Evaluation of Standing-Up Motion from a Forward-Sloping Toilet Seat for Older People

(1) Background: In-home assistive systems to help people with physical disabilities stand up from toilets are necessary, and the ease of the standing-up motion should be evaluated. (2) Methods: This study investigated the ease of the standing-up motion using objective and subjective data from healthy participants to facilitate the development of a toilet-seat-tilting system. Participants were divided into younger and older age groups. Objective data concerning muscle activity (EMG), three-dimensional (3D) body motion, and center of pressure distribution (COP) were collected. The participants also provided subjective data related to standing up from a toilet tilted at three different angles. (3) Results: All participants repeated the motion 25 times for each angle and provided feedback regarding their standing-up experience under each condition. Objective EMG, COP, and 3D body motion analysis results varied across individuals and age groups. The older group exhibited a consistent pattern of head motion while standing up. Thus, older individuals prefer a forward trunk-inclination motion. (4) Conclusions: According to the collected subjective data, all participants found it easier to stand when the seat angle was 5° or 10°; objective data on the ankle dorsiflexion angle, muscle activity, and head motion may be related to the subjective ease of the standing-up motion.


Introduction
In recent years, the number of slips and falls during walking or other movements among older people with sarcopenia has been increasing [1,2]. The U.S. Center for Disease Control and Prevention (CDC) [3] reported that the prevalence of falls associated with toilet use ranges from 19% to 37% in older persons. Slips and falls during bathroom use accounted for over 40% of all in a Japanese hospital [4]; these accidents occur during walking and translation motions as well as during attempted standing-up motions, for example, from a bathtub or toilet seat. In 2016, CDC reported that deaths from falls in the United States increased by 30% over the past 10 years [5]. In 2015, medical costs of fatal falls and fall injuries among older people in the United States were approximately $50 billion [6]. Consequently, it is necessary to develop assistive systems for standing-up motions.
Previous studies have reported on the motion of standing up from a chair (for a review, see [7]). The standing-up motion is affected by the seat height and seat forward-sloping angle [8,9]. Given the influence of the chair seat angle on forward trunk inclination during standing-up and sitting-down motions, a forward-sloping seat angle may make it easier to stand up [9]. Moreover, some previous studies [10,11] focused on the standing-up motion among older people. One study [10] that focused on muscle strength and speed differences between young and older women showed that the ability to rise from a chair is affected by the reduction in muscle strength. Thus, strength-training regimens may be important to maintain this ability in older people. In another study [11], the standing-up motion is influenced by multiple physiological and psychological processes and represents a particular transfer skill, rather than a simple proxy measure of lower limb strength.
In toilet rooms, the motion of standing-up must be executed in a small space. Furthermore, people consider toilet use to be a private and intimate activity. Therefore, the standing-up motion assistive system must be simple and safe so as to be operated by a single person. In a previous study [12], a prototype of a toilet assistive system using an admittance controller consisting of a toilet seat lift module, two actuators, and force sensors was reported, which could measure the distribution of the center of pressure on the seat. This system will help the two actuators to bear the weight of the person (considering a weight of 100 kg). Previously, we have discussed the development of a new assistive system to aid with standing up from toilet seats [13]. The underlying concept of the accessible toilet assistive system presented relies on an active toilet seat whose seat angle can be changed to aid the standing-up motion. The concept of our research is to utilize the method of adding the tilting angle of the toilet seat by attaching an actuator to the rear part of the seat. When the user wants to lift the toilet seat, this system adds the tilting angle of the toilet seat using the user's switch. Namely, this system can be easily attached to the toilet seat of household of old people, and it can be used in a compact and small space. The rear part of the toilet seat can be lifted vertically, thus mitigating the physical and mental loads for physically vulnerable people when standing up. Thus, it is necessary to evaluate the standing-up motion for users when developing a new assistive system. However, it is difficult to quantitatively evaluate the ease of the standing-up motion using transfer aids such as handrails. Previous studies have evaluated objective data (e.g., muscle activity, body motion, center of pressure), but it was difficult to evaluate the ease of the standing-up motion [7][8][9][10][11][12]. Therefore, the ease of the standing-up motion needs to be evaluated with both objective and subjective data. Furthermore, it is not clear that whether the standing-up motion is affected by changes in physical functioning due to aging.
The purpose of this study is to evaluate the standing-up motion in younger versus older people to develop a new assistive system using a forward-sloping toilet seat. As a prior step in the system development, we must elucidate the effect of changes in physical functioning due to aging using forward-sloping toilet seat. In this pilot study, in addition to subjective data, we monitored objective data such as three-dimensional (3D) body motion, center of pressure distribution, and muscle activity, which were analyzed independently. The relationship between objective and subjective parameters and the effect of toilet seat angle on standing-up motion performance was analyzed. Finally, we evaluated the ease of the standing-up motion by age and toilet seat angle and considered a new in-home assistive system to aid people with physical disabilities in standing up from toilet seats.

