## 2 Materials and Methods
### Data Collection  
Athletes performed a standardized HRV test, as previously described by [PERSON] et al. [15] Briefly, the test consisted of 6 min in the supine position in a calm environment, without moving or talking, with their arms positioned along their body. Each test was conducted on Monday morning, just after awakening, and after at least one and half days of rest (no exercise activity). RR intervals (the time elapsed between two successive R-waves of the QRS signal on the electrocardiogram) were recorded with an HR belt (Polar H10, Polar Electro Oy, Kempele, Finland) and transmitted by Bluetooth using a smartphone application (Elite HRV, USA). RR interval recordings were analyzed from the last 240 s of the test. Breathing rate was not guided. During the first step, no Kubios filter was used. The Kubios filter algorithm compared every RR (Gloucester, MA, USA) interval value against a local average interval. Then, a medium filter was used, considering that the strongest filters could over-filter the signal. In the second stage, each data file was visually inspected in shortened windows for artifacts, which were manually corrected. Three time domain indices were used for the HRV analysis: heart rate (HR), Ln rMSSD (natural logarithm of the square root of the mean sum of the squared differences between RR intervals), and SDNN (standard deviation of normal-to-normal RR intervals).  
Training volume was quantified with the athletes' coach before and after the lockdown period (strength training and swimming). All training sessions were monitored by the same coach. The study period comprised seven mesocycles, as follows. Before the lockdown (BEF), swimmers were involved in a training camp, which ended four weeks before the national championships (which were cancelled after the surge of the COVID-19 pandemic). This training camp was characterized by a high training volume and relatively high-intensity work. This period ended with the beginning of lockdown. A self-reported training volume (in hours) questionnaire was conducted on each Monday during the lockdown period (LOCK), including strength training and aerobic training volume (typically including a stationary bike, outdoor cycling or running, indoor rowing, etc.). The swimmers were not able to swim at all, had limited equipment to perform aerobic training, and were required to limit training intensity. The swimmers were requested to complete an online questionnaire to facilitate the data collection. This technique is commonly used by this group of swimmers during the training season. The first week after the lockdown (AFT1) was characterized by a higher aerobic volume, but with absolutely no intensity and still with no swimming sessions. The next two weeks (AFT2-3) were characterized with the resumption of one daily swimming session with no intensity and mainly aerobic and technique training, but no more aerobic land-based training. The next two weeks were (AFT4-5) characterized by an increase in training volume, with a majority of training performed under the first lactate threshold. It was also characterized by the re-turn of twice-daily sessions, three times per week. Then, the last four weeks of the study (AFT6-9) were characterized by a return of the classical 11 weekly swimming sessions. However, the intensity was still limited, and it was very rare for the coach to ask for an intensity above the second lactate threshold. It is also important to note that no competition was organized during that period.