dashed line represents the cosine curve fitted to the data.

function, which is the normal method of analysing circadian rhythms, gives only an approximate fit to the data. Disproportionate losses occurred in the early part of the day, 45% of total loss occurring in the first 90 min of waking. Conversely the majority of gains in stature occurred in the first half of the night's sleep. Recovery in this time resulted in regaining 71% of the height lost.

A better fit to the data is obtained by analysing the daytime values for loss of height, assuming the Kelvin unit model of the disc which has no elastic response (Burns and Kaleps 1980). An experimental regression analysis provided the following equation

where H is loss of height in millimetres and t indicates time. The resultant power function presented in figure 3 fits the data almost perfectly. The value of the constant (-0.21 6) in the equation is similar to that reported in the earlier study of male subjects (Leatt et al. 1986), suggesting that rates of height loss for males and females are similar.

The morning regime produced the greater losses of height, 5.4mm compared to 4.3 mm on average (see the table). This difference was significant (P <0-01). As the disc height is already relatively low in the evening, the greater stiffness of the disc at this time is likely to increase the risk of injury. This question of greater susceptibility to injury in the evening may be addressed by examining epidemiological data from industrial SOU rces.

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Spinal shrinkage in females

i 8 l'2 16 T tME AFTER RISING (hours)

Figure 3. The power function relating time since rising from sleep to the loss of height (mm). The curve overlaps the observed changes in height almost perfectly.

Shrinkage, perceived exertion and back strength at two different times of day. Values are mean (f s.d.).

Morning Evening

Shrinkage (mm) 5.4f 0 3 4 .3+03 Perceived exertion 14-9f1-5 13.8+1-5 Back strength (N) 7402 186 799f 193

The perceived exertion was greater when exercise was conducted in the morning than at night (P < 0.01). This is probably unrelated to shrinkage, but is more likely to be linked with the state of arousal, body temperature and other factors (Reilly 1985). Indeed the correlation analysis failed to show a significant relationship between shrinkage and perceived exertion at either of the two times of day. The observation that a fixed exercise regime is felt to be subjectively harder in the morning than in the evening has been reported by others (Faria and Drummond 1982).

However, the back strength may be related to the amount of shrinkage. Back strength was found to be greater in the evening than in the morning (P ~ 0 0 5 ) . Thus the musculature of the back may be better able to compensate for loss of disc height in the evening than in the morning. Height losses were, in fact, inversely correlated with isometric back strength, the correlation coefficient (r) being - 0-60 (P < 0.05). Back

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52 J. Wilby et al,

strength was, in turn sign~ficantly negatively related to RPE, r being - 0.56 and - 059 in morning and evening respectively (P < 0.05). These relationships are not an artefact of differences in body size between subjects since back strength was not significantly related to height or weight.

There were significant regains in stature when the Fowler position was adopted, the rate of regain in height being inversely related to time (figure 4). This regain was greater in the morning than in the evening, the average being 4.5 mm and 3-4 mm respectively. These represented values of 83% and 79% respectively of the previous losses due to exercise. Again regains were significantly negatively related to isometric back strength, the coefficients being -0.55 and -0.64 for morning and evening data respectively (P < 0.05). There were significant height losses in the subsequent standing posture but no differences between morning and evening.

- - - - F o w l e r psitian - - --. I- - - Standing recovery - - -+ I

TIME ( m i n l

Figure 4. Mean height recovered after circuit weight-training in the morning and in the evening Subjects were in the Fowler position for 20min and then assumed an erect standing posture.

In summary:

(1) The results confirm in females the circadian variation in stature previously reported in males. This has implications for scheduling of heavy exercise that loads the spine and suggests that the normal trend of habitual activity during the day should be taken in consideration when exercise regimes are prescribed.

(2) Results support the hypothesis that height losses will be of smaller amplitude from a given spinal load when body height is in the trough of circadian variation.

(3) Results confirm the benefits of recovery procedures post-exercise in reversing shrinkage, 'with potential application also to pre-exercise contexts.

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Spinal shrinkage in females. 53

Une charge compressive appliquk A la colonne vertibrale amine une contraction de celle-ci, aboutissant i une rkduction de la taille. L'objectif de cette itude itait de diterminer s'il y a une variation circadienne de la taille chez les femmes et d'ivaluer les effets differentiels de la charge spinale en fonction de l'heure du jour. Dix femmes a g k de 20 B 30 am ont i t i examinks en 9 mensurations sur une pkriode de 24 heures. Entre le lever et le coucher, la variation moyenne minima-maxima itait de 15,4mm (492% de la taille). Les sujets effectuaient deux sessions d'entrainement de 20 min sur un circuit avec des poids, suivies d'une position allongk de Fowler pendant 20min, puis d'une position debout pendant 20min. Ces sessions avaient lieu immediatement au lever apks un sommeil de 7 heures et juste avant le coucher A minuit. La perte moyenne de taifle due A I'entrainement sur circuit ttait de $4 mm, le rnatin et de 4,3 mm le soir, (p <0,001). Le gain moyen de taifle dd A la position de Fowler itait respectivement de 4,5 mm et 3,4mm (pc0,05). I1 y avait des pertes de taille significatives dues a la position debout subdquente, mais on n'a pas observk d'effet de l teure du jour. Les pertes de taille dues B l'exercice ont i te mises en relation avec la force isomktrique des muscles du dos et avec I'efTort pequ. Ces risultats confirment l'hypoth&se selon laquetle les pertes de taille dues a une charge spinale donnk sseraient d'une amplitude plus faible lorsque la taille se situe au niveau bas de la variation circadienne.

Druck belastung der Wirbelsiiule fihrt zur Wirbelsiulenschrumpfung und einem Verlust an Kiirpergr6Be. Ziel dieser Studie war es, die circadiane Anderung der Kbrpcrgriik von Frauen zu messen und in ebhingigkeit von der Tageszeit unterschiediiche Auswirkungen bei Wirbcl- dulenbelastung zu untersuchen. 10 Frauen im Alter von 20-30 Jabren wurden in 9 Messungen fiber einen Zktraum von 24 Stunden untersucht. Zwischen dern Aufstehen und dern zu Bett gehen betrug die mit tlere Maximum-Minimum- Abweicbung 15,4 mm (0,92% der K6rperg6-k). Die Versuchspersonen unterzogen sich zwei Serien ein_es j?we!s 2Gminiitigen sogen. Circuit- trainings mit Gewichten, gefolgt von 20min Erholung in der Fowler-Lage und 20min aufrechtem Stehen; dieses wurde morgens a l s erstes nach 7 Studen Schlaf durchgefiihrt und vor dem zu Bett gehen urn Mitternacht. Der mittlere Verlust an Kbrpergrdk durch das Circuittraining betrug 5,4 mm am Morgen und 4,3 mm am Abend (P <0,001): Der Wiederanstieg der K6rpergr613e betrug in der Fowler-Lage jeweils 4,5 mm und 3,4mm (P<O,OS). Bei dem nachfolgenden aufrechten Stehen zeigte sich ein signifikanter Verlust an Kbrpergrbk aber kein Einflufl durch die Tageszeit. Der Verlust an Kbrpergr6De bei Belastung steht im Zusammenhang mit der isometrischen Kraft der Riickenmuskeln und der empfundenen Anstrengung. Die Ergebnisse stiitzen die Hypothese, daB ein Verlust an Kbrpergrhlle durch das Aufbringen eintr spinalen Belastung geringer ausgepragt ist, wenn die Kerpergriik bereits ihr circadianes Minimum aufweist.

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