The Effects of Cross-Legged Sitting on the Lower Limb Muscles and Body Balance and the Implications in Rehabilitation

Abstract

Background: Although a cross-legged sitting (CLS) posture has been commonly practiced as a daily activity, particularly in Arabic, Middle Eastern, and Asian societies, there is no medical study focusing on the effects of cross-legged sitting on body balance and muscular strength. Therefore, this study aimed to investigate the effect of CLS on lower extremity muscular strength, muscular electrical activity, and body balance. Methods: Thirty healthy volunteers participated in this research study by performing CLS for a 20 min duration. The balance tests included a static test, i.e., a single-leg-standing posture with eyes closed, to assess if the centre of the pelvis and centre of the shoulders (CoS) moved, and a dynamic test, i.e., four-square-returning, to assess if the moving speed changed. Regarding the muscular assessment, the electrical activity was assessed depending on the maximal value of activation and rooted mean of squared values, while the muscular strength was assessed according to the maximum force by the lower limbs using a force sensor. The balance and muscular results were statistically compared before and after CLS. Results: The duration of the static balance after CLS decreased by an average of 2.5 s, or approximately 15.64%, compared to before CLS (p < 0.05 *). Further, the Centre of Pelvis moved greater distances in the medial–lateral direction after CLS compared to before, but CoS was not significantly changed in the static balance test. However, in the dynamic balance test, the duration significantly decreased by 0.2 s, or approximately 8.5%, after CLS compared to before, meaning that dynamic balance ability improved. Considering the muscle results, only the lateral gastrocnemius muscle was noticeably electrically activated after CLS, while the hip extensor and knee flexor muscles became significantly stronger after CLS compared to before, roughly by about 14%, and the ankle plantar flexor maximum force increased noticeably, by about 4%, after CLS. Conclusions: CLS had a positive impact on the dynamic balance; the strength of the hip extensor, knee flexor, and ankle plantar flexion; and all lower limb muscles, in terms of electrical stimulation, except for the lateral gastrocnemius post-CLS compared to pre-CLS. Therefore, CLS can be safely included in one’s daily routine and in any rehabilitation programme, except for patients who are suffering from static balance disturbance. Although this posture is commonly used in many societies, because this is the first study focused on the impact of CLS on body balance and muscular status, the results would supply knowledge and new understanding, as well as provide clear insight for sitting posture research.

1. Introduction

Although any prolonged sitting position may be responsible for postural deformities, such as abnormal spinal curvatures, trying to maintain a balanced and comfortable sitting posture may play a vital role in compensating for the negative side effects and securing several body parts during activities of daily living [1,2]. This is due to the significance of balanced sitting, which is considered either as an assessment tool to measure the patient’s coordination or as a treatment goal by increasing the sitting base of support [3]. In one’s daily habits and tasks, the routine sitting position, in a way by which the functional independence might be achieved, can be considered an ideal activity in physical therapy [4]. This point should be taken into account, as sitting consumes more than a third of peoples’ daily lives and more than half of their daily activities during working time or leisure time [5,6,7,8]. Therefore, the improper sitting position might affect the muscular condition negatively, as it might excessively overload the work of muscles, which may lead to hip joint inactivity, thus altering the general postural alignment [9,10]. For instance, computerised occupations that require sitting for a long time in front of technological machines might require the employee to sit forward-leaning, with some degree of neck fixation, to be more concentrated on the computer screens. This might greatly lead to some musculoskeletal problems, like disc prolapse or herniation at the cervical vertebrae, that have a large impact on the quality of daily life [11].

To achieve a good, balanced body condition, which is a basic task for locomotion and communication, it is important to keep normal spinal and pelvic alignments in the sitting position. For instance, a sit-to-stand task can be considered a basic task for performing all activities of daily living, and it needs to be conducted in a specific arrangement of alternative balance degrees to avoid any injuries or abnormal pressure distributions. This is because it includes a combination of several body postures, like bending, lowering down, sitting, raising the hands, standing, and locomotion [12]. Therefore, although a long sitting duration can affect body balance negatively, trying to sit in a healthy posture while making sure the pelvis and trunk form the same line, without any abnormal curvatures, and continuing the sit-to-stand tasks during prolonged static positions, such as sitting, could help in avoiding or at least decreasing the negative side effects [5].

The crossed-leg sitting (CLS) posture is habitually used, particularly in Arabic, Middle Eastern, and Asian societies. However, it is not clear whether CLS for a long duration would affect the lower limb muscles and the balance ability of the body. There is no previous study on this topic. Therefore, this study aimed to investigate the effect of CLS on the lower extremities’ muscles according to electrical activity and muscular strength, as well as assessing its impact on static and dynamic body balance. The research hypotheses were that CLS could overload the lower limb muscles and alter body balance status by comparing the parameters in the following two situations:

• before CLS;
• after 20 min of CLS.

