###### Abstract  
_Background:_ Many athletes worldwide have endured home confinement during the COVID-19 pandemic, and their opportunities to train were strongly limited. This study describes the impact of lockdown on training volume and heart rate variability (HRV) in elite swimmers. _Methods:_ HRV data of seven elite males were collected each Monday morning over 20 weeks, including 8 weeks of lockdown. The training volume was quantified retrospectively. _Results:_ During the lockdown period (weeks 4-11) swimming was not allowed, and the total training volume was reduced by \(55.2\pm 7.5\%\) compared to the baseline volume (from 27.2 to 12.2 training hours). This drop was associated with a decrease in vagal activity (a \(9.2\pm 5.4\%\) increase in resting HR and a \(6.5\pm 3.4\%\) decrease in the natural logarithm of rMSSD from baseline values). After the lockdown (weeks 12-20), the training volume was gradually increased before attaining 68.8% and 88.2% of the baseline training volume at weeks 15 and 17, respectively. Resting HR and Ln rMSSD returned to baseline values four weeks after the lockdown. _Conclusions:_ The lockdown period induced a decreased training volume which was associated with a decrease in vagal activity. However, HRV values returned to the baseline 4 weeks after the resumption of swimming training. physiology; swimming; monitoring; training load; home confinement +
Footnote †: journal: Medicale  
some physical impairment in elite athletes, with a reduction in endurance capacity in elite handball players [7] and an increase in body mass in combat sports athletes [8]. This was in line with another study which described an increase in resting heart rate after 4 weeks of lockdown in elite swimmers [9], which was associated with a reduction in training volume and a decrease in the state of well-being. The monitoring of heart rate, and especially heart rate variability (HRV), could be considered as one of the most informative and easy to obtain tools to track fitness status in elite athletes, as testing \"on-the-field\" was not possible during the lockdown. HRV is the analysis of the time intervals between heart beats, used to assess the functioning of the sympathetic and parasympathetic branches of the nervous system. Ultimately, this allows inferences to be made on the athlete's adaptations to training in relation to training type, training volume, or training intensity [10]. The root mean square of the difference between adjacent normal RR intervals and a time interval (rMSSD) index seems to be the most relevant index to assess parasympathetic activity in elite athletes [10], whereas the standard deviation of normal-to-normal RR intervals (SDNN) is the gold standard for medical stratification of cardiac risk [11]. In the short term, both indices could give an indication of athlete fatigue and their wellness state. The association between heart rate and HRV indices could help coaches and support staff to interpret long-term training adaptations [10]. The HRV assessment seemed to be useful during the lockdown, where important changes in training characteristics occurred for elite athletes, which was particularly the case for swimmers who were unable to complete any sport-specific training. Previous studies investigated changes in the autonomic cardiac balance in line with training changes in swimmers. [PERSON] et al. showed a positive correlation between parasympathetic indices and performance in elite swimmers [12]. Conversely, two studies found a shift towards sympathetic dominance during the taper period, associated with a high pre-competition anxiety level and reduced parasympathetic activity [13; 14]. [PERSON] et al. also demonstrated that training volume and training intensity have a considerable impact on HRV responses, depending on the mesocycle plan [15]. Therefore, the objective of this brief report is to describe the effects of the COVID-19 crisis in elite swimmers, with a focus on (1) training characteristics and (2) HRV changes before, during, and after an 8-week lockdown period.