In this study, non-elderly healthy hospital staff were randomly assigned to a resistance training group and a control group, and the effects of the sukubara® intervention were investigated. As a result, significant changes in body composition and physical function (i.e., skeletal muscle mass, knee extension strength, and OFS) were observed in the resistance training group, compared to the control group. In addition, in the pre and post intervention comparison between the resistance training group and control group, an increasing trend in skeletal muscle mass, and a significant increase in knee extension muscle strength and OFS were observed in the resistance training group, suggesting the intervention effect of sukubara®. These results supported the hypothesis.
A particularly important finding was that the sukubara® intervention was effective, despite being entirely self-administered. In addition, a significant correlation was observed between the total YouTube playback time and changes in knee extension strength and OFS variables. These results suggest that sukubara® is a completely self-administered training system with guaranteed accuracy. In addition, it was suggested that the total YouTube playback time could be converted into actual training time, which would provide a meaningful clinical index. The YouTube features adopted by the sukubara® system are default applications on most smartphones, and are free and easy to use. Therefore, sukubara® is a convenient training system for wide-spread use in the region. In addition, by collecting the total YouTube playback time, the training practice can be managed indirectly and sukubara® can be established as a telemedicine system. The sukubara® system could have the same effects for community-dwelling older adults as it does for non-elderly healthy people.
In this study, the effect was produced because the load by eccentric contraction was incorporated into resistance training. Eccentric contraction is an effective form of muscle contraction that can increase muscle strength[11] and recruit motor units with high thresholds even at low tension[12]. In other words, resistance training with sukubara® is considered a rational method aimed at muscle strengthening effects, despite the low load. In addition, Chen et al. reported that stair descent training intended for eccentric contraction in obese women not only increased muscle strength but also significantly increased muscle hypertrophy[13]. For these reasons, the implementation of sukubara® was considered to produce muscle hypertrophy and muscle strengthening effects.
The second reason is that participants’ frequency in using sukubara® was properly observed. Braith et al. reported that knee extension training produced a conspicuous muscle strength-enhancing effect in the group that used sukubara® 3 times a week rather than 2 times a week[14], and the possibility that the more frequent the training, the greater the effect. Assuming that participants in the intervention group averaged 1 hours of total video viewing time and that the program completion time per session was 14.5 minutes, then the frequency was 4.2 sessions/week (12 weeks). Before the intervention, participants were encouraged to perform sukubara® at least 3 times a week, and every day if possible. In fact, 6 out of 8 people (75%) performed it more than 3 times a week. In this study, as in the previous study, it is possible that a sufficient frequency of 3 times/week or more was ensured, resulting in the muscle strength-enhancing effect. In addition, the participants of this study may have performed sukubara® at an appropriate frequency because they were aware that the daily video playback time was captured in real time by the research administrator. Several studies have pointed out that people may change their behaviors when they are monitored[15, 16]. In other words, it is possible that the participants fell into a mental state of being "remotely monitored" on a daily basis and diligently practiced every day, even though they were not actually being monitored.
This study has two limiting factors. First, owing to the impact of the recent COVID-19 pandemic, the number of applicants for the study was insufficient, and the final number of participants was half that of the pre-calculated sample size. Therefore, bias could not be eliminated completely when the participants were assigned to the resistance training and control groups, resulting in a corresponding difference in the participants’ attributes. A pre-intervention comparison of the two groups showed that, compared to the control group, the resistance training group tended to have a lighter body weight and significantly higher IPS, which is the ability to balance. These results may have influenced the amount of change in each variable. In the next training system verification, an intervention study will be conducted after recruiting the number of participants according to the sample size calculation. The results obtained will be interpreted using the significance test, enabling the final verification. Second, in the control group, skeletal muscle mass tended to decrease while body fat mass increased, and a significant decrease in knee extension strength occurred despite the approximately 12-week control period. Although these changes were attributed to decreased physical activity due to the influence of COVID-19 restrictions, the participants’ physical activity level was no different from the general physical activity level among Japanese individuals of the same generation[17]. During the study period, no significant difference in physical activity was noted between the two groups. A previous study that examined the amount of physical activity performed by workers revealed that those who met the recommended amount of physical activity while walking were more likely to increase the amount of physical activity at work[18]. Workers spend most of their time in the workplace. Participants in this study continued to work as essential workers at the hospital, even in an environment where activity was restricted due to the COVID-19 pandemic; therefore, a certain level of physical activity was ensured during the study period. However, the participants may have refrained from leisure activities such as outdoor activities and exercise on holidays. A previous study has shown that workers’ leisure-time exercise is more strongly associated with Health-Related Quality of Life than with physical activity, including work[19], and is associated with favorable health checkup results[20]. Furthermore, leisure activities among workers are an important factor linked directly to health benefits. Thus, the decline in leisure activities may have affected body composition and physical performance. In the future, it will be necessary to examine the effects of leisure activities on body composition and physical performance in other situations.
In conclusion, a self-administered training system (sukubara®) was devised which incorporated low-load exercise and resulted in muscle hypertrophy and improvement in physical function. Importantly, muscle hypertrophy, knee extensor strength, and balance ability were improved in this completely self-administered training intervention.