## 2 Physical Activity during the COVID-19 Pandemic
The resting heart rate decreased from 55.09 \(\pm\) 0.81 to 54.2 \(\pm\) 0.55 bpm, and the heart rate variability increased from 65.85 \(\pm\) 2.28 to 66.82 \(\pm\) 1.4 ms during the physical distancing period in 2020; however, the increase in heart rate variability was also observed during the control period in 2019, and it was larger than that during the physical distancing period. The authors speculated that the increased exercise intensity during the physical distancing period may increase the effect of the sympathetic nervous function. [PERSON] et al. [29] investigated the impact of social distancing and lockdown on daily physical activity, sleep, and training mode and intensity in highly trained athletes, kayakers, and canoeists in Germany. This retrospective observational study enrolled 14 young athletes, and their data concerning sleep duration, daily physical activity, and heart rate during each training session were collected using a multi-sensor smartwatch (Polar M430). Sleep duration was 30 (6.7%) min longer (from 451 \(\pm\) 22 to 481 \(\pm\) 17 min) during the lockdown. The time spent lying down was significantly increased (from 623 \(\pm\) 60 to 729 \(\pm\) 21 min; 17%), while the time spent on light-intensity physical activity (from 205 \(\pm\) 25 to 190 \(\pm\) 5 min; \(-\)7.3%), time spent on moderate-intensity physical activity (from 59 \(\pm\) 15 to 48 \(\pm\) 14 min; \(-\)18.6%), and sitting time (from 360 \(\pm\) 33 to 326 \(\pm\) 21 min; \(-\)9.4%) were significantly decreased during the lockdown. The vigorous-intensity physical activity did not change between before and during the lockdown. Decreases in overall training time per week (\(-\)27.6%) and average duration of each training session (\(-\)15.4%) were also observed during the lockdown. The total training time spent at \(<\)60%, 82-88%, 89-93%, or 94-100% of the individual peak heart rate was 2.8-17.5% higher and that spent at 60-72% or 73-83% of the individual peak heart rate was 4.3-18.7% lower during the lockdown. Athletes were engaged in high-intensity training longer during the lockdown than before the lockdown; however, the training time and time spent on light- to moderate-intensity physical activity were decreased. Overall, athletes also became inactive during the lockdown. [PERSON] et al. [30] quantified the effect of lockdown on physical activity in patients with heart failure. Physical activity data were collected from modern cardiac implantable electronic devices (CIEDs) in the Triage HF Plus Evaluation Study in the UK. CIEDs have multiple built-in sensors to monitor real-time physiological data including physical activity, thoracic impedance, heart rate, heart rhythm, and atrial arrhythmias. A total of 311 patients who completed the collection of 8-week activity data (pre-lockdown for 4 weeks and post-lockdown for 4 weeks) were enrolled in this study. Daily physical activity was measured by an accelerometer within the CIED. A total of 246 patients (79.1%) showed a reduction in physical activity post-lockdown. The median physical activity per day was significantly decreased from 134.7 min/day during pre-lockdown to 113.9 min/day during post-lockdown. Daily physical activity was immediately decreased within 2 weeks after the implementation of lockdown. There were no characteristics of patients such as age, sex, BMI, frailty, and severity of heart failure that were clearly associated with the reduction in physical activity due to the lockdown. Table 1 summarizes the studies investigating the change in daily physical activity during the COVID-19 pandemic. It is evident that there is heterogeneity among the studies. First, the types of wearable activity trackers differ among the studies. Although there is evidence supporting inter-device validity and reliability between different types of wearable activity trackers [31], the method for measuring physical activity should be standardized. [PERSON] et al. [32] developed a system to record data on physical activity from different types of wearable activity trackers.