Participants
Twenty healthy participants were divided into two age groups, each consisting of 10 participants (five males and five females). The younger participants had an average age of 41.7 ± 12.9 years, average height of 165.1 ± 7.5 cm, and average weight of 59.2 ± 6.2 kg. The older participants had an average age of 71.6 ± 1.8 years, average height of 159.2 ± 11.1 cm, and average weight of 59.7 ± 6.7 kg. Details of the participant characteristics of each group are shown in Table 1. All participants were monitored while using a portable toilet (Panasonic, PN-L30201V, Osaka, Japan) with a variable seat angle (Figure 1a). The study was conducted according to the guidelines of the Declaration of Helsinki and approved by the institutional review board of the National Institute of Advanced Industrial Science and Technology (2015-198E). All participants provided informed consent.

Measurement Items
In this study, we evaluated the ease of the standing-up motion using both objective and subjective data in healthy participants. The objective data included 3D body motion, center of pressure distribution, and muscle activity. Subjective data were also collected.
The body motion was measured using a 3D optical motion capture system (Natural Point, OptiTrack V120: Trio, Corvallis, OR, USA). Five markers were attached to the body, above the (i) head, (ii) acromion, (iii) iliac crest, (iv) head of the fibula, and (v) lateral malleolus on the right side ( Figure 1b). The distribution of foot pressure was measured using a pressure sensor plate (Medicapteurs, Win-Pod, Balma, France). During each task, the motion capture system and the foot pressure platform collected data at a rate of 120 and 100 samples per second (120 and 100 Hz), respectively.
The muscle activity was measured using a portable electromyography (EMG) device (Cometa Systems, Wave Wireless EMG, Bareggio, Italy). Four EMG channels were recorded using an analog-to-digital converter (ADInstruments, PowerLab 4/26, Dunedin, New Zealand) with disposable electrodes. The electrodes were attached to the skin on the right side of the body above the following four muscles: (1) erector spinae, (2) rectus femoris, (3) tibialis anterior, and (4) gastrocnemius ( Figure 1c). The skin was cleaned before the electrodes were applied with an electrode center-to-center spacing of 20 mm in the direction of the muscle fibers. The EMG device collected data during each task performed at a rate of 1000 samples per second (sampling frequency of 1.0 kHz). Muscle activity capture was performed during a standing-up motion of 5 s.
After these measurements, all participants provided feedback concerning the perceived ease of movement for each seat angle in response to a close-ended verbal question. In addition, participants gave a score for each seat angle using a five-point Likert scale. Questions regarding the ease of the standing-up motion refer only to this motion. We also evaluated the comfort score for each toilet seat angle. The ease of the standing-up motion was affected by the duration of seat-off and sitting time. The comfort score by toilet seat angle took into account the entire duration of the motion and was based on a Likert scale.

Protocols
A portable toilet was used in this study. Toilet seat angles of 5 • , 10 • , and 15 • were used. To remove biases due to body habitus and sitting position from the experimental results, the height from the floor to the top of the seat was individually fixed for each participant. It was based on the length of the participant's lower leg to obtain a similar leg position while seated. For the seat angle of 0 • , the angle between the thigh and lower leg and the angle between the lower leg and foot were each set to 90 • (Figure 1d). During all experiments and for all toilet seat angles examined, the foot placement when the seat angle was 0 • was used as the reference, because the standing-up motion was a controlled experimental condition.
The standing-up motion procedure was outlined in a guide provided to the participants. They sat on the toilet seat with their hands crossed over their chest, which meant they could not use them while standing up. They performed the standing-up motion until they reached a full upright position. They repeated the procedure 25 times for each toilet seat angle and were allowed to rest between each standing-up motion. The measurements for each person are shown in Table 1.

