Heart Rate Variability (HRV) in Sport: A Review of the Research

Heart Rate Variability (HRV) in Sport: A Review of the Research

Most athletes and sport science personnel understand the importance of recovery after exercise, which is defined as the return of body homeostasis after training to pre-training or near pre-training levels [28].

Recovery involves getting adequate rest in between training sessions/competition to allow the body to repair and strengthen itself in preparation for the subsequent bout. Optimal athletic performance is supported when recovery to pre-training or near pre-training levels is allowed. If recovery is insufficient, hindrance of physiological adaptation and reduced athletic performance should be expected [29, 55-57].  Recovery plays a major role in minimizing the negative effects of training (fatigue) while retaining the positive effect (improved fitness/strength/performance). If recovery is not monitored following exercise, fatigue may accumulate and become excessive prior to competition, resulting in reduced athletic performance and, potentially, overtraining syndrome. In its essence, overtraining syndrome is characterized by a combination of excessive overload in training stress and inadequate recovery, leading to fatigue and decreased performance [30]. Heart rate variability (HRV) is a non-invasive method that is thought to provide valuable data about recovery and physiological adaptation changes that occur in response to physical activity. Using HRV as a monitoring tool for these purposes will be discussed in the following sections.

Heart Rate Variability and the Autonomic Nervous System

Heart rate variability (HRV) involves measurement of the variation in time between individual heart beats across consecutive cardiac cycles, which can estimate the activity level of a person’s autonomic nervous system (ANS), [1]. The ANS works to maintain homeostasis during and following exercise; examining ANS responsiveness to changes in training stresses may indicate the body’s ability to tolerate or adapt to an exercise stimulus [2, 3]. The ANS controls cardiovascular function through sympathetic and parasympathetic modulation [4]. Since this ANS-controlled sympathetic-parasympathetic balance can be altered following changes in training stress [5, 6], monitoring indices of HRV (indirect estimate of ANS function) has been used to better understand training adaptation/maladaptation in athletes [7-11]. In fact, HRV has been shown to be a valid and reliable predictor of ANS function [22]. Effectively managing training stress through HRV monitoring may enhance training periodization, which can result in improved athletic performances. HRV has been collected at various time points within a day (post-exercise, at night, upon waking, etc.); the data described in this article will focus on studies where HRV was collected upon waking or at rest prior to exercise.

Note: You may have noticed that a few words are highlighted in blue; abbreviations for these words are used extensively in this article.

Deciphering HRV: Increases are Better?

Generally, an increase in HRV indicates a beneficial training adaptation and better recovery status, whereas a reduction in HRV reflects stress and worse recovery status. Acute decreases in HRV have been reported to occur following intense endurance training [12, 26], resistance training [13], combined training [14], sport-specific training [15-19], and competition [20, 21]. Given these reports, and others, low HRV is commonly thought to provide a reflection of acute fatigue from training or competing. For example, reduced HRV was observed in elite rowers during a 26-week, intensified training period leading up to the 2012 Olympic Games [12]. In elite male weightlifters with >6 years of participation in national or international competitions, HRV decreased following training, followed by a return to baseline after time was given for recovery [13]. The combination of low HRV and high acute training load was associated with increased injury risk in CrossFit athletes [14]. HRV was negatively correlated with training load in NCAA Division I Football players, and larger players experienced greater HRV reductions during intensified training than their smaller counterparts [15]. In collegiate female soccer players, lower fitness and greater perceived fatigue were associated with reduced HRV [18]. NCAA Division I swimmers also exhibited reduced HRV and perceived wellness during two weeks of overload training. Wellness ratings and HRV increased back to baseline levels during the subsequent two weeks of tapering (i.e. de-intensified training) leading up to a championship competition [19]. In a recent meta-analysis and systematic review, athletic performance improvements were associated with concurrent increases in measures of resting HRV [9]. The authors suggest that these observed increases were facilitated by positive adaptations to training and associated parasympathetic HR modulation [9].

Flatt et al. (2017). Heart rate variability and psychometric responses to overload and tapering in collegiate sprint-swimmers. Journal of Science and Medicine in Sport, 20(6), pp.606-610.

Deciphering HRV: Decreases are Better?

