HRV & Exercise

Methods

Systematic review and meta-analysis of altitude-HRV studies

Key Findings

• RMSSD and pNN50 decrease at altitude (parasympathetic drop)
• Shift from vagal to sympathetic predominance
• Individual variability in responses significant
• Acclimatization duration affects adaptation

Limitations

Heterogeneity in altitude levels, exposure durations, study designs

Methods

Binary logistic regression model on pre-sleep HRV data from national-level male athletes

Key Findings

• 96% accuracy in predicting chronic insomnia from pre-sleep HRV
• Pre-sleep HRV reflects autonomic state affecting sleep onset
• Athletes with insomnia show distinct pre-sleep HRV patterns
• Model more accurate than self-reported sleep quality measures

Limitations

Male athletes only, specific to elite populations

Methods

9-week high-intensity functional training intervention

Key Findings

• HRV had 95% sensitivity for detecting T:C ratio changes
• Higher HRV associated with favorable hormonal balance
• Changes in HF power correlated with T:C ratio
• Specificity was low (5%), limiting diagnostic use alone

Limitations

Small sample, high-intensity training context only

Methods

Multiple studies on 64km to 100-mile ultra-marathons

Key Findings

• HRV suppressed 1 day post-race for 64km events
• HRV returned to baseline by day 2 for shorter ultras
• 100-mile races: HRV may not recover within one week
• Faster runners showed greater HRV disruption
• Perceptual fatigue took 5 days to resolve

Limitations

Individual variation significant, training status matters

Methods

Exercise challenge with continuous HRV monitoring

Key Findings

• ME/CFS patients took ~8 minutes for HRV recovery vs controls
• Sympathetic nervous system remained overactivated post-exercise
• Parasympathetic system failed to engage during recovery
• Demonstrates reduced functional capacity for exercise
• May explain post-exertional malaise mechanism

Limitations

Small sample, short exercise challenge

Methods

Longitudinal HRV monitoring in concussed athletes

Key Findings

• pNN50 decreased at 3 days post-concussion
• HRV recovered to baseline by ~3 weeks
• Persistent low HRV may indicate incomplete recovery
• Autonomic dysfunction common after mild TBI
• HRV may complement symptom-based return protocols

Limitations

Individual recovery timelines vary significantly

Methods

HRV assessment before ACFT in military cadets

Key Findings

• HRV indices predict ACFT performance outcomes
• Acute exercise alters HRV in predictable patterns
• Higher baseline HRV associated with better performance
• Can inform daily training readiness decisions
• Useful for monitoring adaptation to military training

Limitations

Specific to military fitness tests, young population

Methods

Systematic review and meta-analysis of altitude HRV studies

Key Findings

• Acute altitude markedly reduces time and frequency domain HRV
• LF/HF ratio increases (sympathetic predominance)
• Effects more pronounced above 3,500m
• Trained individuals show better vagal preservation
• Gradual ascent helps maintain autonomic balance

Limitations

Individual acclimatization rates vary significantly

Methods

Review of combined heat and altitude training protocols

Key Findings

• Both heat and altitude stress autonomic system
• HRV helps monitor adaptation progress
• Metrics stabilize as acclimatization occurs
• Heat may provide similar hematological benefits to altitude
• Combined training increasingly common in elite sport

Limitations

Optimal protocols still being refined

Methods

Heat acclimatization studies with HRV monitoring

Key Findings

• Acute heat exposure reduces HRV during exercise
• Sympathetic dominance increases in hot conditions
• 10-14 days of heat exposure improves tolerance
• HRV stabilizes as acclimatization progresses
• Hydration status affects autonomic response

Limitations

Individual heat tolerance varies significantly

Methods

Review of HRV applications in youth sports

Key Findings

• About 30% of young athletes experience overtraining
• Reduced HRV may signal overreaching before symptoms
• HRV helps optimize training loads for young athletes
• Burnout is primary reason for youth sports attrition
• AI models integrating HRV show promise for risk prediction

Limitations

Age-appropriate norms still being established

Methods

Randomized trials comparing HIIT vs moderate intensity continuous training (MICT)

Key Findings

• Both modalities improved resting HRV after 8-12 weeks
• HIIT showed faster initial HRV improvements
• MICT produced more stable, sustained HRV gains
• Combination training may be optimal

Limitations

Individual response varies; recovery capacity matters

Methods

Systematic review of exercise training studies measuring HRV

Key Findings

• Measurable HRV increases begin at 2-3 weeks of consistent training
• Peak improvements of 15-30% in RMSSD at 8-12 weeks
• Both aerobic and resistance training improve HRV
• Overtraining temporarily suppresses HRV before recovery

Limitations

Wide variation in training protocols across studies

Methods

Systematic review and meta-analysis of HRV-guided training studies

Key Findings

• HRV-guided training produced larger improvements in VO2max
• Athletes training on low-HRV days showed impaired adaptation
• Flexible programming based on HRV outperformed rigid periodization
• Benefits most pronounced in endurance sports

Limitations

Heterogeneous study designs, varying HRV thresholds used

Methods

Meta-analysis of exercise intervention studies up to November 2024

Key Findings

• Significant improvements in RMSSD and SDNN with exercise training
• Both aerobic and resistance training effective
• Benefits seen across age groups and fitness levels
• Effect size varies by baseline fitness and training type

Limitations

Exercise type, duration, and intensity varied widely across studies

Methods

Systematic review of 19 studies on HRV and overtraining in soccer players

Key Findings

• Reduced RMSSD associated with fatigue and overreaching
• HRV decline often precedes performance decrements
• Sustained low HRV for 3-4 weeks may indicate overtraining syndrome
• Psychological symptoms (mood, sleep) often appear alongside HRV changes

Limitations

Soccer-specific findings may not generalize to all sports

Methods

Cross-sectional analysis of sleep duration, exercise, and HRV relationships

Key Findings

• Sleeping 6+ hours had protective effect on HRV
• Short sleep impaired restorative processes
• Inadequate exercise + short sleep = lowest HRV
• Sleep may be more foundational than exercise for HRV

Limitations

Cross-sectional design cannot establish causation