Data Analysis
For this study, the time interval for the analysis of the standing-up motion started when the acromion marker moved forward and ended when the iliac crest marker stopped moving. The locus of the 3D body motion capture system was used to calculate the ankle dorsiflexion angle, which was defined as the angle between the head of the fibula, lateral malleolus, and floor in the sagittal plane. Before the standing-up motion, the motion capture system was also used to calculate the forward trunk inclination angle when the participant's buttocks were barely touching the toilet seat. The minimum angle between the acromion, iliac crest, and caput fibulae in the sagittal plane during the standing-up motion was defined as the forward trunk inclination angle. To compare the effect of the toilet seat angle on head motion, we focused on peak-to-peak displacement of the head in the sagittal plane. The peak-to-peak displacement of the head for each participant was the ensemble average normalized by dividing each displacement value by the participant's body length, as measured before the experiments. Averaging was performed for all motions (Figure 2). The center of pressure (COP) was calculated based on foot pressure distribution. To compare the influence of toilet seat angle on participant motion, we focused on peak-topeak COP displacement in the X-axis and Z-axis (Figure 1a). EMG signals were passed through a high-pass filter using zero-phase-lag finite impulse response (FIR) with a 60 Hz cutoff frequency before analysis. To compare the rate of change, each participant's EMG signal ensemble average was normalized as follows: (a) the average EMG signal amplitude was calculated for each toilet seat angle (5 • , 10 • , and 15 • ) and (b) the mean of the participant ensemble averages was taken. The signals were then rectified to obtain the root-mean-square (RMS) value of the muscle activity signal during the standing-up motion.
For the objective data, differences by age group and toilet seat angle were determined using two-way analysis of variance (ANOVA) with Bonferroni test. Each participant's average normalized RMS EMG value for each of the four muscles was used. Average COP displacement and body motion were also calculated for each participant.
For subjective data, differences according to age group were determined using the one-way non-parametric ANOVA. The Kruskal-Wallis test was used to compared scores for ease of the standing-up motion and overall score of the standing-up motion for each angle relative to a toilet seat angle of 0 • . The Bonferroni and Kruskal-Wallis tests were used to calculate the statistical significance at the 0.01 and 0.05 levels for multiple comparisons performed in this study. The Statistical Product and Service Solutions (SPSS) software package (IBM, SPSS Statistics version 24, Armonk, NY, USA) was used for statistical analyses. Figure 3 shows an example of raw data of the standing-up motion among older participants with a toilet seat angle of 5 • .  Figure 4 shows the average ankle dorsiflexion angle and average forward trunk inclination angle according to the toilet seat angle. As toilet seat angle increased, both angles increased, which was observed for all participants. ANOVA results for each age group indicated that ankle dorsiflexion angle was affected by toilet seat angle, with statistically significant differences observed between 5 • and 10 • and between 5 • and 15 • (p < 0.01 with the Bonferroni test). Forward trunk inclination angle was also affected by toilet seat angle, especially between 5 • and 15 • (p < 0.01). On the other hand, the interaction between age group and toilet seat angle did not have a significant effect on either average ankle dorsiflexion angle or average forward trunk inclination angle. Figure 5 shows the average head position in the sagittal plane during the standing-up motion. ANOVA revealed that head posture was affected by age group for all toilet seat angles. Both forward-backward ( Figure 5a) and up-down (Figure 5b) head motion during the standing-up motion decreased with higher seat angles, with statistically significant differences being observed between younger and older participants (p < 0.01). Moreover, ANOVA indicated that forward-backward head motion were not affected by toilet seat angle.

COP
The average peak-to-peak COP displacement in the X-axis and Z-axis during the standing-up motion is shown in Figure 6. COP displacement in the X-axis decreased as toilet seat angle increased in younger participants ( Figure 6a). However, in older participants, average COP displacement in the X-axis increased slightly as toilet seat angle increased. Figure 6b shows that COP displacement in the Z-axis tended to decrease as toilet seat angle increased among all participants. Statistically significant differences between younger and older participants at toilet seat angles of 5 • and 15 • (p < 0.05) were observed. ANOVA results indicated that the COP displacement in the X-axis and Z-axis was not affected by toilet seat angle in either age group. The interactions of participant age group and toilet seat angle did not have a significant effect on the COP displacement. Figure 6. Peak-to-peak COP displacement in the X-axis (a) and Z-axis (b) according to toilet seat angle. Vertical lines denote the standard error of the mean. Asterisks indicate a statistically significant difference (based on the Bonferroni test) between younger and older participants. Bonferroni multiple comparison test (*) p < 0.05.

