Participants
This study involved 12 adolescents with mild to moderate mental retardation (BAL group)[1] in Qingdao Sanjiang special education school and 12 healthy peers (CON group) in Qingdao Laoshan No. 3 middle school. All participants’ health status and medical history were examined, and they were certified to have no cardiopulmonary disease. Both test subjects and their guardians were briefed on the research objectives and procedures. They also read and signed the informed consent form prior to the study. This study was approved by the Special Committee on Scientific Ethics of Ocean University of China (Ethics approval: OUC-HM-2021-012) (see Appendix A).
Study Design
Basic information such as participants’ age, weight, intelligence degree, and plantar pressure distribution were collected pre-intervention. After 12 weeks of Swiss ball intervention, the aforementioned indexes were collected again in the BAL group.
Intervention Programme
According to the previous Swiss Ball intervention programme[14], the Swiss ball exercise intervention for adolescents with ID was conducted for 12 weeks. The exercise was divided into three stages, four weeks in each stage, and three training sessions were conducted weekly for 40 minutes, including the warm-up part (5 minutes), the exercise part (30 minutes) and the cool-down part (5 minutes). Both the warm-up and the cool-down parts were performed (jogged) at the speed selected by each participant for five minutes on the treadmill. Swiss ball exercise intervention (see Appendix B) was divided into sitting exercise (No: 1–4), kneeling exercise (No: 5), and lying exercise (No: 6–8). The number of repetitions and groups increased gradually with the intervention duration. A one-minute interval was inserted between each set. According to the 6–20 Borg scale, exercise intensity is the level that subjects feel "A little hard-13". Subjects were informed to stop the exercise and rest if they feel dizzy or have any difficulty during the process (Appendix B)
Intelligence Assessment
Participants’ intelligence quotients (IQ) was evaluated using a pre-validated Wechsler Intelligence Scale test pre and post-exercise interventions[18–19]. Participants’ IQ measurements were categorised as normal, mild, and moderate if the IQ score ranged from 90 to 114, 50 to 70, and 35 to 49, respectively.
Experimental Instrument
The experimental setup included the Footscan 7 plantar force test system (RSscan international, Belgium), comprising a 2 m × 0.5 m plantar pressure plate, data acquisition box, data line, and analysis software. Four pressure sensors were built in the plantar pressure plate per square centimetre and the sampling frequency was 250 Hz. A high-speed camera (JVC gc-px10ac, China), with a camera height of 1.2 m, a frame rate of 50F/s, and a shutter speed of 1/200s was used in this study.
Gait Test
The subjects’ height and weight in the standing state were measured using the calibrated height ruler and weight scale. The plantar pressure test plate was installed in the middle of a 10 m long rubber sidewalk. In order to prevent the subject from being affected by the position of the plate[20], the sidewalk was covered with a top layer made of EVA material (hardness: shore a 70; thickness: 2 mm). The camera was located at an included angle of 90 ° in the walking direction and 5 m vertically away from the flat plate.
Participants walked barefoot through the plantar pressure plate in a natural gait. The test was repeated until three valid data are collected. Participants start walking 5 m in front of the slab. Upon walking through the slab, they walked for another 2 to 3 m. They were asked to familiarise themselves with gait tests at least twice before the formal test. The subjects’ spatiotemporal parameters during walking were extracted by the video analysis system (Simi motion V9.2), including complex stride length, stride time, and walking speed. The shorter and longer sides of the stride length were defined as the weak and strong sides, respectively[21].
Parameters
The plantar force test and analysis system were employed to extract the subjects’ pressure parameters, such as peak pressure and impulse. The whole plantar area was divided into 10 zones (Fig. 1): Toe 1 (T1), Toe 2 to Toe 5 (T2–T5), Metatarsal 1 (M1), Metatarsal 2 (M2), Metatarsal 3 (M3), Metatarsal 4 (M4), Metatarsal 5 (M5), Midfoot (MF), Heel medial (HM), and Heel lateral (HL).
In order to make the parameters comparable, some parameters were standardised as a percentage of body weight[22]. The calculation formula is as follows:
$$\text{R}\text{e}\text{l}\text{a}\text{t}\text{i}\text{v}\text{e} \text{p}\text{e}\text{a}\text{k} \text{p}\text{r}\text{e}\text{s}\text{s}\text{u}\text{r}\text{e}=\frac{ \text{P}\text{e}\text{a}\text{k} \text{p}\text{r}\text{e}\text{s}\text{s}\text{u}\text{r}\text{e}}{\text{W}\text{e}\text{i}\text{g}\text{h}\text{t}}$$
The following formula is used to calculate the symmetry index (SI) of gait variables[21] to evaluate the symmetry of gait.
$$\text{S}\text{y}\text{m}\text{m}\text{e}\text{t}\text{r}\text{y} \text{I}\text{n}\text{d}\text{e}\text{x}=\left|1-\frac{ \text{v}\text{a}\text{r}\text{i}\text{a}\text{b}\text{l}\text{e}\text{s} \text{i}\text{n} \text{s}\text{t}\text{r}\text{i}\text{d}\text{e} \text{l}\text{e}\text{n}\text{g}\text{t}\text{h} \text{s}\text{h}\text{o}\text{r}\text{t} \text{s}\text{i}\text{d}\text{e}}{\text{v}\text{a}\text{r}\text{i}\text{a}\text{b}\text{l}\text{e}\text{s} \text{i}\text{n} \text{s}\text{t}\text{r}\text{i}\text{d}\text{e} \text{l}\text{e}\text{n}\text{g}\text{t}\text{h} \text{l}\text{o}\text{n}\text{g}\text{e}\text{r} \text{s}\text{i}\text{d}\text{e}}\right|$$
Statistical analysis
All the data were analysed using the Statistical Package for Social Science (IBM SPSS, Version 23.0). The data were summarised and expressed as mean ± standard deviation. Paired sample t-test was used to compare the gait symmetry index, gait temporal and spatial parameters, peak plantar pressure, and plantar impulse between adolescents with ID and normal peers pre and post-balance training. Statistical significant differences were considered present for P-values less than 0.05.