Heart Rate Variability Applications in Strength and Conditioning: A Narrative Review

The paper addresses the increasing use of heart rate variability (HRV) to quantify and monitor athletes’ psychophysiological loads in order to optimize training adaptations. HRV, a surrogate of autonomic balance, has become widely accessible via wearable devices, prompting a need for clear guidance on its application in strength and conditioning beyond endurance-only contexts. The primary aim is to propose evidence-based preliminary guidelines for implementing HRV within strength and conditioning, synthesizing research on the effects of resistance training on HRV, HRV’s capacity to assess physiological stress from resistance training, and the use of HRV-guided resistance training, recognizing gaps in existing literature and the need for standardized practices.

Prior reviews have evaluated HRV in sports physiology and endurance athletes, highlighting HRV’s promise for monitoring training status and adaptation. Recent data underscore the necessity to extend discussion to strength and conditioning modalities that combine resistance and endurance training. Existing literature shows HRV’s association with performance and training status in endurance athletes, but fewer studies address resistance training, overreaching detection across sport types, and HRV-guided programming outside endurance. This review consolidates findings across athletic populations, identifies methodological considerations, and proposes preliminary guidelines tailored to strength and conditioning.

A narrative literature review was conducted using PubMed and Scopus with keywords: heart rate variability, strength, conditioning, overreaching, overtraining, and HRV-guided training. Inclusion criteria: original research and review papers discussing HRV, specifically time-domain measures, and applications in strength and conditioning. Exclusion criteria: case reports and studies not including time-domain HRV measures. The review emphasizes measurement considerations (device type: ECG vs PPG; body position; recording duration), standardization (measurement time, methodology, artifact removal, respiration control), and interpretation using individual baselines and smallest worthwhile change (SWC).

- Measurement and interpretation: - HRV analysis spans time-domain (e.g., RMSSD, SDNN, pNN50) and frequency-domain metrics. RMSSD is preferred in strength and conditioning due to sensitivity and lower influence from respiration and recording duration; commonly log-transformed. - Frequency-domain bands: HF 0.14–0.40 Hz (parasympathetic), LF 0.04–0.15 Hz (sympathetic/baroreflex modulation in longer recordings); LF and HF are confounded by respiration in short-term recordings. - No universal normal HRV range; interpretation should be relative to individual baseline. Stability within SWC (±0.5 to 1 SD) and use of rolling 7-day averages provide meaningful context. - Measurement best practices include consistent timing, body position, and duration (clinical recommendation ~5 min; ultra-short 1 min used in athletes). PPG is acceptable at rest but diverges from ECG under stress, high heart rates (>160 bpm), and motion; potential reduced accuracy with darker skin tones. - Training adaptations: - Endurance-trained athletes often show increased night-time RMSSD and correlations with VO2 max, maximum running velocity, and time-trial performance after training; higher baseline HRV relates to training status and faster return to baseline post-exercise, whereas delayed return may indicate reduced training age/adaptability. - Strength/resistance-trained athletes: Limited evidence but likely correlations exist between HRV and strength-based adaptations; gaps remain regarding associations with maximum strength, rate of force development, critical torque/power, and muscle cross-sectional area. - Untrained/moderately trained individuals: Physical activity associates positively with HRV independent of adiposity; unclear link between HRV and VO2 max; limited data on strength and hypertrophy associations. - Overreaching/overtraining: - Endurance athletes show mixed HRV responses: instability outside SWC in overtrained states vs no change or paradoxical increases in some studies. Rolling 7-day averages may be more informative than daily values. - Team/other sports (soccer, wrestling, football, rowing) demonstrate HRV instability with increased workload, potentially signaling overreaching; effects vary by training age and sport. - Resistance-trained athletes: Increased volume/intensity can reduce HRV and 48 h performance, resolving after multi-day recovery; individual variability in recovery (HRV normalization as quickly as 1 h in some cases). Reduced HRV alongside high acute–chronic workload ratio may raise overuse injury risk. - Untrained/moderately trained: Greater HRV reductions with overtraining; endurance sessions of 90 min perturb HRV more than 30–60 min sessions; consistent training may improve tolerance. - HRV-guided programming: - Endurance athletes: HRV-guided training improves maximum running velocity, VO2 max, ventilatory threshold metrics, peak power output, HRV, and serum testosterone versus predefined/block periodized programs across running, skiing, cycling; likely utility in elite athletes. - Resistance/multimodal: No consistent HRV differences across resistance methods (traditional, paired set, superset, circuit, multiple set). CrossFit athletes tolerate higher acute–chronic workload ratios better when 7-day HRV is normal/high. HRV-guided programming may individualize intensity based on recovery but shows no clear advantage for muscle hypertrophy/strength over fixed programming. - Untrained/moderately trained: HRV-guided programs can yield similar or superior improvements in hypertrophy, strength, VO2 max, body composition, and fitness with fewer high-intensity days, with differential benefit favoring endurance outcomes; potential sex differences warrant further study. - Factors influencing HRV: - Age, sex, BMI, sleep quality, stress, alcohol/nicotine, dehydration, illness, vaccination responses, pain, concussion, travel, medications (e.g., beta-blockers, ACE inhibitors, contraceptives, antidepressants), and various diseases can reduce HRV. Weight loss can improve HRV. HRV varies with menstrual cycle and may relate to sleep, stress, injury, motivation, and enjoyment. - Preliminary guidelines: - Use individual baselines; aim for stability within SWC; interpret reductions cautiously, especially in endurance athletes where HRV may be less sensitive to overreaching; HRV-guided training shows utility particularly in aerobic contexts; consider non-training factors and act within professional scope.

The review’s findings support HRV as a practical, non-invasive tool for monitoring training status, adaptability, and recovery across athletic and general populations. By centering interpretation on individual baselines and SWC, HRV can inform day-to-day training decisions and readiness, complementing other performance and recovery markers. Evidence suggests HRV-guided programming enhances endurance-related adaptations and training efficiency compared to predefined models, though benefits for hypertrophy and maximal strength are less clear. Heterogeneity across sports and training ages implies HRV’s sensitivity to overreaching/overtraining is population- and context-dependent; endurance athletes may require additional markers. Standardized measurement protocols and trend-based analyses (e.g., 7-day averages) improve interpretability, while accounting for numerous physiological and psychological factors that influence HRV ensures appropriate application within professional scopes.

Heart rate variability is a helpful metric in strength and conditioning to assess training status and to inform programming across various sports and fitness activities. Utilities differ by athletic population for identifying or predicting overreaching and/or overtraining. HRV-guided programming likely offers advantages over predefined approaches in several contexts, particularly endurance training. As this field evolves, more high-quality, standardized research is needed to clarify best practices for HRV application in strength and conditioning, including resistance training outcomes and sport-specific implementation.

The reviewed studies often have small sample sizes and methodological heterogeneity, limiting generalizability and standardization. Much of the literature focuses on adult and endurance populations, with fewer studies in youth and resistance-trained athletes. Variability in HRV measurement techniques (device type, duration, position, artifact handling) complicates comparisons and meta-analytic synthesis. HRV changes with HRV-guided training may be small, necessitating large samples that are challenging in athletic cohorts. Further research is required to validate HRV’s sensitivity to overreaching/overtraining, delineate sport-specific utility, and determine associations with resistance training adaptations (e.g., maximal strength, rate of force development, critical power/torque, muscle cross-sectional area).