PURPOSE:To quantify position-specific associations between heart-rate-variability profile characteristics and external-workload parameters in American college football players during preseason camp while accounting for injury-related participation status. METHODS:Twenty-seven players categorized into skill (n = 15) and midskill (n = 12) position groups participated in the study. Players were further categorized as having "full go" (n = 15) or "go as can" (n = 12) participation status. The camp mean and coefficient of variation (CV) (indexing daily variation) for various workload parameters and resting heart rate and the natural log of the root mean square of successive differences (LnRMSSD) were quantified. RESULTS:Skill players who consistently performed higher total workloads (total distance and player load) exhibited the most stable LnRMSSD (ie, lower CV; P < .05), while midskill players with the highest total workloads exhibited greater daily variation in LnRMSSD (ie, higher CV; P < .05). These associations persisted after injury-status adjustment. However, consistent between position groups, players with greater daily variation in select workload parameters also showed greater daily variation in LnRMSSD (P < .05). No between-groups differences for injury status were observed (P > .05), although effect-size analysis showed that LnRMSSD was moderately greater in "full go" players (effect size = 0.73, P < .07). CONCLUSIONS:Daily LnRMSSD variation better reflected position-specific workload responses, whereas mean LnRMSSD may affect or be affected by injury status. While skill players tolerated high total workloads with stable LnRMSSD, midskill players exhibited less day-to-day variation in LnRMSSD and high-intensity outputs at moderate workloads.

PURPOSE:To track cardiac-autonomic functioning, indexed by heart-rate variability, in American college football players throughout a competitive period.METHODS:Resting heart rate (RHR) and the natural logarithm root mean square of successive differences (LnRMSSD) were obtained throughout preseason and ∼3 times weekly leading up to the national championship among 8 linemen and 12 nonlinemen. Seated 1-minute recordings were performed via mobile device and standardized for time of day and proximity to training.RESULTS:Relative to preseason, linemen exhibited suppressed LnRMSSD during camp-style preparation for the playoffs (P = .041, effect size [ES] = -1.01), the week of the national semifinal (P < .001, ES = -1.27), and the week of the national championship (P = .005, ES = -1.16). As a combined group, increases in RHR (P < .001) were observed at the same time points (nonlinemen ES = 0.48-0.59, linemen ES = 1.03-1.10). For all linemen, RHR trended upward (positive slopes, R2 = .02-.77) while LnRMSSD trended downward (negative slopes, R2 = .02-.62) throughout the season. Preseason to postseason changes in RHR (r = .50, P = .025) and LnRMSSD (r = -.68, P < .001) were associated with body mass.CONCLUSIONS:Heart-rate variability tracking revealed progressive autonomic imbalance in the lineman position group, with individual players showing suppressed values by midseason. Attenuated parasympathetic activation is a hallmark of impaired recovery and may contribute to cardiovascular maladaptations reported to occur in linemen following a competitive season. Thus, a descending pattern may serve as an easily identifiable red flag requiring attention from performance and medical staff.

Flatt, AA, Esco, MR, Allen, JR, Robinson, JB, Bragg, A, Keith, CM, Fedewa, MV, and Earley, RL. Cardiac-autonomic responses to in-season training among Division-1 college football players. J Strength Cond Res 34(6): 1649-1656, 2020-Despite having to endure a rigorous in-season training schedule, research evaluating daily physiological recovery status markers among American football players is limited. The purpose of this study was to determine whether recovery of cardiac-autonomic activity to resting values occurs between consecutive-day, in-season training sessions among college football players. Subjects (n = 29) were divided into groups based on position: receivers and defensive backs (SKILL, n = 10); running backs, linebackers, and tight-ends (MID-SKILL, n = 11) and linemen (LINEMEN, n = 8). Resting heart rate (RHR) and the natural logarithm of the root mean square of successive differences multiplied by 20 (LnRMSSD) were acquired at rest in the seated position before Tuesday and Wednesday training sessions and repeated over 3 weeks during the first month of the competitive season. A position x time interaction was observed for LnRMSSD (p = 0.04), but not for the RHR (p = 0.33). No differences in LnRMSSD between days was observed for SKILL (Tuesday = 82.8 +/- 9.3, Wednesday = 81.9 +/- 8.7, p > 0.05). Small reductions in LnRMSSD were observed for MID-SKILL (Tuesday = 79.2 +/- 9.4, Wednesday = 76.2 +/- 9.5, p <= 0.05) and LINEMEN (Tuesday = 79.4 +/- 10.5, Wednesday = 74.5 +/- 11.5, p <= 0.05). The individually averaged changes in LnRMSSD from Tuesday to Wednesday were related to PlayerLoad (r = 0.46, p = 0.02) and body mass (r = -0.39, p = 0.04). Cardiac-parasympathetic activity did not return to resting values for LINEMEN or MID-SKILL before the next training session. Larger reductions in LnRMSSD tended to occur in players with greater body mass despite having performed lower workloads, although some individual variability was observed. These findings may have implications for how coaches and support staff address training and recovery interventions for players demonstrating inadequate cardiovascular recovery between sessions.

