## 4 Discussion  
After collecting training load and HRV data on seven elite swimmers before, during, and after 8 weeks of lockdown without any swimming activity, it was noted that swimmers encountered a 55.2% training volume reduction during the lockdown, which was associated with a decrease in vagal activity (+9.2% in RHR and \(-6.5\%\) in Ln rMSSD) and HRV global activity (\(-5.3\%\) in Ln SDNN). The lockdown period induced a decrease in weekly training volume from 27.2 to 12.2 h. The present data show a large increase in resting HR during this period, that was mainly mediated by a decrease in vagal tone and vagal control of the heart [18]. A recent study described the HRV profile of a world class open-water swimmer and demonstrated that HRV vagal-related indices were increased after a 50% increase in mean training volume [15]. Previous studies in elite endurance athletes have shown that a reduction in training load can lead to a decrease in vagal activity indices [10; 18; 19]. Other studies highlighted the impact of training cessation on the cardiac autonomic function with a reduced parasympathetic activity in endurance athletes [20; 21; 22]. Considering the association between the decrease in parasympathetic activity and the decline in cardiorespiratory fitness [23], these changes in HRV may reflect an alteration of one or more of the components of endurance performance [10]. In contrast, the changes in resting HRV and Ln rMSSD observed in this study also indicate an increase in sympathetic activity. A similar response has also been reported following a taper period which preceded an optimal triathlon race performance, brought about by a reduction in training volume and possible pre-race anxiety [19]. While these HRV responses may indicate a decline in aerobic fitness, although this was not measured in the present study, it is important to note that similar HRV responses can also be indicative of an optimal taper and readiness to perform. These similar HRV patterns can occur due to different contextual factors, and that reinforces the importance of interpreting HRV changes with additional markers of training status [24]. It is possible that swimmers experienced higher levels of stress or anxiety during the lockdown period due to a lack of certainty around training and competition schedules with the postpement of the Olympic Games, which may have contributed to the increase in sympathetic activity. This hypothesis could also be argued by the decrease observed in SDNN values during the lockdown period. Indeed, it has already been shown that anxious people have lower SDNN values compared with controls [25]. However, following a gradual increase in training volume and a return to swimming training after the lockdown, a progressive increase in vagal modulation was observed, highlighted by a decrease in resting HR and an increase in Ln rMSSD. It seems that despite the reduced training volume, mostly at the expense of swim-specific training, these swimmers were able to avoid excessive declines in cardiovascular function during the lockdown period, as recently described by [PERSON] et al. [26] The maintenance of land-based aerobic training during the lockdown likely helped to prevent this decline and to restore baseline HR and HRV values. Another key insight from these results is the observation that vagal activity appears to be higher in the last weeks of the study in comparison with the baseline values. Indeed, despite a lower training volume than during the baseline, rMSSD values are higher and HR values are lower than at baseline. During this post-lockdown period, the aerobic volume was likely sufficient to increase parasympathetic activation, but the total intensity level was possibly limited, which would explain a lesser dominance of orthosympathetic activity. This case report presented some shortcomings that were difficult to overcome given the inability to have physical contact with the swimmers and to maintain a standardized training plan during this time.