One of the serious problems in modern society is the aging problem. Due to aging, the proportion of the elderly is rapidly increasing[1], and one of the problems experienced by the majority of the elderly is sarcopenia[2]. Sarcopenia is a phenomenon in which muscle mass decreases due to aging, which can lead to various problems such as spinal disc problems, knee arthritis, and falls[2]. Therefore, strength training is very important to prevent these secondary diseases. Strength training can improve physical and mental health, prevent various injuries and diseases, and increase lifespan[3].

In particular, the quadriceps femoris is an important muscle that particularly requires strength training. The quadriceps femoris is a major muscle used for walking, sitting, and standing, which is essential for daily life, and the thickness of the quadriceps femoris is proportional to health[4, 5]. One effective method for strengthening the quadriceps muscles is through stair climbing exercises[6]. Stairs are easy to find in modern society, and in the movement of climbing stairs, the quadriceps muscle overcomes the person's weight and repeats contraction and relaxation. Additionally, climbing stairs is a low-cost exercise method that increases human lifespan[7]. According to previous studies, when climbing stairs, heart rate, cholesterol, lactic acid concentration, and average oxygen intake decreased compared to the control group[8].

Meanwhile, wearable robots are emerging, and in particular, Samsung Electronics developed Bot fit (Samsung Electronics Co., Ltd., Suwon, Republic of Korea), an exercise robot that helps with hip joint movements[9]. Bot Fit helps the user walk or induce exercise by providing assistance or resistance torque in the direction of flexion and extension of the wearer's hip joint[10]. According to a previous study, there was a partial improvement in lower extremity muscle strength as a result of performing an exercise program that included climbing stairs and walking exercise while wearing the Bot Fit for 4 weeks[10]. In addition, as a result of interval walking exercise for 6 weeks wearing Bot Fit, it was confirmed that the elderly’s walking function improved and rectus abdominis muscle activity increased[9].

However, for improving muscle strength, an exercise program focused on improving muscle strength may be more effective than complex exercise. Considering the results of previous research, it can be expected that higher-intensity resistance will be needed to improve muscle strength in anaerobic exercise using Bot fit, especially for young adults. Combining Bot Fit's resistance mode with stair climbing exercises, which can be easily incorporated into daily life, is anticipated to result in a higher-intensity workout. This effect requires analysis, particularly for users aiming to enhance muscle strength. Therefore, the purpose of this study is to confirm the exercise effect when combining Bot Fit's resistance mode and stair climbing.

Participants

A total of 53 subjects were recruited and data were collected, and the subject selection conditions were as follows: 1) Normal adults in their 20s to 50s with no history of central nervous system disease. 2) Normal elderly people over 65 years of age with no history of central nervous system disease. The purpose was to evenly distribute young and old adults in order to investigate the effectiveness of the robot used in this study for various age groups. The exclusion criteria are as follows: 1) People who have difficulty walking independently due to problems such as visual field defects or fractures. 2) Those with severe arthritis or orthopedic problems that limit passive range of motion (ROM) of the lower extremities (knee flexion contracture> 10°, knee flexion<90°, hip flexion contracture> 25°, ankle plantar flexion contracture>15°). 3) Cases where it is difficult to understand the exercise program performance due to severe cognitive decline (Korean-Mini-Mental State Examination, K-MMSE≤ 10)[11]. 4) People who have difficulty participating in exercise programs due to adult diseases such as severely uncontrolled high blood pressure or diabetes. 5) People at risk of falling when walking due to severe dizziness. 6) Those whose height is less than 140 cm or more than 185 cm and are not suitable for wearing the walking assistance robot. 7) Severely obese with a body mass index (BMI) of 35 or higher. The sample size calculation standard was set at 95% target power, 5% significance level, and 0.41 clinically effective effect size as a result of sample size calculation using the G*Power (3.1.9.2) program, resulting in 55 people. The review of safety and ethical suitability for the overall clinical trial was approved in advance by the Sahmyook University Institutional Review Board (SYU IRB NO.2023-12 -013-003).