We aimed to characterize cardiac-autonomic responses to a 13-day preseason camp in the heat among an American college football team. Players were categorized as linemen (n=10) and non-linemen (n=18). RHR, natural logarithm of the root-mean square of successive differences multiplied by twenty (LnRMSSD), and subjective wellbeing (LnWellness) were acquired daily. Effect sizes±90% confidence interval showed that for linemen, LnRMSSD decreased (moderate) on day 2 (71.2±10.4) and increased (moderate) on day 12 (87.1±11.2) relative to day 1 (77.9±11.2) while RHR decreased (small-moderate) on days 6, 7, and 12 (67.7±9.3-70.4±5.5 b∙min-1) relative to day 1 (77.1±10.1 b∙min-1). For non-linemen, LnRMSSD increased (small-large) on days 3-5, 7, 12, and 13 (83.4±6.8-87.6±8.5) relative to day 1 (80.0±6.5) while RHR decreased (small-large) on days 3-9, 12, and 13 (62.1±5.2-67.9±8.1 b∙min-1) relative to day 1 (70.8±6.2 b∙min-1). Decrements in LnWellness were observed on days 4-10 and 13 for linemen (moderate) and on days 6-9, 12, and 13 for non-linemen (small-moderate). Despite reductions in LnWellness, cardiac-autonomic parameters demonstrated responses consistent with heat-acclimation, which possibly attenuated fatigue-related decrements.

This case report demonstrates the effects of sport-related concussion (SRC) on heart rate variability (HRV) in an American college football player. Daily measures of resting, ultra-short natural logarithm of the root mean square of successive differences (LnRMSSD), subjective wellbeing, and Player Load were obtained each training day throughout a 4-week spring camp and 4 weeks of preseason training. SRC occurred within the first 2 weeks of the preseason. During spring camp and preseason pre-SRC, the athlete demonstrated minimal day-to-day fluctuations in LnRMSSD, which increased post-SRC (LnRMSSD coefficient of variation pre-SRC ≤ 3.1%, post-SRC = 5.8%). Moderate decrements in daily-averaged LnRMSSD were observed post-SRC relative to pre-SRC (Effect Size ± 90% Confidence Interval = −1.12 ± 0.80), and the 7-day rolling average fell below the smallest worthwhile change for the remainder of the preseason. LnRMSSD responses to SRC appeared similar to trends associated with stress and training fatigue. Therefore, performance and sports medicine staff should maintain regular communication regarding player injury and fatigue status so that HRV can be interpreted in the appropriate context. Detection and monitoring of autonomic dysregulation post-SRC may require near-daily assessment, as LnRMSSD showed greater daily fluctuations rather than chronic suppression following the head injury.

Abstract Flatt, AA, Esco, MR, Allen, JR, Robinson, JB, Earley, RL, Fedewa, MV, Bragg, A, Keith, CM, and Wingo, JE. Heart rate variability and training load among National Collegiate Athletic Association Division 1 college football players throughout spring camp. J Strength Cond Res 32(11): 3127–3134, 2018—The purpose of this study was to determine whether recovery of cardiac-autonomic activity to baseline occurs between consecutive-day training sessions among positional groups of a collegiate football team during Spring camp. A secondary aim was to evaluate relationships between chronic (i.e., 4-week) heart rate variability (HRV) and training load parameters. Baseline HRV (lnRMSSD_BL) was compared with HRV after ∼20 hours of recovery before next-day training (lnRMSSDpost20) among positional groups composed of SKILL (n = 11), MID-SKILL (n = 9), and LINEMEN (n = 5) with a linear mixed model and effect sizes (ES). Pearson and partial correlations were used to quantify relationships between chronic mean and coefficient of variation (CV) of lnRMSSD (lnRMSSD_chronic and lnRMSSDcv, respectively) with the mean and CV of PlayerLoad (PL_chronic and PL_cv, respectively). A position × time interaction was observed for lnRMSSD (p = 0.01). lnRMSSD_BL was higher than lnRMSSDpost20 for LINEMEN (p < 0.01; ES = large), whereas differences for SKILL and MID-SKILL were not statistically different (p > 0.05). Players with greater body mass experienced larger reductions in lnRMSSD (r = −0.62, p < 0.01). Longitudinally, lnRMSSDcv was significantly related to body mass (r = 0.48) and PL_chronic (r = −0.60). After adjusting for body mass, lnRMSSDcv and PL_chronic remained significantly related (r = −0.43). The ∼20-hour recovery time between training sessions on consecutive days may not be adequate for restoration of cardiac-parasympathetic activity to baseline among LINEMEN. Players with a lower chronic training load throughout camp experienced greater fluctuation in lnRMSSD (i.e., lnRMSSDcv) and vice versa. Thus, a capacity for greater chronic workloads may be protective against perturbations in cardiac-autonomic homeostasis among American college football players.

Wilkerson, GB, Gupta, A, Allen, JR, Keith, CM, and Colston, MA. Utilization of practice session average inertial load to quantify college football injury risk. J Strength Cond Res 30(9): 2369-2374, 2016-Relatively few studies have investigated the potential injury prevention value of data derived from recently developed wearable technology for measurement of body mass accelerations during the performance of sport-related activities. The available evidence has been derived from studies focused on avoidance of overtraining syndrome, which is believed to induce a chronically fatigued state that can be identified through monitoring of inertial load accumulation. Reduced variability in movement patterns is also believed to be an important injury risk factor, but no evidence currently exists to guide interpretation of data derived from inertial measurement units (IMUs) in this regard. We retrospectively analyzed archived data for a cohort of 45 National Collegiate Athletic Association Division 1-football bowl subdivision football players who wore IMUs on the upper back during practice sessions to quantify any associations between average inertial load measured during practice sessions and occurrence of musculoskeletal sprains and strains. Both the coefficient of variation for average inertial load and frequent exposure to game conditions were found to be strongly associated with injury occurrence. Having either or both of the 2 risk factors provided strong discrimination between injured and noninjured players (χ = 9.048; p = 0.004; odds ratio = 8.04; 90% CI: 2.39, 27.03). Our findings may facilitate identification of individual football players who are likely to derive the greatest benefit from training activities designed to reduce injury risk through improved adaptability to rapidly changing environmental demands.