EMG
The average RMS of EMG amplitude during the standing-up motion is shown in Figure 7. The erector spinae muscle activity decreased as the seat angle increased (Figure 7a), while the tibialis anterior muscle activity decreased (Figure 7c). There were statistically significant differences between younger and older participants in both erector spinae and tibialis anterior muscle at a seat angle of 5 • (p < 0.01). Rectus femoris and gastrocnemius muscle activity remained unchanged (Figure 7b,d, respectively). ANOVA indicated that erector spinae muscle activity was affected by the toilet seat angle in both age groups, with statistically significant differences between toilet seat angles of 5 • and 15 • (p < 0.01). For tibialis anterior, ANOVA showed that muscle activity was affected by toilet seat angle only in the older age group, between 5 • and 10 • as well as 5 • and 15 • (p < 0.01). In both age groups, muscle activity in the rectus femoris and gastrocnemius was not affected by toilet seat angle. The interaction of age group and toilet seat angle had a significant effect on erector spinae and tibialis anterior muscle activity (p < 0.01 and p < 0.05, respectively).

Perceived Ease of the Standing-Up Motion
The perceived ease of the standing-up motion was assessed for all tested toilet seat angles using subjective data ( Figure 8). As shown in Figure 8a, the ease of the standing-up motion increased as seat angle increased. The overall score for the standing-up motion was high for all participants at 5 • or 10 • , meaning they found it comfortable to perform the standing-up motion. The score was lower for a seat angle of 15 • (Figure 8b). However, ANOVA indicated that neither the score for ease of the standing-up motion nor the overall toilet seat score was affected by toilet seat angle in either age group. Furthermore, the interaction of age group and toilet seat angle did not have a significant effect on perceived ease of the standing-up motion. Finally, in Table 2, the quantitative performance of statistically significant difference between younger and older participants in this study is summarized. Table 2. Summary of the quantitative performance of statistically significant differences between age groups ((**) p < 0.01 and (*) p < 0.05, respectively).

Score
Ease score Overall score

Effect of Toilet Seat Angle on the Standing-Up Motion
Previously, the standing-up motion was evaluated based on objective data, such as EMG, 3D body motion, and COP, to support the analysis of physical loads (for reference, see [7]). Other studies have measured the effect of forward trunk inclination angle during the standing-up motion, but the ease of the motion was not measured [8,9]. In this study, both objective data (muscle activity, body motion, and COP based on foot pressure distribution) and subjective data (perceived ease) associated with the standing-up motion after toilet use were evaluated. Based on the subjective data provided by the participants, younger participants found it easy and comfortable to stand when the seat angle was 5 • , while older participants indicated ease and comfort when it was 10 • (Figure 6b). Objective EMG, COP, and 3D body motion analysis results varied across individuals and age groups. Taken together, these results show that the perceived ease of the standing-up motion cannot always be explained with objective data only.
Furthermore, ankle dorsiflexion angle decreased and forward trunk inclination angle increased monotonically with seat angle for all participants. The relationship between ease of the standing-up motion and ankle dorsiflexion angle was also evaluated. It has been reported that decreasing the stress of the standing-up motion requires ankle dorsiflexion, meaning that a person could easily stand up from a sitting position if there was little ankle dorsiflexion involved [14]. In another previous study [15], the dependence of physical loads experienced during the standing-up motion on knee flexion and ankle dorsiflexion angles was measured using a height-adjustable chair. In that study, the physical load during the standing-up motion was affected by foot position (i.e., ankle dorsiflexion), but not the seat height. As shown in Figure 3, ankle dorsiflexion angle decreased with increases in toilet seat angle for all participants, even though the height of the rear part of the toilet seat changed vertically to increase the toilet seat angle. Therefore, ease of the standing-up motion was affected ankle dorsiflexion angle, not by the (partially) higher toilet seat. We suggest that ankle dorsiflexion angle can indeed affect the standing-up motion; hence, when ankle dorsiflexion angle decreased with the increase in toilet seat angle, participants found it easier to stand up from the seat. Ease of the standing-up movement depends on various characteristics such as body height, body weight, and leg length, not just the difference in ankle dorsiflexion angle. In future work, we plan to determine the ease of the standing-up movement in terms of the contribution of various factors such as EMG, COP, and body motion, and discuss the individual characteristics of the standing-up motion.
There is a clear need to evaluate the ease of the standing-up motion not only with measured objective data, but also with subjective data. There is a hypothesized relationship between ankle dorsiflexion angle and ease of the standing-up motion. According to the EMG analysis, erector spinae and tibialis anterior muscle activity was each affected by toilet seat angle. However, the subjective data showed that all participants perceived a high level of comfort when standing up from toilets with a seat angle of 5 • or 10 • , but less for a seat angle of 15 • . Consequently, with a high toilet seat angle, participants were afraid of standing up when they started to get off the seat. This result was not apparent with the objective data; thus, subjective data may play an important role in the development of a new assistive device. However, subjective parameters varied substantially according to various human factors. Hence, subjective data alone are not sufficient to help develop a toilet seat tilting system for accessible toilet facilities. Both objective and subjective data should be considered in all future studies.