We hope this study will contribute to the knowledge and add a new understanding of sitting posture, as well as give a clear insight into sitting posture research. While this posture is commonly used as a daily habit in many societies, as there is little research analysing the impact of CLS on body balance and muscular status, the results of this study can be considered as a supplement to this new field.

After the Section 1, this article will be structured as follows:

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Section 2 presents the Materials and Methods, including participants, inclusion and exclusion criteria, the equipment used in data collection, how the data were collected, and what statistical method was used to compare two sets of data before and after CLS.

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Section 3 presents the results of the comparison between static balance and dynamic balance data before and after CLS, with the results assessed according to gender.

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Section 4 discusses the findings of the current study based on comparing the current study results to the previous studies that focused on different healthy sitting postures.

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Section 5 points out the limitations in the current study.

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Section 6 suggests some recommendations for future studies.

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Section 7 describes the clinical relevance of the current research and how the results of this study can be considered as a supplement to this new field.

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Section 8 offers a conclusion.

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The last section is the list of references in Harvard style.

2. Materials and Methods

This study is considered to be a longitudinal study, which involves repeated tests of the same people over a short period of time. A longitudinal study is usually used to study and monitor rapid fluctuations in behaviours from moment to moment in the same people. This type of study allows medical researchers to distinguish between short phenomena among fewer than 100 participants [13,14,15,16].

This analysis was conducted according to the maximum value of activation and rooted mean of squared values (RMS) for calculating the muscular electrical activity, the maximum force to estimate the muscular strength, the Centre of Pelvis and Centre of Shoulders for static balance, and only according to the duration of the dynamic balance.

The data were collected in the Motion & Gait Analysis Laboratory at the Tayside Orthopaedic and Rehabilitation Technology (TORT) Centre, Ninewells Hospital & Medical School, during the periods of September 2021 and September 2022. The data collection process commenced after obtaining ethical approval from the School of Medicine and Life Sciences Research Ethics Committee at the University of Dundee (SMED REC Number 21/74) on the 6th of September 2021.

Before commencing with data collection, a Participant Information Sheet (PIS) was read, a consent form was signed by the participants, and the study protocol was explained in detail to each volunteer who agreed to participate in this study. Each participant was informed that all equipment and techniques are used routinely in the laboratory, with no risks or side-effects to participants

Generally, 90 min were taken for a single session of data collection, including 5 static and 5 dynamic balance trials, and 5 walking trials before and after CLS, respectively. The analysis of walking trials was carried out in another paper [17].

2.1. Subject Data

The sample size included 30 healthy adults (15 male and 15 female) with an age range of 18–40 years. The inclusion criteria included any participant who could walk unaided and perform daily life activities independently without any prior surgeries or postural deformities that could affect lower limb ability. The exclusion criterium was any participant suffering from an abnormal spinal curvature, such as Kyphosis, Lordosis, or Scoliosis.

Disabled, obese, and pregnant individuals, as well as any volunteers who suffered from any deformities or bone fractures, or those who were unable to walk without orthoses or prosthetic limbs, were also excluded from the study sample. In addition, any subject with any musculoskeletal disease or neuropathy that could affect general body performance, particularly of the lower limbs, was excluded.

2.2. Laboratory Equipment

2.2.1. Vicon^® Nexus Motion Capture System

Vicon^® Nexus Motion Capture system (Vicon Motion System Ltd., Oxford, UK) is a device consisting of 15 infrared digital cameras which connect to Nexus software version 2.12.0 and are conducted within the capture volume area in order to capture data at a frequency of 200 Hz. Each camera has an optical filtration tool with a strobe head unit used to capture the movement of markers on the trunk during dynamic balance testing.

2.2.2. Force Plates

Four force plates (Advanced Mechanical Technology, Inc., Watertown, MA, USA, AMTI^®, BP 600 mm × 400 mm) connected to the Vicon and were used to collect the ground reaction force (GRF) in all three planes simultaneously, with a frequency of 1000 Hz. The force plate data were used to determine balance durations.

2.2.3. Electromyography

Electromyography (Delsys^® Inc., Natick, MA, USA) is a wireless system (Delsys Trigno TM Wireless system) was used to calculate the muscular electrical activities using wireless electrodes that were adhered to the skin of the participant’s dominant leg with double-sided adhesive tape. The electrodes can collect the muscular electrical activity if they are put on the muscular sensor location following the European Biomedical Health and Research Program instructions, called SENIAM (Surface Electromyography for the Non-Invasive Assessment of Muscles) [13]. The Vicon, AMTI, and Delsys systems collected the data synchronously.