Bot Fit

Bot Fit (Samsung Electronics Co., Ltd., Suwon, Republic of Korea) is a wearable robot that provides assistance or resistance to the hip joint (Figure 1). Bot Fit is divided into sizes based on the width of the wearer's pelvis, and is fixed using straps on the waist and both thighs. Bot Fit was developed to assist users in exercising with a focus on their core and lower extremity muscles and features two distinct modes. Bot Fit's assist mode is a mode that provides assistance in proportion to the user's hip movement and walking speed in the same direction. Conversely, resist mode, which feels like walking in water, provides resistance in the opposite direction to the user's hip movements. In this study, to increase the effectiveness of strength training, the resist mode that provides resistance was used. Each mode is divided into five levels depending on the size of the providing power. In this study, the strength that provided the strongest resistance was applied. The control pack located on the waist has a rechargeable battery and a central processor, Therefore, it can be operated by linking with a mobile phone or mobile watch.

Procedures

All 53 subjects were measured maximal voluntary contraction (MVC) after attaching EMG electrodes to gastrocnemius (GCM), rectus femoris (RF), biceps femoris (BF), erector spinae (ES), and vastus medialis oblique (VMO) before climbing stairs. For all muscles, maximum muscle contraction was performed three times for 5 seconds and the average MVC was analyzed. After measurement of MVC, to get used to the Bot Fit, the subject wore the Bot Fit and walked a 20m round trip. In order to adapt to the Bot Fit, after wearing the Bot Fit, the assistance mode and resistance mode of the Bot Fit were applied one by one and the subjects walked naturally on flat ground to achieve sufficient adaptation.

After that, the subject took off the Bot Fit and wore foot pressure sensor equipment and climbed 48 stairs. While climbing stairs, plantar pressure values were measured throughout the entire section. Additionally, muscle activity occurring during the stair climbing section was measured in five muscles. Afterwards, a sufficient rest of 5 minutes was taken to eliminate the effect between conditions. Subsequently, the subjects wore the Bot Fit device and ascended 48 stairs with the resistance mode set to intensity 5 (on a scale from 1 to 5), which provides resistance to hip joint movement in the direction opposite to that intended by the wearer. Finally, we compared and analyzed the muscle activity and plantar pressure generated during stair climbing with and without the Bot Fit resistance mode.

Measurements

Foot Pressure

W-insole Science System (Letsense group, Bologna, Italy) was used to measure average foot pressure while climbing stairs. After wearing insole-type sensors on the left and right sides of shoes that fit the subject's foot size, calibration was performed with both feet lifted off the ground. Foot pressure measurements were taken from the time they started climbing the stairs until they reached the end, and the average data was collected. Foot pressure was assessed by dividing it into the front, middle, and back parts of the foot, and the overall average foot pressure and the difference between left and right foot pressures were also analyzed.

Muscle Activity

Before attaching electrodes, hair was removed from the attachment area and cleaned with an alcohol swab to minimize noise. Afterwards, surface electromyography (sEMG; FreeEMG, BTS Bioengineering, Italy) was used to measure the MVC of each subject’s GCM, BF, RF, ES, and VMO. Electrodes were attached according to SENIAM guidelines[12]. MVC was measured three times and averaged, and the data was sampled at 1,000 Hz and amplified 1000 times. Additionally, root mean square was conducted with a 50 ms mobile window and all data were removed outliers using box plot. Data were analyzed using a software, EMG Analyzer v2.9.37.0 (BTS Bioengineering, Milano, Italy) and band passed at 20-450 Hz[13]. The final muscle activity value was calculated by applying the RVC (reference voluntary contraction) concept as data ‘muscle activity wearing Bot Fit/muscle activity without wearing Bot Fit’ (%)[14].

Statistical analysis

The Shapiro-Wilk test was used to check whether all data satisfied the normal distribution form. After confirming that the normality of the data was satisfactory, a paired t-test was used to compare the two conditions. When performing statistical analysis on specific groups, the Wilcoxon signed rank test, a non-parametric method, was used. For statistical verification, program PASW statistics 18 (SPSS, Inc., Quarry Bay, Hong Kong) was used, and the statistical significance level was set at 0.05.

Baseline participant characteristic

The baseline characteristics of the 53 subjects are summarized in Table 1.

