Monitoring heart rate variability in elite college football players throughout the preparatory and competitive season
Vagally-mediated heart rate variability (lnRMSSD) reflects cardiac-parasympathetic modulation and may be a useful marker for reflecting recovery status and training adaptation in football players. Three studies were conducted to evaluate lnRMSSD responses to training based on playing position (SKILL, MID-SKILL and LINEMEN) among an elite college football team during three distinct phases of training including the off-season (Spring camp), preseason camp and the in-season competitive period. The first study evaluated daily and chronic lnRMSSD responses to training during Spring camp. Following ~20 h of recovery from a football practice, a significant reduction in lnRMSSD for LINEMEN was observed, while lnRMSSD for SKILL and MID-SKILL returned to near or within baseline values. Individual changes in lnRMSSD from baseline to 20 h post-training were significantly related to body mass with greater lnRMSSD reductions occurring among heavier players and vice versa. Chronic responses showed that individual coefficient of variation of lnRMSSD derived from the entire 4-week Spring camp was significantly inversely related to individual mean training load after adjusting for body mass. The second study evaluated daily lnRMSSD and perceived wellness responses to a 13-day intensive preseason training camp in hot and humid conditions. After the first few days of training, decrements in lnWellness and increases in lnRMSSD, peaking on Day 12 following a day of passive rest among SKILL and LINEMEN were observed. The peak in lnRMSSD was associated with the return of lnWellness to Day 1 values. MID-SKILL showed no meaningful changes in lnRMSSD while their lnWellness remained chronically suppressed throughout the duration of preseason camp. The third study evaluated the daily lnRMSSD response to the most intense training session of the week across three separate weeks during the first month of the competitive season. Compared to resting values, lnRMSSD ~20 h following training was significantly reduced for LINEMEN and MID-SKILL but not for SKILL. The individual change in lnRMSSD from rest to 20 h post training was significantly related to both body mass and training load. In conclusion, the lnRMSSD response to training among elite football players depends on playing position, body mass, training load and training phase. Subjects with greater body mass and lower mean training loads tend to show the largest reductions in lnRMSSD while subjects with lower body mass and higher mean training loads tend to show smaller daily changes in lnRMSSD. While these lnRMSSD responses to training were consistent during Spring camp and during the early in-season competitive phase, this trend was obscured during preseason training in hot and humid conditions. Rather than decreasing with accumulated fatigue, lnRMSSD tended to increase. Thus, heat acclimatization responses during preseason camp may prevent typical training-induced reductions in lnRMSSD observed during Spring camp and in-season.