2.2.4. Force Sensor

The force sensor (Interface Force Measurements Ltd.) is an aluminium and stainless-steel device containing 3 axes connected with 6 channels which can be used to measure the muscular strength of any of the body segments by calculating the forces and moments in all directions. Force sensor (Model 6A68D, Scottsdale, AZ, USA) specifications and its components are seen on the website of Interface Force Measurement Solution [18]. The device was validated by a previous study [19]. As the device is portable, it was installed on the wall and chair with a base so that participants can perform different postures to test muscle strength in the lower limbs [19]. Though there were other wearable devices reported [20,21], those instruments are not suitable for this study.

2.3. Procedure

2.3.1. Anthropometric Measures

Anthropometric parameters, including knee and ankle width, and leg length for both the right and left sides, were measured. In addition, the distance between the right and left anterior superior iliac bony prominences, body mass, height, and some general information, such as gender and age, were collected.

2.3.2. T-Pose

The participant stood on the force plate whilst maintaining the abduction position, with the arms and the legs slightly separated. The T-pose is necessary to confirm that all adhered markers are visible with the Vicon^® Nexus version 2.12.0 (Figure 1).

2.3.3. Static and Dynamic Balance

Retro-reflective markers used to collect the data from the Vicon^® 3D motion capture system (Vicon, Oxford, UK) were adhered to the skin surface of specific bony prominences of the body using double-sided adhesive medical tape following the Vicon^® Plug-in-Gait model. A set of markers was applied to the participants’ bodies to test balance, i.e., the moving of the Centre of Shoulders and Centre of Pelvis. According to the requirements of the project, markers were placed on the following bony landmarks:

• To obtain the Centre of Shoulders: two markers were placed on the right and left shoulder bony prominences.
• To obtain the Centre of Pelvis: four markers were placed on the anterior and posterior superior iliac spinae for both the right and left sides.

The Centre of Pelvis was defined as the centre of four markers on the pelvis and was used to approximate the Centre of Mass of the whole body. The Centre of Shoulders was defined as the middle point between two shoulder markers and was used to assess movement in the upper trunk.

The test used to assess the static balance was the single-leg stance test with eyes closed, while the four-square step test was used to assess dynamic balance [22]. Participants were required to stand on their dominant leg with their eyes closed for as long as possible to test static balance. In addition, the participants were required to walk for four steps in two opposite directions (clockwise and return) and complete the sequences as quickly as possible. Five trials were collected before and after CLS, using the Vicon^® 3D motion capture system with four force plates. Ten trials were the total captured trials of each static and dynamic balance test for each volunteer.

The collection of data is shown in Figure 2.

The static balance test was performed using the dominant leg. The dominant side was determined by asking the participant which leg is preferred to stand on or which hand they used when writing. During data processing, the start and finish events were determined by the non-dominant foot off and on the ground during the static balance test. For example, if the participant preferred standing on the right leg as the dominant side, then the first foot strike (the last moment of touching the ground), foot-off, and the second foot strike (the first moment of touching the ground again) were collected from the left leg. For the static balance test, the time between the first and last heel strikes on the force platforms was used to calculate the balance duration, as shown in Figure 3.

For the dynamic balance trials, right and left contexts (which are called clockwise and return events) were labelled for all trials, with events including first foot touch, foot-off, and second foot touch in each. The full sequence of walking along the force plates is the main aspect of assessing the dynamic balance by calculating the time of each context, either the right or left leg. For the right leg (clockwise cycle), the context was labelled depending on the full sequence, from the first touch of the second force plate, foot-off until the first strike of the last force plate (the first force plate), which was called the clockwise cycle, while the left leg cycle (return cycle) was from the first touch of the fourth force plate in the opposite direction, foot-off until the first strike of the last force plate (the first force plate), as shown in Figure 4.

Currently, there are many different tests available. The two selected tests are commonly used. These tests are easy to perform and reflect balance ability very well. It is also recognised that Centre of Pelvis is a reasonable approximate of the CoM. In addition, the marker set is compatible with gait analysis, which was another aim of the project.

2.4. Muscular Electrical Activity

Electromyography (EMG) electrodes were attached to the dominant leg skin on the muscular placement points after shaving the target areas and applying NuPrep gel to enhance the electrical stimulation and connection. The target muscles and their electrode positions were referred to as follows [13]:

• The rectus femoris, which is responsible for hip joint flexion and knee joint extension. The electrode was placed 50% of the distance between the anterior superior iliac spine (ASIS) and the proximal part of the patella.
• The biceps femoris, which is mainly used for flexion and lateral rotation of the knee joint and acts as assistance in the extension and lateral rotation of the hip joint. The electrode was placed 50% of the distance between the ischial tuberosity and the lateral epicondyle of the tibia.
• The vastus medialis, which is responsible for knee joint extension. The electrode was placed 80% perpendicular on the line between the anterior superior iliac spine and the anterior medial ligament of the knee.
• The vastus lateralis, which is responsible for the knee joint extension. The electrode was placed 2/3 on the line between the anterior superior iliac spine and anterior lateral patella in the same direction as the muscle fibres.
• The medial and lateral gastrocnemius, whose main function is the ankle joint flexion, and they act as assistant muscles in the flexion of the knee joint. Electrodes should be placed on the lateral muscle bulge of the posterior leg at 1/3 of the distance from the fibula’s head to the heel.

