Daily Heart Rate Variability Among College Football Players Throughout Preseason Camp in The Heat

Document Type

Article

Publication Date

7-11-2019

Publication Title

Journal of Strength and Conditioning Research

DOI

10.1519/JSC.0000000000003450

Abstract

Heat stress and high training loads during preseason camp may serve as a potent stimulus for improving fitness and thermophysiological adaptations that can reduce resting heart rate (HR) and increase HR variability (HRV) in football players. Alternatively, the intense physical demands of training may overtax the athletes, resulting in converse HR and HRV responses. Purpose: To monitor daily HR and HRV in college football players throughout 13 days of preseason camp in hot and humid conditions. Methods: Division-1 players were categorized as linemen (n = 10, height = 191.6 ± 4.7 cm, weight = 131.2 ± 12 kg) and non-linemen (n = 18, height = 187.4 ± 4.4 cm, weight = 96.2 ± 9.4 kg). HR and the natural logarithm of the root mean square of successive differences (LnRMSSD) were acquired 60–90 minutes before training via finger-pulse plethysmography. A questionnaire regarding perceived sleep, fatigue, soreness, stress and mood was administered daily and averaged intra-individually for a single wellness rating (LnWellness). Day 11 was reserved for passive rest. Linear mixed models and effect sizes (ES) ± 90% confidence intervals were used to assess changes in outcome variables. Results: A main effect for day was observed for LnWellness (p < 0.0001). As a group (n = 28) LnWellness on day 1 was greater than days 3, 6–9 and 13 (p < 0.0001–0.039, ES = 0.38 ± 0.45–0.62 ± 0.45). Main effects for position and day (p < 0.001) were observed for RHR. RHR for non-lineman was lower than lineman (66.4 ± 7.2 vs. 73.8 ± 8.6 b·min−1, p = 0.002, ES = −0.93 ± 0.68). As a group (n = 28), RHR on day 12 was lower than days 1–3, 9 and 10 (p < 0.001–0.038, ES = −0.40 ± 0.41–1.05 ± 0.47). Additionally, RHR on day 1 and 2 was higher than days 4, 7, 8 and 13 (p < 0.001–0.048, ES = 0.66 ± 0.45–0.77 ± 0.46). A position × day interaction was observed for LnRMSSD (p = 0.036). LnRMSSD on day 12 was greater than days 2–5, 8, 9 and 10 for linemen (p = 0.0001–0.042, ES = 0.76 ± 0.76–1.41 ± 0.82). Between-groups, LnRMSSD on day 2 for lineman was lower than days 7 and 12 for non-linemen (p = 0.007–0.027, ES = −1.35 ± 0.71–1.73 ± 0.76). Mean ± standard deviation for HR, LnRMSSD and LnWellness are displayed in Figure 1. Conclusion: Despite some reductions in subjective wellbeing throughout camp, HR parameters demonstrated responses consistent with heat acclimation. Non-linemen exhibited favorable changes in HR and HRV earlier than non-linemen. For both groups, peak increases and decreases in HRV and HR, respectively, were observed on Day 12 following a day of rest. Practical Applications: Linemen should be monitored closely within the first few days to ensure that decrements in HRV are not sustained throughout camp. While the observed changes in HR and HRV were considered a positive adaptation to training in the heat, practitioners should be aware that heat acclimation responses may attenuate typical fatigue-related decrements in HR-derived parameters.

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© 2019 National Strength and Conditioning Association

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