Foot pressure

In Table 2, there was no significant difference in foot pressure between conditions for all 53 subjects. However, in 17 healthy young adults, the front (p<0.01) and middle (p<0.05) areas of the foot significantly increased when wearing Bot Fit (Table 3). Additionally, COP deviation within the sole decreased. In elderly people with knee arthritis or obesity, the pressure in the front area significantly increased (p<0.05) (Figure 2).

Muscle activity

In table 4, the muscle activity of all subjects significantly decreased in the gastrocnemius muscle compared to without Bot Fit (p<0.01). Additionally, the muscle activity of the rectus femoris muscle in elderly people with knee arthritis was significantly decreased compared to without Bot Fit (p<0.05), and in obese elderly people, the muscle activity of the gastrocnemius muscle was significantly decreased compared to without Bot Fit (p<0.05).

This study was conducted to determine how effective stair climbing exercise using the resistance mode of Samsung Electronics' wearable robot 'Bot Fit'. To confirm the effectiveness of the exercise, a comparative analysis was conducted between conditions with and without Bot Fit by measuring foot pressure and the activity of major muscles. In the results of foot pressure, the pressure in the front and middle parts significantly increased compared to the rear foot in adults wearing Bot Fit. It is implied that the significantly increased foot pressure in adults compared to the elderly was because they performed more stably when walking on stairs. On the other hand, it is assumed that older people have less stability when climbing stairs due to lower limb and core strength and decreased balance compared to younger people, making it difficult to confidently push up with their feet.

In particular, pressure increase in the front and middle areas is a common phenomenon when climbing stairs[15, 16]. This means that when subjects wore Bot Fit and applied resistance mode, they climbed the stairs with a more correct posture. additionally, it was confirmed that the trajectory deviation of the pressure center line decreased when wearing the Bot Fit, which serves as additional evidence for stable stair walking[17]. In addition, there was no significant change in all seniors, but among them, there was a significant increase in frontal pressure in elderly people with knee arthritis or obesity. In general, people with knee arthritis or obesity have an increased risk of falling[16, 18, 19]. Therefore, these people need to exercise with correct posture more than healthy people. In this respect, the results are clinically meaningful. However, in the case of seniors, there was no significant increase in forefoot pressure, so it is expected that voice coaching in the form of telling them to climb stairs by applying force to the front of the foot would be effective.

In the muscle activity results, the muscle activity of the gastrocnemius muscle was significantly decreased in all subjects. This indicates that both adults and seniors emphasize the use of their thigh muscles when climbing stairs with the Bot Fit resistance mode. Given that the resistance provided by Bot Fit targets the flexion and extension movements of the hip joint, the thigh muscles are directly impacted by this resistance[9, 10]. Additionally, although it was not significant, erector spinae activity tended to decrease. As a result of these results, it is expected that the Bot fit's control pack located at the back to support the lower back, thereby relieving the burden on the lower back. In young adults, rectus femoris activity tends to increase compared to seniors, which means that they walk stably on stairs and perform knee extension movement smoothly. On the other hand, seniors tend to have an increase in the biceps femoris muscle, which means that they perform control of knee thrust movements with relatively less stability[20, 21]. Additionally, in individuals with knee arthritis, the activity of the erector spinae muscles increases. This is because patients with knee arthritis generally tend to use their lower back muscles to compensate for knee pain during walking[22, 23]. Therefore, it is necessary to determine the appropriate intensity of the resistance mode to identify the point at which compensatory action of the back muscles is mitigated.

Stair climbing exercise using a wearable robot that provides resistance centered on the hip joint in the same form as Bot Fit increases muscle strength due to the resistance of the robot, and allows the user to exercise in the correct posture through symmetrical movement and design of the left and right lower limbs. When developing wearable robots for exercise in the future, more effective methods from an exercise perspective can be explored based on the results of this study.

Based on the results of this study, it is possible to induce correct stair climbing posture when climbing stairs using Bot fit resistance mode. In particular, it is expected to be an effective exercise for strengthening muscle endurance by increasing the activity of the rectus femoris muscle.

This study was supported by Samsung Electronics (202400020001).

The authors declare no conflict of interest.