Then, the subject was asked to walk along the walkway at their normal walking speed to avoid any subconsciousness or alteration that could happen in the measured parameters. The participants were asked to walk for at least five trials, two times, before and after CLS, to collect the muscular electrical activity of the gait by using the electromyography channels and electrodes.

2.4.1. Muscular Strength

Regarding the muscular strength data, they should be collected using the force sensor instrument by pushing the force sensor device, which was fixed on the wall, in all sagittal plane movements of all lower limb joints, as much as possible. The collection of three trials for each muscle group was the main aspect of muscle strength data collection. During each trial, the foot pushes with a reset to zero in between was the basic step to collect the data properly. Given that the muscular strength measured by the force sensor has been validated [17], it must be pointed out that all body segments, including upper and lower extremities, may participate in the measured movement and cause some reading interference.

To illustrate this, the method of measurement for muscular strength has been clarified in the following steps [19]:

1-

To measure the hip muscle strength, the participant was required to be in a comfortable standing position on a wooden step, and then to

• Push the force sensor device by the toes of their dominant leg forward and upward (standing while trying to keep the lower limbs as straight as possible while the force sensor was in front of the participant) as much as they can with an extended knee to collect the strength data of the hip flexor muscle group (Figure 5A).
• Push the force sensor device backward and upward by the heel of the dominant leg (standing with extended hip and knee joints while the force sensor was behind the participant) as much as they can with an extended knee to collect the strength data of the hip extensor muscle group (Figure 5B).

2-

To measure the knee muscle strength, the participant was required to be in the sitting position on a chair that is put on a wooden step to keep the hip and knee in 90-degree flexion and to support the ankle on the step simultaneously. Then, the participant should try to

• Push the force sensor device by the foot of the dominant leg forward and upward as much as they can while being in a sitting position, with the force sensor in front of the participant to collect the strength data of the knee extensor muscle group (Figure 6A).
• Push the force sensor device by the foot of the dominant leg backward and upward as much as they can while being in a sitting position, with the force sensor under the chair to collect the strength data of the knee flexor muscle group (Figure 6B).

3-

To measure the ankle muscle strength, the participant was required to be in a comfortable or natural sitting position on a chair while keeping the hip and knee in 90-degree flexion and the ankle in the neutral position; the heel should be maintained on another higher wooden step (sitting while the force sensor is in front of the participant). Then, the participant tried to

• Push the force sensor device by the forefoot or toes of the dominant leg upward to the maximum level of the participant’s ability to collect the strength data of the ankle dorsiflexor muscle group (Figure 7A).
• Push the force sensor device by the forefoot or toe of the dominant leg downwards to the maximum level to collect the strength data of the ankle plantar flexor muscle group (ankle extensor muscle group) (Figure 7B).

2.4.2. Cross-Legged Sitting

In the CLS position for 20 min, no consideration was taken regarding which leg was dominant or which leg should be on the top or bottom. Participants were required to perform the CLS position naturally on the carpeted floor (Figure 8).

2.5. Data Collection

The total number of collected trials are as follows:

• 300 static trials for each balance test were collected from 30 volunteers (150 before CLS and 150 after).
• 300 dynamic trials for each balance test were collected from 30 volunteers (150 before CLS and 150 after).
• 300 trials of walking were the total number of collected trials for 30 participants (150 before CLS and 150 after), which was reported previously [17].
• 1080 trials were the total number of trials collected from the force sensor for 30 participants to calculate the muscular strength, 540 trials collected before CLS, and 540 after. The total number of muscular strength trials that should be collected for each participant is 36; 18 trials for the sagittal muscle group were collected before CLS, and 18 after.

2.6. Data Analysis

The demographic descriptive statistic variables were analysed using Microsoft Excel before obtaining the basic results. Gender, body mass index (${\mathrm{k}\mathrm{g}/\mathrm{m}}^2$), height (cm), and age (years) were included as demographics for the analysed parameters.

The following variables were analysed using SPSS^® version 28 (SPSS^® Inc., Chicago, IL, USA):

• Static balance.
  • Duration (s), which is the time between the last touch moment of the first foot strike to the first touch moment of the last foot strike.
  • The RoM of the Centre of Shoulders in anterior/posterior, medial/lateral, and vertical directions (mm)
  • The RoM of the Centre of Pelvis in anterior/posterior, medial/lateral, and vertical directions (mm)

Out of the 300 collected static balance trials, only 60 trials (30 before CLS and 30 after) were used in the data analysis process after selecting the longest trials for each participant. Two of the longest static trials for each participant were selected, one before CLS and the other one after, after calculating the time between the first and last heel strikes for all trials and then detecting which one was the longest in each situation.

