Differences in lumbar spine intradiscal pressure between standing and sitting postures: a comprehensive literature review

Data management

In order to compare the IP of the same posture among studies, unification of the study results was necessary. Data from each included study was entered onto a data extraction form, which assigned the value of the variable that measured IP to each individual body posture and study. It should be emphasized that body position is understood as the position and the value of external force. This means that the external force also differentiated the considered cases defined as body position. Then, for every posture within the study a relative intradiscal pressure (rIP) was calculated as IP for this posture, and juxtaposed to the IP for the standing upright posture. The rIP calculation process was dependent on the type of data provided by the article. In cases where a relative value of IP was provided, data were not further transposed, e.g., this ratio was accepted for analysis as rIP. Some study provided absolute values of IP as means and standard deviations for groups of study participants or as one value for each participant and each posture. In those papers that provided IP for individual participants separately, the rIP for each participant was calculated first. According to this, the mean and standard deviation (SD) for this particular posture for all study participants was calculated. If the study presented absolute measures for defined postures as a mean and SD of a group of participants, the ratio of means of rIP was calculated with the application of the Ratio of Means (RoM) method, expressed in the following way (Friedrich, Adhikari & Beyene, 2008): ${\displaystyle \text{RL}=\left({\text{mean}}_{\mathrm{posture}}/{\text{mean}}_{\mathrm{standing}}\right)}$ ${\displaystyle {\mathrm{SD}}_{\mathrm{RL}}=\mathrm{sqrt}\left\{\left(1/n\right)\ast \left[{\left({\mathrm{SD}}_{\mathrm{posture}}/{\mathrm{mean}}_{\mathrm{posture}}\right)}^2+{\left({\mathrm{SD}}_{\mathrm{standing}}/{\mathrm{mean}}_{\mathrm{standing}}\right)}^2\right]\right\}.}$ Expressing load in the lumbar spine as a relative value (rIP measure) makes results of different studies comparable regardless of the individual characteristics of the group of study participants or the measurement technique. If more than one study presented data for the same posture, the final step included grouping data of rIP from various studies together and presenting it as one representative measure. In order to do so, weighted means and pooled SD was calculated, as proposed by Killeen (2005): ${\displaystyle {rIP}_{\mathrm{pooled}}=\left(n_1\cdot rI{P}_1+\dots \dots .+n_{\mathrm{k}}\cdot rI{P}_{\mathrm{k}}\right)/\left(n_1+\dots n_{\mathrm{k}}\right)}$ ${\displaystyle S{D}_{\mathrm{pooled}}=\mathrm{sqrt}\left[\left\{\left(n_1-1\right)\cdot S{D}_1^2+\dots ..+\left(n_{\mathrm{k}}-1\right)\cdot S{D}_{\mathrm{k}}^2\right\}/\left(n_1+n_2+\dots \dots +n_{\mathrm{k}}--k\right)\right]}$ where n is the number of participants in a given study.

Intradiscal pressure was measured in nine studies with altogether 37 sitting postures and 46 standing ones (excluding standing upright). There were postures, for which IP was presented in a number of studies, while others in only one. Postures have been described in various ways in different studies. Some descriptions were quantitative by presenting the values of the angles, while others were qualitative presenting a qualitative description in terms of “flexion”, “bending”, “twisting” or “extension”. This means that, in some cases, details that would allow for a clear definition of the posture were not presented. In those cases where postures were defined quantitatively and there was enough data, mathematical equations that expressed rIP as a function of angles or external force were developed.

Models of changes in rIP were developed using regression analysis of the Statistica 10 package. As a model of rIP changes, a formula was adopted that expressed a function of two variables (back flexion angle and exerted force) and ensured the best fit of the model calculations to the experimental data. The fit was tested with the value of the Spearman’s correlation coefficient.

Data obtained from reviewed studies

Most of the studies presented absolute values of IP for each person and each posture (Andersson, Ortengren & Nachemson, 1977; Nachemson, 1966; Nachemson & Morris, 1964; Nachemson & Elfstrom, 1970; Sato, Kikuchi & Yonezawa, 1999). In those cases, for each person and each posture, rIP was calculated first followed by the mean and standard deviation for a group of this study participants. Schultz et al. (1982) performed a study on four volunteers and presented mean values of IP for each posture. The study by Wilke et al. (1999) and Wilke et al. (2001) presented IP for one person as both absolute and relative values (in relation to standing upright). Wilke et al. (1999) and Wilke et al.’s (2001) results concerned the same single participant. As some postures differ between these articles, both were included. Wilke et al. (1999) and Wilke et al. (2001) measured intradiscal pressure over a broad range of postures and activities. Tables 2 and 3 report the IP of lumbar back, as calculated in the reviewed papers, for both standing and sitting postures respectively.