Although increased Heart Rate Variability (HRV) typically relates to improved recovery and performance, this is not always the case [23, 24, 28, 31-33]. In elite endurance athletes, decreased performance on a maximal incremental exercise test was associated with increased weekly HRV values following a 3-week overload period [24]. An interesting small study was performed on 3 high-level tennis players [23]. Following a 30-day overload period, HRV was reduced, as expected, but improvements in aerobic capacity (VO2max), single-leg jump, and drop jump index performances were observed, as well [23]. In elite female wrestlers, researchers were able to identify athletes who were functionally overreached (the fatigue state that precedes overtraining) and over-trained (i.e. excessively fatigued) using HRV metrics [31]. However, the identification factors for overreaching and overtraining included both increases and decreases in various HRV indices [31]. It was clear that periods of excessive training and inadequate recovery resulted in ANS imbalance, but since the drastic HRV perturbations shifted in either direction, it’s difficult to decipher the practical application of the results. Additionally, there have been reports that changes in HRV do not occur in overtrained athletes with short-term training (6 days) or long-term (6 months) overtraining periods [32, 33].

Adding to the Complexity: The Variance

There is extreme variance in HRV responses between individuals, which may contribute to the varying cohort-based results [39, 40]. For example, there were substantial differences observed among World-Class rowers [39], and also among professional baseball pitchers [40]. In addition to athletic conditioning, age, gender, and ethnicity are known contributors to the differing HRV responses between people [42-45].

Additionally, it appears that the way an individual’s HRV responses change over time is influenced by certain factors, such as training intensity and body mass. Research supports that intra-individual HRV changes are far more sensitive during periods of intensified training, compared with baseline. The variance of intra-individual HRV responses increased during more intense training periods in teams of NCAA Division I football players and swimmers, and in a small case study of elite endurance athletes  [8, 15, 16, 19]. Significant relationships between individual HRV response variance and body mass was also observed in the same group of NCAA Division I football players [15, 16]. HRV responses are sensitive to even slight changes in psychological stress [46-48, 54], emotional and attentional state [46, 49, 50, 54], and anxiety [51-54], adding to the complexity of individual athlete HRV interpretation.

Trying to determine whether HRV increases or decreases are “better” reminds me of this great scene from Billy Madison.

Evidence of HRV-Prescribed Training

There is some evidence that manipulating training variables based on HRV can be an effective strategy for maintaining or improving athletic performance. A few studies investigated HRV-guided training vs. pre-planned training. In HRV-guided training, if the athlete’s HRV is normal or higher than normal, then that athlete will be prescribed an intense training session, but if the athlete’s HRV is lower than normal, the athlete will be prescribed a lower-intensity session. With pre-planned training, athletes performed the programs as prescribed, regardless of HRV status. Positive results were observed with HRV-guided training, compared with pre-planned training [25, 27], but the differences in one of these studies were not statistically significant [27]. A separate group of researchers lowered exercise intensity when a reduced athlete HRV was observed [6, 26]. Modulating the exercise intensity based on HRV maintained fitness levels compared to control groups, indicating the potential utility of HRV use in athletes [6, 28]. Additionally, a group of elite Nordic skiers recently benefited from HRV-guided training [41].

Applicability and Conclusion

Heart rate variability (HRV) is a valid, reliable indicator of autonomic nervous system (ANS) function, which indicates the status of body homeostasis [22]. Although the research is still young, it appears that, more likely than not, decreased HRV represents a shift toward sympathetic dominance, indicating increased training stress and worse recovery status [12-21, 27]. Multiple studies have found shifts of HRV from vagal to sympathetic dominance when athletes become over-trained [34-36]. However, results are mixed [23, 24, 28, 31-33]. Although companies have worked hard to make HRV accessible via mobile applications [37], the validity of using such devices for HRV monitoring is considered questionable [38]. There has been an extraordinary variance in the methodology used to quantify HRV responses in the research thus far, including differences in athlete positioning during measurement, duration, and time of day, making practical application difficult to support, currently [6, 28].

Although monitoring HRV is becoming increasingly attractive due to increasing availability of technology able to measure it [28], its application beyond research settings is up for debate.HRV analysis may be an inexpensive, fast, and noninvasive method to monitor exercise recovery and readiness to train. However, given the paucity of research, mixed results, and variance in methodologies used to assess morning and pre-exercise HRV as an athlete monitoring tool to date, I would not advise prescription of training recommendations based solely on HRV analyses, at this time. If/when HRV is used in conjunction with other collected indices to determine athlete recovery or physiological adaptation status, interpretation of results should be carefully and cautiously integrated and analyzed.


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