2.

Dynamic balance.

• Duration (s): For the right leg (clockwise cycle), the time was calculated depending on the full sequence that was labelled from the first touch on the second force plate, foot-off, until the first strike of the last force plate (the first force plate). For the left leg cycle (return cycle), the time was calculated from the first touch of the fourth force plate in the opposite direction, foot-off, until the first strike of the last force plate (the first force plate).

In all, 296 trials (out of 300 collected trials) were used in the data analysis process (148 before CLS and 148 after). Some trials were excluded from the data analysis process due to having gaps in the markers or missing force plate data at specific moments.

3.

The muscular electrical activity (the rectus femoris, biceps femoris, vastus medialis, vastus lateralis, gastrocnemius medialis, and gastrocnemius lateralis). EMG data were filtered by 5–500 Hz and normalised using the baseline of the EMG signals for each muscle. The baseline was defined as the absolute mean of 40% of the lowest EMG in the cycle of gait. This method allowed us to compare the EMG data among individuals. Then, the two key values from the normalised EMG were calculated as shown below.

• Maximum value of activation in a 24 ms time window.
• Rooted mean of squared values,

$$RMS=\sqrt{{\displaystyle \frac{\sum _{i=1}^n{s}_i^2}{n}}}$$

(1)

where s is the signal and n is the number of signals in a trail. The RMS was calculated for the whole gait cycle.

4.

The muscular strength for the following muscles (hip flexors and extensors force, knee flexors and extensors force, and ankle dorsiflexors and plantar flexors force)

• The maximum force values (N).

5.

The difference between the two groups of data (before and after 20 min of CLS) according to gender.

2.7. Statistical Analysis

Statistical analysis was performed using SPSS^® version 28 (SPSS^® Inc., Chicago, IL, USA) software. The static balance data were analysed using a t-test and non-parametric test, depending on whether the variables were normally distributed. For the dynamic balance trials, muscle electrical activity and muscular strength, a general linear model for repeated measures, were used. This model allows for multi-way comparison, including gender and BMI as interactive factors. Thus, gender was used as the between-subject factor and body mass index was used as the covariate, while the main factor to be compared was the within-subject variable, i.e., before and after CLS. This method allowed for analysis of the within-subject, between-subject, and interactive factors together. This method not only tested the difference between before and after CLS situations, but also highlighted the differences among the selected factors. Results were reported by using the estimated means and standard error of means (SEM) with the significance level from SPSS to Excel, which produced the resultant graphs. The adjustment option was Bonferroni. The significance level was p < 0.05 (p-value) between the two groups of data, which was reasoned as a significant difference if p ≤ 0.05, symbolised as *; as a high confidence in the difference if p ≤ 0.01, symbolised as **; and as an extremely high confidence in the difference if p ≤ 0.001, symbolised as ***. However, if the p > 0.05, there was no significant difference.

2.8. Power Analysis

As this is a new study and no previous data are currently available, we checked whether the sample size was large enough by doing a posterior power analysis. Given that β is 0.2, i.e., power = 1-β = 0.8 or 80%, α = 0.05, and the clinical difference is 20 mm in the range of motion of the Centre of Pelvis in the medial–lateral direction, with an approximate standard deviation of 40 mm from the collected data, the sample size was 31 [14]. Therefore, although this study used a reasonable sample size of 30, it is still considered to be a pilot study.

3. Results

3.1. Demography and Gait Parameters

The mean of the demographic measures was as follows: age 26.8 years, height 167 cm, body mass 70.42 kg, and body mass index (BMI) 25.06 ${\mathrm{k}\mathrm{g}/\mathrm{m}}^2$, as shown in

Table 1. Demographic parameters.

	Mean Std. Error		Minimum	Maximum
Gender (Male/Female)	30 (15/15)	__	__	__
Body mass (kg)	70.42	3.52	42.40	123
Height (cm)	167	1.54	150	185
$\mathrm{BMI} ({\mathrm{k}\mathrm{g}/\mathrm{m}}^2$)	25.06	1.04	16.77	38.4
Age (years)	26.86	0.86	20	39

.

3.2. Static Balance Results

3.2.1. Duration in Static Test

A longer static balance time can be considered as a sign of better balance. However, CLS affects the static balance negatively, as the balance time decreased significantly, with an average of 2.5 s and 15.64% compared to before (p ≤ 0.05 *), as shown in Figure 9.

3.2.2. Centre of Pelvis

Although the static balance results could be intuitively obvious due to the significant difference in the balance duration, analysing the Centre of Pelvis may provide an evidence-based statement of the outcomes.

Considering the opposite relationship between the balance Centre of Pelvis and balance efficiency, increasing the range of motion in the Centre of Pelvis could be proven as a negative character of the static balance. Therefore, CLS has a negative impact on the Centre of Pelvis in the medial/lateral direction (Y-direction), as it increased noticeably compared to before, with a p-value < 0.050 *, as shown in Figure 10.