In general terms, IP in the lumbar spine is higher in a sitting posture that in upright standing. There are cases, however, when IP is lower. Slouching on a chair, relaxing and reclining on a table are postures that induce lower IP than standing upright by 50–60% (Table 3). Also, sitting bent foreword in a 40° angle with the elbows resting on the thighs lowers lumbar back IP by 14% compared to standing upright. There are observable differences in IP between sitting on a chair and on a ball. When sitting on a ball with a straight back, IP is very similar to standing upright. When sitting on a ball in a flexed posture, IP increases by 30% (Table 3). The Wilke et al. (1999) and Wilke et al. (2001) study reports an increase in IP when sitting flexed with the head in front of the body. As shown in Table 3, this posture increases IP by about 80% compared to sitting upright.

Relationship of IPs dependent on a specific posture

Relative IP values (rIP) obtained for each of postures that are presented in Tables 2 and 3 were grouped when referring to the same posture. After pooling rIP values from different studies for the same posture, rIP was presented for 27 different sitting postures and 26 standing ones. For each posture, the aggregated relative intradiscal pressure was calculated. Some of the results of these calculations were used in order to determine a mathematical equation that expresses pooled rIP as a function of back flexion angle and exerted force.

In the case of standing posture for exerted force close to 200 N (10 kg in each hand), experimental data were obtained for six back flexion angles. Data were also available for seven back postures without external load, and for 30° back flexion with 100 N and 300 N. This determined a second degree polynomial equation of rIP as a function of back flexion angle and exerted force (Eq. 1).

Relative IP values allowed to develop an equation expressing rIP as a function of back angle flexion and exerted force for the sitting posture as well (Eq. 2). For the sitting posture, experimental data were available for four back positions without external force, 0° back flexion with 90 N and 230 N force exerted, and 20° back flexion with 200 N force. Due to the rough definition of postures, in the first step data that referred to the upright sitting posture were analysed separately from those that referred to a relaxed/slumped sitting. Data for those two cases showed lack of differences in rIP values, which proved that both of those postures can be treated as one and data can be pooled. (1)${\displaystyle rI{P}_{\mathrm{standing}}=1.08+0.005\ast A+0.0012\ast F+0.0006\ast A^2+0.0001\ast A\ast F}$ (2)${\displaystyle rI{P}_{\mathrm{sittin}}=1.35+0.007\ast A+0.0032\ast F+0.0001\ast A^2+0.0001\ast A\ast F}$ where

A–back flexion angle (^∘)

F–exerted force (N)

Figure 2 presents the experimental data (mean and sd) and the rIP values calculated with the Eq. (1) for standing posture. Changes in rIP with back flexion angle for 0N and for 200 N can be approximated by second degree curves with increase for 200 N steeper than when the external force equals to zero. rIP calculated with the equation showed high Spearman correlation coefficient (equal to 0.974) with experimental data. An extension posture, compared to standing upright, does not significantly change lumbar back load. Nachemson & Elfstrom (1970) calculated rIP for flexed postures, without providing the value of angle flexion, as equal to 2.54 (0.73). According to the calculations with the equation, this would suggest back flexion of about 45°.

Figure 3 presents rIP at the lumbar spine for different angles of back flexion and different values of loads carried in the hands for sitting posture. The experimental data showed high Spearman correlation coefficient (equal to 0.964) with calculations with the model. Changes due to back flexion are smaller than those obtained due to standing. Figure 3 presents data for back flexion with 20° and external force of 0 N and 200 N. Adding this external force, increased rIP by about 50%, which is a similar increase to adding 200 N to the standing posture flexed by 20°. The figure shows IP as a linear function of external force.

In order to express differences in IP between sitting and standing postures, the ratio of sitting to standing for the same angle of back flexion was calculated. Figure 4 presents the values of this ratio and illustrates that for back flexion angles equal to 0° and 10°, IP is higher for sitting than for standing. When the angle is 20°, there are no differences in IP, while for bigger angles, IP in sitting is lower than in standing for the same angle of back flexion.

Figure 5 presents differences in IP between cases with and without external force for standing postures, as well as a quantitative increase in IP due to differences in posture when lifting a load. Adding 8 kg of force to standing in 30° flexion increases the lumbar load by about 50%. The same effect is achieved by adding 10 kg to each hand in a twisting or bending posture. The figure also shows about 50% increase in lumbar back IP when a load of 20 kg is lifted with back flexion in comparison to when the load is lifted while bending the knees. Lifting 10 kg gives only about 10% increase in IP. The highest increase in IP was observed when a load of 8 kg was added to the upright standing posture with arms extended. The arms extended posture, however, has not been defined more precisely.

Schultz et al. (1982) measured IP in the sitting posture for four different upper limb positions. Position A required the adduction of an extended upper limb by moving in front of the body by about 75°. In positions B and C, the angle of adduction was 45°. Position B was closer to the body. In position C, the upper limb was fully extended. For each position, IP was measured in two variants, without external force and with 4 kg at hand. In order to express quantitatively the increase of IP under the influence of external force, for each position, a comparison of the ratio of IP with external force to IP without external force was drawn (Fig. 6). In most postures, adding 4 kg nearly doubles IP measured for a posture without external force. It can be noted that addition of external force in posture B (closer to the body) increases IP at the lumbar spine less significantly than in other postures.