Effect of Age on Toilet Seat Angle during the Standing-Up Motion
In previous studies, the standing-up motion in older individuals showed a consistent pattern of trunk and lower extremity motion, with two distinct upper extremity movement strategies [16]. Older individuals used a technique consisting of forward trunk inclination motion. To stand up easily, it was thought that the center of mass for the forward trunk inclination motion at the time of getting off the seat was important. However, it is unclear whether the forward trunk inclination motion is affected by the toilet seat angle, which in turn affects the ease of the standing-up motion. In these studies, the effect of head posture on the standing-up motion with different seat angles was not measured. In this study, forward-backward and up-down head motions during the standing-up motion decreased as the seat angle increased. More head motion among older participants was seen as compared to younger participants. Regarding characteristics of the standing-up motion, older participants had less head motion than younger participants. However, muscle activity and COP displacement were the same in both groups. Forward trunk inclination angle and head motion in two directions affected the standing-up motion. Specifically, older participants had a larger forward trunk inclination angle than younger participants. We think that older individuals prefer to use a forward trunk inclination motion (which moves the center of gravity) for motions like standing up because physical strength decreases with age. In this study, static parameters, such as peak-to-peak displacement of head movement and COP, were analyzed instead of temporal changes during the standing-up motion. Future work will focus on determining the ease of the standing-up motion for older individuals while taking into account time elements and dynamic data.

Limitation of This Study
In this study, we evaluated the ease of standing-up motion due to the difference with a forward-sloping toilet seat using both objective and subjective data. As a prior step in the system development, we analyzed all objective data of 3D body motion, center of pressure distribution, and muscle activity independently. For this reason, we elucidated the changes in physical functioning due to aging using forward-sloping toilet seat. In this study, we determined that younger participants deemed it easy and comfortable to stand when the seat angle was 5 • , while older participants indicated ease and comfort when it was 10 • (Figure 8b).
However, these results were obtained without considering the relation among all objective data. In future works, we will develop the assistive system, and then we will measure the standing-up motion, using this system to make the tilting motion by timesynchronizing all objective data.

Conclusions
The aim of this study was a quantitative evaluation of the effect of toilet seat angle on the performance of healthy younger and older participants attempting standing-up motions. Results will be used in the development of a toilet-seat-tilting device for accessible toilet facilities. The main conclusions of this study can be summarized as follows: • Older participants showed a consistent pattern of head motion and upper extremity movement strategies during the standing-up motion.

•
The perceived ease of the standing-up motion is not always explained using objective data alone. For the same subjective comfort level, measured objective data exhibited considerable variations across individuals. • Base on the subjective data, all participants found it easy to stand when the seat angle was 5 • or 10 • . Objective data on ankle dorsiflexion angle, muscle activity, and head movement may be related to the ease of the standing-up motion. Informed Consent Statement: Informed consent was obtained from all subjects involved in the study.
Data Availability Statement: Not applicable.