Frequency Analyses of Postural Sway during Prolonged Sitting on a Large Gymnastics Ball and Stool

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2.3. Assessment of Sitting Posture

The measurement technique has been described and evaluated in detail in previous studies ^[13]. In brief, the sitting posture of each child was recorded from a lateral view using a digital video camera (Figure 1). The camera was leveled on a tripod, and therefore the vertical side of the image was parallel to the direction of gravity. It was located 5 m from the child at a height of 1 m and positioned perpendicular to the plane of motion to decrease errors of perspective. Five markers constructed of a LED diode were fixed on the free end of a 2 cm long frame, as shown in Figure 2, and were positioned at the most superior parts of the spinal prominence of lumbar vertebrae L1, L3, and L5, of sacrum S2, and at the anterior superior iliac spine (ASIS).

Figure 2. Definition of angles

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The intensity of the light in the room was low; therefore, the optical markers were clearly visible on the recorded image. The images, which were digitized and fed into the computer with a spatial resolution of 320 × 280 pixels and with an acquisition rate of 3 frames per second, were analyzed to determine the position of each marker. For this purpose, the researchers developed motion analysis software, which found the position of the marker in the first image and traced its position during the rest of the session.

Optical skin markers L1, L2, and L3 were used to define the lumbar lordosis (LL) angle; markers ASIS and S2 together with a horizontal line were used to define pelvicinclination (PI).

2.4. Definition of Angles

The marker’s coordinates were used to determine two angles, which served to describe the sitting posture. The PI angle was defined as the sharp angle between the line connecting markers S2, SAIS, and the horizontal line (Figure 2). The LL angle was defined with markers L1, L3, and L5, as shown in Figure 2. The angle was considered negative in the case of kyphotic lumbar curvature.

2.5. Data Analysis

For each session measured, the researchers determined a power frequency spectrum (PFS) for the PI and LL time courses. The frequency range between 1/90 s⁻¹ and 3/5 s⁻¹ was then normalized. The median split PFS into two intervals so that the power in both was equal.

The researchers then determined the fraction of the power in four intervals: 1) 1/90–1/20 s⁻¹, 2) 1/20–1/10 s⁻¹, 3) 1/10–1/3 s⁻¹, and 4) 1/3–3/5 s⁻¹. Paired t-test statistics were used to compare groups between sitting on a LGB and sitting on a stool and between PI and LL.

3. Results

Altogether, 5,400 consequent values of the PI and LL angles were recorded for each session (Figure 3). The LL median of the PFS on the LGB occurs at higher frequencies (Figure 4), which means that sitting on a LGB is characterized by higher frequency postural dynamics (LL median: LGB = 0.17 s⁻¹ 0.03 s⁻¹, stool = 0.14 s⁻¹ 0.03 s⁻¹, p < 0.05). The difference of the PI median of the power frequency spectrum is not significant (PI median: LGB = 0.13 s⁻¹ 0.01 s⁻¹, stool = 0.13 s⁻¹ 0.01 s⁻¹, p > 0.05).

Considering LL, the fraction of the power (Figure 5) in the intervals of PFS is larger in the third interval when sitting on a LGB (LGB: 4.09 0.61, stool: 3.39 0.44, p < 0.05) and in the first interval when sitting on a stool (LGB: 1.83 0.61, stool: 2.67 0.81, p < 0.05). There is no significant difference in the second and fourth intervals and there is no significant difference in PI for any interval.

Figure 3. PI time course

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The PI angle during 30 minutes of sitting on a stool (Figure 3, top line) and on a LGB (Figure 3, bottom line) was measured with an acquisition rate of 3 s⁻¹. Note that there is less fluctuation on the stool.

Figure 4. Frequency spectra median

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Figure 4 shows that the median value of LL was found to be at significantly (*) higher frequencies when sitting on a LGB. There is no difference in PI.

Figure 5 shows the power of each interval with respect to a LGB and a stool. Intervals in units of s⁻¹ are 1) 1/90–1/20, 2) 1/20–1/10, 3) 1/10–1/3, and 4) 1/3–3/5. Note the statistically significant (*) difference in the first and third intervals.

4. Discussion

This study showed that while sitting on a LGB only the upper part of the body adopts a more dynamic posture (as indicated by the LL median), whereas the PI dynamics remained fairly similar whether sitting on a LGB or a stool.

Regarding the sway of the upper body, this study found that while sitting on a LGB there is more sway with a typical period of 5 s as compared to a stool, where more sway was found with a period of 33 s. These two periods represent a pattern of loading and unloading of tissues.

The nature of sitting on a LGB is that it requires maintenance of balance ^[14]. Posture balance maintenance resembles a standing or walking posture, in which balance must also be continuously maintained ^{[12, 15, 16]}. It is a matter of speculation whether the same mechanisms for balance maintenance are used in both cases.

Figure 5. Four intervals of LL angle power frequency spectra4. Discussion

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