This study evaluated relationships between changes in resting cardiac autonomic markers derived from a novel smartphone device within the first 3 weeks of a 5-week conditioning program and the eventual change in intermittent running performance among a collegiate female soccer team. The 2 main findings were as follows: (a) the very large relationship between [INCREMENT]Yo-Yo and [INCREMENT]Ln rMSSDcv, which indicated that the participants who showed a decrease in Ln rMSSDcv from weeks 1 to 3 experienced a greater improvement in Yo-Yo performance posttraining and (b) acquisition of daily HRV data with a smartphone application using ultrashort recordings provided meaningful training status information. This supports the efficacy of shortened and more convenient HRV recording procedures (<2 minutes) compared with traditionally used methodology that requires up to 10-minute HRV recordings for evaluating training adaptation in athletes (6,15,16,29).
The CV reflects the daily fluctuation in Ln rMSSD across a training week and is believed to reflect the stress and recovery process in response to training (7). For example, Schmitt et al. (36) found that fatigued elite endurance athletes displayed lower HRV and higher interindividual variation in HRV parameters compared with those who were nonfatigued. Buchheit et al. (11) reported higher Ln rMSSDcv in less fit athletes throughout a 3-week training camp in young soccer players. It was found that in contrast to other HR measures (e.g., exercise heart rate and heart rate recovery), only Ln rMSSDcv was associated with a faster MAS (r = 0.52, p = 0.002) (11). Athletes characterized with lower day-to-day variability (i.e., Ln RMSSDcv) experienced less homeostatic perturbation likely because of a greater capacity for training stress (11). However, these HR measures were derived from postexercise recordings (11) unlike this study that evaluated resting HR measures. In a recent case study, Ln rMSSDcv was associated with a faster 8-km race time in a collegiate male runner (19). However, along with a decrease in the weekly mean, a reduced Ln rMSSDcv was associated with the development of nonfunctional overreaching in an elite female triathlete (32). Based on these previous studies (11,36), it is conceivable that the decrease in Ln rMSSDcv observed in this study indicated adequate recovery, appropriate training loads, and positive adaptation. In contrast, the athletes characterized with an increased Ln rMSSDcv were possibly experiencing higher fatigue and inadequate recovery.
Of importance to the strength and conditioning coach are the potential factors that influence daily HRV changes and thus Ln rMSSDmean and CV values. Acute decreases in vagal-HRV indices seem to be mode and intensity related (31,38). Low-intensity aerobic work can stimulate parasympathetic activity within 24 hours (13), whereas intense conditioning or RT can require between 48 and 72 hours to return to baseline (13,31,38). Recovery modalities such as cold-water immersion have been shown to stimulate parasympathetic reactivation after training and are associated with improved rating of perceived sleep quality in athletes (3). Additionally, hydration status and plasma volume changes (10) and non–training-related factors such as sleep quality and psychosocial stress may also contribute to greater acute fluctuation in HRV (7,39). Ensuring adequate nutrition and sleep practices in addition to limiting non–training-related stressors may therefore help facilitate recovery and training adaptation.
Worth mention is the effectiveness of the MAS-based conditioning program adapted from Baker (4) for inducing significant improvements in Yo-Yo performance in just 5 weeks. The Yo-Yo test is useful for evaluating one's capacity to perform repeated bouts of intense running and to assess changes in performance (5). This test has also been shown to serve as an indicator of physical match performance in female soccer players (26). The MAS-based conditioning protocol seems to be an effective and easily implemented program that coaches may consider adopting to improve intermittent running capacity in female soccer players in relatively short time periods (e.g., 5 weeks).
The limitations of this study pertain to the small sample size and acquisition of HRV data from only 2 of the 5 weeks. Future research with larger sample sizes and more frequent HRV data collection may provide more insight into the evolution of HRV and its association with changes in fitness. Additionally, future research should assess the effectiveness of manipulating training load or recovery modalities for athletes demonstrating unfavorable HRV changes to support training adaptation. This study showed that a reduced Ln rMSSDcv after 3 weeks of training was largely associated with eventual changes in intermittent running performance. It seems that a reduction in daily fluctuations in Ln rMSSD is indicative of positive adaptation to training in female collegiate team-sport athletes. Acquisition of daily HRV data with a smartphone application using ultrashort recordings seems suitable for athlete monitoring.
The authors would like to thank the athletes for volunteering their time to participate in this study. The authors have received HRV tools from HRV Fit Ltd., for future research projects, established after the completion of this study.
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