3.2.3. Centre of Shoulders (CoS)

There was no significant difference between the static balance before and after CLS according to the range of motion in the Centre of Shoulders in all of the anterior/posterior, medial/lateral, and vertical directions (p > 0.05) as Figure 11.

3.2.4. Gender Comparison in Static Balance

According to the gender comparison of the significantly changed parameters, the static balance duration decreased noticeably after CLS among both genders among females and among males (Figure 12 and Figure 13). However, there was no significant difference between the static balance before and after CLS in terms of the Centre of Pelvis and Centre of Shoulders in all directions, except for females in the Y-direction of the Centre of Pelvis, as shown in Figure 12 and Figure 13.

3.3. Dynamic Balance Results

3.3.1. Duration

Depending on the dynamic balance assessment, the fastest trial had the best balance. Therefore, the significant decline in the dynamic balance duration after CLS compared to before, in both the clockwise and return directions, can be considered a sign of a positive effect of CLS on dynamic balance. The dynamic balance duration decreased at an average of 0.2 s, or approximately 8.5%, after CLS in both the clockwise and return directions compared to before, as highlighted in Figure 14.

3.3.2. Gender Comparison

The dynamic balance duration had a significant decrease after CLS among both females and males in all clockwise and return directions (p ≤ 0.001 ***), as shown in Figure 15.

3.4. Electromyography Results

3.4.1. Maximal EMG Activation (MAX)

The maximal activation percentage for all of the measured muscles (Rectus femoris, Biceps femoris, Vastus medialis, Vastus lateralis, Gastrocnemius medialis, and Gastrocnemius lateralis) increased on the dominant leg during walking after 20 min of CLS compared to before. However, there is no significant difference between the gait before and after CLS according to the maximal activation percentage in all muscles, except for the gastrocnemius lateralis muscle, which was activated by 6.4% more after CLS than before (p ≤ 0.05 *), as shown in Figure 16.

3.4.2. Root Mean Square (RMS)

According to RMS, only the gastrocnemius lateralis muscle increased significantly, at an average of 1.7% after CLS, which is more than before (p ≤ 0.01 **), as shown in Figure 17.

3.4.3. Gender Comparison

Although only the gastrocnemius lateralis was activated significantly after CLS compared to before, according to the maximal activation percentage and RMS, there is no significant difference between walking before and after CLS in all muscles depending on gender, except for the vastus lateralis MAX and RMS among females (p ≤ 0.05 * and p ≤ 0.01 **, respectively), as shown in Figure 18 and Figure 19.

3.5. Muscular Strength Results

3.5.1. Maximum Force

According to the maximum force of the lower limb sagittal muscles that was measured using the force sensor, the force values of the hip flexors, knee extensors, and ankle dorsiflexion muscles were not significantly changed after CLS compared to before (p > 0.05); conversely, after CLS there was a significant increase in the maximum force values of the hip extensors, knee flexors, and ankle plantar flexion muscles, at roughly about 14% in both the hip and knee muscles and 4% in the ankle (Figure 20).

3.5.2. Gender Comparison

Depending on the muscles that represented a significant increase after CLS, only females achieved a stronger hip extensor and knee flexor after CLS compared to before, with the p-value equalling p ≤ 0.001 *** on the hip extensors and p ≤ 0.001 *** on the knee flexors. In contrast, there is no significant difference according to the gender comparison in the ankle plantar flexor muscle, as shown in Figure 21.

4. Discussion

To the best of our knowledge, this study is the first one focused on the impacts of CLS on body balance and lower limb muscles; we cannot find any previous studies to compare it to. Therefore, the discussion was written by comparing the current study results to previous studies that focused on different healthy sitting postures, such as yoga, unsupported sitting, slumped or relaxed sitting, and forward- or backward-leaning sitting, and how body balance and lower limb muscles are affected by each position.

4.1. Balance Discussion

The positive impact of CLS on dynamic balance is consistent with what Zettergren et al. (2011) illustrated in terms of the effect of yoga posture on dynamic balance using the four-square step test; the same test is used in the current study. However, static balance was affected negatively after CLS, depending on the balance duration and Centre of Pelvis. Therefore, the current study results are compatible with the previously mentioned study on dynamic balance, while the opposite is true of the static balance results [10].

The results demonstrated by Sohrabi et al. (2020) regarding the effects of yoga exercises on balance match those of the current study, as both yoga and CLS can maintain dynamic balance by decreasing the duration recorded after both were compared to the balance before the test, and can thus protect normal postural alignment [11]. This could be a result of positively increasing the base of support during the positions that affect all daily activity by preserving a good balance status [3,8,23].

