Altitude adaptations

At elevations above 3,900 feet (1,200 meters), your body makes physiological adjustments to compensate for reduced oxygen availability.

Acute altitude responses:

• Increased pulmonary ventilation (hyperventilation)
• Elevated submaximal heart rate
• Slight decrease in stroke volume
• Increased blood lactate concentration

Long-term altitude adaptations:

• Increased red blood cell production (polycythemia)
• Higher hematocrit and hemoglobin levels
• Enhanced capillary density in muscles
• Greater mitochondrial efficiency

These adaptations can help endurance athletes perform at altitude, but full acclimatization may take weeks.

Adjustments to altitude

System	Immediate adjustments	Longer-term adaptations
Pulmonary	Hyperventilation	Increased ventilation rate stabilizers
Acid-base balance	Body fluids become more alkaline (due to CO₂ loss from hyperventilation)	Excretion of bicarbonate (HCO₃⁻) by the kidneys
Cardiovascular	Increased cardiac output at rest and during submaximal exercise	Continued elevation in submaximal heart rate
	Slight decrease in stroke volume	Lower resting and maximal heart rate
	Maximal cardiac output remains the same or slightly lower	Decreased maximal cardiac output
Hematologic	None	Increased red blood cell production (polycythemia)
	None	Increased hematocrit and blood viscosity
	None	Decreased plasma volume
Local tissue	None	Increased capillary density in skeletal muscle
	None	Increased number of mitochondria
	None	Greater use of free fatty acids as fuel, sparing muscle glycogen

Together, these changes improve oxygen delivery and support endurance performance at altitude.

Hyperoxic breathing

Hyperoxic breathing means inhaling oxygen-enriched gas mixtures during exercise or during post-exercise recovery. This method has been proposed to:

• Enhance aerobic performance
• Speed up recovery
• Reduce lactate accumulation

However, research is still inconclusive about its long-term benefits for endurance athletes.

Smoking and aerobic performance

Research on smoking and exercise performance is limited, but available evidence suggests that smoking impairs:

• Lung function, leading to reduced oxygen delivery.
• Airway resistance, due to nicotine-related bronchoconstriction.
• Ciliary function, limiting the clearance of mucus and debris from the respiratory tract.

In addition, carbon monoxide (CO) in cigarette smoke binds to hemoglobin with a higher affinity than oxygen. This reduces oxygen transport and increases cardiovascular strain.

Blood doping

Blood doping is the artificial increase of red blood cell (RBC) mass to improve oxygen-carrying capacity and aerobic performance. It can be achieved by:

• Infusion of stored red blood cells (autologous or homologous transfusion).
• Administration of erythropoietin (EPO), a hormone that stimulates RBC production.

Effects of blood doping:

• Increased maximal oxygen uptake (VO2 max) by up to 11%.
• Lower heart rate and blood lactate levels at standardized workloads.
• Enhanced tolerance to submaximal workloads at altitude.

However, blood doping carries serious health risks, including increased blood viscosity, hypertension, and a higher risk of clotting-related events (e.g., stroke, myocardial infarction).

Genetic potential and aerobic adaptation

Genetic potential plays a key role in determining maximum aerobic capacity and how much you adapt to endurance training.

• Some athletes experience rapid gains, while others plateau despite similar training loads.
• Elite endurance athletes typically have a higher percentage of Type I muscle fibers and more efficient oxygen utilization.

Because performance margins are small in elite competition, program design and monitoring are crucial for optimizing results.

Age and sex differences in aerobic adaptations

• Aerobic power decreases with age, primarily due to declining muscle mass, mitochondrial efficiency, and cardiovascular function.
• Men generally exhibit higher absolute aerobic power than women, though sex-based differences in relative training responses are minimal.

Aging is associated with decreased VO2 max and endurance capacity, but consistent training can reduce many of these declines.

Overtraining syndrome (OTS)

Overtraining syndrome (OTS) occurs when training stress exceeds recovery capacity. It leads to performance decline, fatigue, and increased injury risk.

Stages of overtraining:

1. Functional overreaching (FOR) - Short-term decline in performance, followed by supercompensation with proper recovery.
2. Nonfunctional overreaching (NFOR) - More prolonged performance decline with insufficient recovery.
3. Overtraining syndrome (OTS) - Chronic performance impairment that may require weeks or months of recovery.

Cardiovascular responses to OTS

• Increased resting heart rate
• Reduced heart rate variability
• Decreased maximal heart rate response

These changes reflect heightened sympathetic activation and altered autonomic function.

Biochemical responses to OTS

• Elevated creatine kinase (CK) levels, indicating muscle damage.
• Altered blood lactate response, suggesting metabolic dysregulation.
• Reduced glycogen storage, which contributes to fatigue and impaired performance.

Endocrine responses to OTS

• Reduced testosterone levels
• Increased cortisol secretion
• Altered catecholamine (epinephrine/norepinephrine) patterns

These hormonal shifts reflect a chronic stress response that can negatively affect recovery, mood, and metabolism. Because no single marker can diagnose overtraining syndrome (OTS), coaches and practitioners should consider a combination of performance measures, mood state, and physiological indicators when assessing an athlete’s status.

Strategies for preventing overtraining syndrome

• Monitor training loads to avoid excessive accumulation of stress.
• Implement structured periodization with adequate recovery phases.
• Track subjective markers (e.g., sleep quality, mood, motivation).
• Utilize heart rate variability (HRV) and lactate thresholds as physiological indicators.
• Ensure proper nutrition and hydration to support recovery.

Early identification of OTS helps prevent long-term performance decline.

Potential markers of overtraining syndrome

Marker	Impact
Performance	Decreased performance, reduced maximal oxygen uptake (VO2 max), increased fatigue.
Body composition	Decreased muscle glycogen, altered body fat levels.
Heart rate	Elevated resting heart rate, reduced heart rate variability.
Biochemical markers	Increased creatine kinase (CK), altered cortisol and testosterone levels.
Psychological state	Mood disturbances, reduced motivation.

No single marker reliably predicts OTS, but a combination of factors can provide useful insight.

Detraining and aerobic performance

Detraining is the loss of training-induced adaptations when exercise is reduced or stopped.

• Short-term detraining (≤4 weeks): Minor reductions in cardiovascular and muscular endurance.
• Long-term detraining (>4 weeks): Significant declines in VO2 max, stroke volume, and mitochondrial function.

Effects of detraining:

• VO2 max decreases by 4-14% within weeks, and by up to 20% with long-term inactivity.
• Reduced stroke volume and cardiac output lower aerobic performance.
• Increased reliance on anaerobic metabolism leads to greater lactate accumulation.

To limit detraining effects, athletes can use maintenance training, including reduced-frequency endurance workouts.