## 2 Physical Activity during the COVID-19 Pandemic
These two studies assessed the influence of the pandemic on mental health of humans as well as physical activity, both of which indicate that the pandemic itself and/or physical distance measures, such as lockdown and self-isolation, negatively affect people's mental state. The COVID-19 pandemic is associated with anxiety, depression, and insomnia in the general population and healthcare workers, and this stress-related mental state is associated with suicidal behavior [25]. Wearable activity trackers may also play a useful role in monitoring daily physical activity in individuals who are at high risk of such mental illness. [PERSON] et al. [26] examined the impact of mobility restrictions during the COVID-19 pandemic on physical activity and sleep pattern using data from the Health Insights Singapore study. Physical activity and sleep duration were measured using a Fitbit. The daily step count on weekdays/weekends (from 9344 \(\pm\) 3634/8992 \(\pm\) 4437 to 5284 \(\pm\) 4283/5432 \(\pm\) 4692 steps, respectively), moderate- to vigorous-intensity activity minutes on weekdays/weekends (37.6 \(\pm\) 27.9/40.9 \(\pm\) 37.5 to 24.4 \(\pm\) 28.2/27.2 \(\pm\) 32.9 min, respectively), and resting heart rate on weekdays/weekends (65.7 \(\pm\) 7.3/65.8 \(\pm\) 7.3 to 64.2 \(\pm\) 7.3/64.1 \(\pm\) 7.4 bpm, respectively) were significantly decreased during the lockdown. Sleep duration on weekdays/weekends was increased from 6.92 \(\pm\) 0.95/7.49 \(\pm\) 1.18 to 7.28 \(\pm\) 0.96/7.73 \(\pm\) 1.18, respectively; however, sleep efficiency was not changed during the lockdown. These changes were proportional to the level of mobility restrictions. French researchers examined whether wearable activity trackers could detect adherence to home confinement policies during the COVID-19 pandemic [27]. The daily step count was monitored using a wristwatch with an embedded accelerometer (Withings). Data from approximately 742,000 subjects were analyzed. Overall, the number of steps significantly decreased (7.8-55.6% of baseline steps per day) in countries with a total lockdown. However, a decrease in the number of steps in countries with a partial lockdown was 1.5-22.1% of baseline steps per day, and an increase (1.3% of baseline steps per day) in the number of steps was observed in Germany. In countries without lockdown, the number of steps also seemed to be decreased; however, it was significantly increased in Sweden (5.7% of baseline steps per day). These findings suggest that people adhered to the home confinement policies during the pandemic, but these raised an issue that such strong policies could harm cardiometabolic health. Although this study showed that wearable activity trackers were useful to detect adherence to the pandemic policies, the strategy to make use of data from wearable activity trackers for health management and prevent damage to cardiometabolic health is not exhibited. [PERSON] and [PERSON] [28] assessed changes in cardiovascular indicators of health as well as sleep and exercise behaviors due to physical distancing during the COVID-19 pandemic. A total of 5436 individuals who used a wearable device, the WHOOP strap, in the United States (US), were retrospectively analyzed. The WHOOP strap can automatically detect exercise, and the following variables were identified: sleep duration, sleep onset, sleep offset, social jet lag, exercise type, exercise frequency, exercise intensity, resting heart rate, and heart rate variability. Sleep duration was significantly increased from 7.65 \(\pm\) 0.20 to 7.89 \(\pm\) 0.18 h after the implementation of physical distancing. People have went to bed earlier and got up later. Exercise frequency was increased from 59.9 \(\pm\) 4.04% to 61.91 \(\pm\) 4.36% during physical distancing compared with baseline in 2020. However, exercise frequency was decreased from 65.62 \(\pm\) 10.44% to 62.83 \(\pm\) 8.22% in subjects aged 18-25 years. A large proportion of subjects were engaged in regular exercise and increased exercise intensity during the physical distancing period.