When comparing our results to Darwish et al. (2019), it must be noted that only static balance was affected negatively, as the participants’ ability to preserve body balance while standing on only one foot with their eyes closed decreased noticeably when comparing the situation after 20 min of CLS to before CLS. However, dynamic balance efficiency was better after CLS than before [12]. Unlike the current study results, the increase in the internal/external rotation at the hip joint was the main cause of balance disturbance analysed by Freddolini et al. (2014) during only 1 s of balance maintenance after the unsupported sitting posture [24].

However, when comparing our results to Wang et al. (2016), it must be highlighted that yoga differs from CLS in terms of its effect on balance status. Wang et al. (2016) reported that balance does not change significantly after the yoga position, while in the current study it appears that CLS has a positive impact on dynamic balance and an adverse impact on static balance [9].

This result ties in well with those of Zhou et al. (2020) and Sun et al. (2020), regarding the effect of the slumped or relaxed sitting position on balance status, wherein the centre of gravity (CoG) was shifted anteriorly, leading to balance disturbance as a result of increasing C-curvature of the spine, including thoracic kyphosis, the trunk tilting backwards, and an increasing pelvic leaning angle with some degree of paraspinal muscle overloading [5,25]. Similarly to the current study results, Waongenngarm et al. (2015) summarised that 20 min of a forward-leaning sitting position or an unsupported sitting position—such as sitting without back supporting—can affect body balance negatively due to these postures shifting the centre of gravity anteriorly and increasing the level of discomfort, particularly at the hip joint [26]. Another similarity between the aforementioned studies and the current one is that the Centre of Pelvis increased significantly during static balance after CLS in the medial/lateral direction, and this change can be considered a sign of the negative impact of CLS on static balance.

In-line with the previous study that was conducted by Hey et al. (2017) regarding the effectiveness of the slumped or relaxed sitting position for a period of 5–12 h, it is summarised that CLS influences could be similar to what occurred after the slumped sitting position in terms of disturbing static balance due to changing the spinal posture to compensate for an excessive weakness that might take place in the back muscles [1].

Considering the healthy sitting terms and conditions, proper balance can be an important sign that can help to determine a proper or healthy posture. The dynamic balance findings are directly in-line with other study findings which summarised that proper balance can be considered the main sign of increasing the base of support and comfort level. We can therefore consider CLS as a suitable technique to improve dynamic balance efficiency; however, it should be avoided for any patients suffering from static balance disturbances [27,28].

4.2. EMG Discussion

CLS can be considered a healthy position for all measured muscles, except the gastrocnemius lateralis, according on what Schult et al. (2013) and Hallman et al. (2016) estimated in terms of muscular electrical activity. To clarify, Schult et al. (2013) and Hallman et al. (2016) pointed out that increasing muscular electrical activity can be considered a sign of muscular overloading, which can consequently lead to muscular problems [3,29]. In addition, this is consistent with the study reported by Martin et al. (2014) regarding the characteristics of the unhealthy sitting position, in which the gastrocnemius muscles overloading can be considered the main tool to determine improper sitting [30].

When comparing our results to the study conducted by Waters and Dick (2014) regarding the prolonged improper sitting characteristics and what was discussed by Hofmann et al. (2016) about the hazards of the backwards-leaning sitting position in increasing discomfort level, CLS had a negative effect only on the posterior leg muscle, which is responsible for the flexion of the ankle and knee joints due to the excessive overloading of gastrocnemius lateralis muscle. Contrastingly, CLS can affect the rectus femoris, biceps femoris, vastus medialis and lateralis, and medial gastrocnemius positively, as there is no significant change found [31,32].

Contrary to the findings of Lee and Yoo (2011), Snijders et al. (2006), and Jung et al. (2020) regarding the effect of sitting with one leg over another on muscular status, we did not find any excessive electrical activation for all lower limb muscles, except the lateral portion of the gastrocnemius muscle. Therefore, only gastrocnemius lateralis may be supposed to have a weakness that results from the excessive elongation, which might lead to asymmetrical knee and ankle flexion angles between the right and left sides [2,6,33].

4.3. Muscle Strength Discussion

A similar pattern of results was obtained in the current study to those found by Wang et al. (2016) and Zettergren et al. (2011) regarding the effect of yoga posture on lower limb muscular strength. To clarify, increasing the lower limb strength, particularly the hip extensor, knee flexor, and ankle plantar flexor, after CLS compared to before can be considered one of the similarities between the effect of CLS and yoga posture on lower limb muscles [9,10].

A similar conclusion was reached by Martin et al. (2014), Waclawski et al. (2015), and Merry et al. (2021) regarding the effectiveness of the unhealthy sitting position on muscular strength due to the weakness of all lower limb muscles, particularly the knee flexors and ankle plantar flexors, which might increase the discomfort and pain level. Therefore, in-line with the idea of muscular weakness, it can be concluded that CLS is a healthy sitting position due to it increasing the muscular strength of the lower extremities [30,34,35].

Although there is a significant increase in hip extensor strength after CLS compared to before, this increase might match what Waongenngarm et al. (2015) demonstrated regarding the effect of an improper sitting position on the lower limb muscles. In detail, Waongenngarm et al. (2015) concluded that weakness of the hip muscles can be considered a sign of unhealthy sitting because the hip joint is the most susceptible joint to muscular problems due to decreasing blood flow [26].

According to the healthy sitting terms and conditions that were documented by Liu et al. (2016) and Waongenngarm et al. (2015), good muscular status, particularly of the hip extensor and knee flexor, which occurred after CLS, can play a significant role in considering CLS a healthy posture for the lower limb muscles [26,36].

5. Limitation

This study has a few of limitations, including the lack of different age groups and real patients. Another limitation is the small sample size; therefore, this study could be considered a pilot study, indicating that a further study with a larger sample size should be considered in the future. Moreover, one of the current study limitations is using only the single-leg stance test with eyes closed to assess static balance and only the four-square step test to assess dynamic balance. Further, this study has not assessed longer periods of CLS, e.g., more than 1 h, which is usually used in many countries. In addition, this study did not compare CLS with other sitting positions (control group) and thus cannot state which sitting position is better in terms of balance.

In addition, one of the current study limitations is using only sagittal plane muscles, which are responsible for flexion and extension of the lower limb to assess the effect of CLS on the muscular strength of the lower extremities. Future studies are recommended to add new understanding to the knowledge of this field.

Regarding EMG processing, the potential noise in EMG measurements might have affected data quality, and thus the noise should be reduced through advanced filtering techniques, such as wavelet denoising in the future.

6. Future Studies

Future studies should be conducted by applying the same procedure on patient groups, and also considering several age groups, such as elderly people, children, and adults, to assess CLS’s broader applicability. These studies would assess the effect of CLS on the balance condition with clearer understanding.

Furthermore, it is necessary to consider the daily life routine in future work to compare those who used CLS occasionally to those who use CLS long-term, looking into the lower limb muscular strength, muscular electrical activities, and balance analysis. Moreover, assessing the effects of CLS over longer durations (e.g., 1 h or more) would provide a clearer understanding of its long-term impact.

In addition, future studies might be necessary to widen the understanding of how CLS can affect the muscular strength in the front and transverse planes, and of how it can clearly impact the static and dynamic balance. Therefore, evaluating muscles responsible for abduction, adduction, and rotation would offer a more comprehensive understanding of CLS’s impact on lower limb function.

Regarding the balance tests, incorporating additional static balance tests, such as the Romberg test or balance platform assessments, would strengthen the study’s conclusions. In addition, including a control group engaging in different sitting postures (e.g., chair sitting or kneeling) would comtextualise CLS’s effects and provide a benchmark for its benefits and drawbacks.

7. Clinical Relevance

As a result of not analysing the impact of CLS on body balance and lower limb muscular status, while this posture is commonly used as a daily habit in many societies, the results of this study can be considered as supplemental to this new field. CLS can be considered a reasonable position and can be included in common programmes to improve the dynamic balance and the muscular strength that is responsible for hip extension, knee flexion, and ankle plantar flexion, and to increase the range of motion accordingly. However, CLS should be avoided for any patients suffering from static balance disturbances.

8. Conclusions

CLS’s impact was analysed by comparing the following factors before and after 20 min of CLS among 30 healthy participants: muscular electrical activity; muscle strength; and balance status (static and dynamic).

• Balance: Decreasing the duration of both the static and dynamic balance can be considered as a negative side effect of CLS for static balance, while this decrease may be viewed as a positive impact for dynamic balance. In addition, the COP in the medial/lateral direction displayed a significant increase post-CLS compared to pre-CLS. Thus, increasing the COP in any direction can be considered another negative effect of CLS on the static balance.
• EMG: regarding the lower limbs’ muscular status, only the lateral gastrocnemius muscle was electrically activated after CLS, with a significant difference compared to before.
• Limb strength: the hip extensor; knee flexor; and ankle plantar flexor became stronger when assessed using the force sensor after CLS compared to before.

Generally, 20 min of CLS had a positive impact on dynamic balance, while static balance was affected negatively post-CLS compared to pre-CLS. In addition, CLS has a positive influence in terms of electrical stimulation on all lower limb muscles, except for the lateral gastrocnemius, which is responsible for the flexion of the ankle joint and acts as an assistant muscle in the flexion of the knee joint. Therefore, CLS can be safely included in one’s daily routine and in any rehabilitation programme to improve the dynamic balance and muscular strength that is responsible for hip extension; knee flexion; and ankle plantar flexion, and to increase the range of motion accordingly. However, CLS should be avoided for patients who are suffering from static balance disturbance. In general, CLS does not need any prevention means if the personal sitting duration is short, e.g., 20 min.