What could be causing my perceived decrease in cardiac contractility, given my history of regular physical activity and normal sinus rhythm (NSR) with rapid heart rate during exercise, as a middle-aged adult with 10-20 years of heavy weight training experience?

Perceived Decrease in Cardiac Contractility During Exercise

Your perceived decrease in the "pounding" force of your heartbeat during vigorous exercise, despite maintaining normal sinus rhythm and appropriate heart rate response, most likely represents age-related changes in cardiac contractility and diastolic function that are common in middle-aged adults, even those with a history of regular exercise.

Understanding the Physiological Changes

Normal Contractile Response to Exercise

During high-intensity dynamic exercise like heavy squats, the heart normally increases cardiac output through both rapid heart rate and forceful contractions, with stroke volume rising from approximately 70 mL at rest to 100 mL at peak exercise in healthy individuals 1. This increase is achieved through:

• Increased end-diastolic volume (Frank-Starling mechanism) - the primary mechanism in athletes 1
• Decreased end-systolic volume through enhanced contractile state 1
• Elevated heart rate and arterial pressure 1

The sensation of forceful "pounding" you remember reflects this robust increase in contractility and stroke volume during maximal effort 1.

Age-Related Contractile Changes

The diminished contractile force you're experiencing is consistent with well-documented age-related cardiac changes, particularly affecting contractility at rapid heart rates:

• Impaired rate-dependent contractility: Research demonstrates that even with normal baseline function, the heart's ability to increase contractile force at rapid rates diminishes with age and can be affected by subclinical hypertrophy 2
• Altered calcium handling: The force-frequency relationship—where faster heart rates normally increase contractile force—can become blunted or even inverted, meaning contractility fails to increase (or actually decreases) at higher heart rates 3, 2
• Diastolic dysfunction: Slowed diastolic relaxation and filling reduce the ability to augment end-diastolic volume during exercise, limiting stroke volume increase 2

Most Likely Explanations for Your Symptoms

1. Subclinical Left Ventricular Hypertrophy or Remodeling

This is the most probable cause given your history of decades of heavy weight training:

• Heavy static exercise (like squats) causes a pressure load on the left ventricle, potentially leading to concentric hypertrophy over time 1
• Even mild hypertrophy can display markedly diminished contractile responses at rapid heart rates (100-150 bpm) while maintaining normal resting function 2
• The contractile impairment becomes most apparent during exercise stress, exactly matching your description 2
• Diastolic dysfunction typically accompanies this but may not worsen further with rapid pacing 2

2. Age-Related Calcium Handling Abnormalities

Independent of structural changes, calcium cycling deteriorates with age:

• The force-frequency relationship shifts from positive (stronger contractions at faster rates) to flat or negative 3
• Systolic calcium release and diastolic calcium reuptake slow down, particularly at elevated heart rates 3
• This explains why your heart rate increases appropriately but the force sensation is diminished 3

3. Normal Aging Process

Even without pathology, cardiac contractility naturally declines with age, though this is accelerated by the factors above 1.

What This Means Clinically

Reassuring Features

• Normal sinus rhythm on multiple ECGs indicates no arrhythmia 1
• Appropriate heart rate response suggests intact chronotropic function 1
• Ability to exercise vigorously indicates preserved functional capacity 1

Concerning Possibilities to Exclude

You should undergo evaluation to rule out:

• Tachycardia-induced cardiomyopathy: Persistent rapid rates can cause dilated cardiomyopathy, though this typically requires sustained rates ≥130 bpm 1
• Ischemic heart disease: Reduced contractility during exercise can indicate myocardial ischemia, particularly in someone with decades of intense physical stress 1
• Subclinical heart failure: Low cardiac output from decreased contractility can present as exercise intolerance before overt heart failure symptoms develop 1
• Thyroid dysfunction: Both hypothyroidism and hyperthyroidism affect cardiac contractility and exercise response 4, 5

Recommended Evaluation

Obtain the following studies to clarify the diagnosis:

1. Echocardiogram with Doppler: Assess for left ventricular hypertrophy, systolic function (ejection fraction), diastolic function parameters, and wall motion abnormalities 2

2. Exercise stress test with imaging: Evaluate contractile reserve during exercise, looking for:

   • Failure of ejection fraction to increase with exercise 2
   • Regional wall motion abnormalities suggesting ischemia 1
   • Abnormal blood pressure response 1

3. Thyroid function tests (TSH, free T4): Exclude thyroid dysfunction as a reversible cause 5

4. Consider cardiac MRI: If echocardiography is inconclusive, MRI can better characterize myocardial tissue and detect subtle fibrosis 2

Clinical Implications

If evaluation confirms reduced contractile reserve:

• This represents a limitation in cardiac functional capacity that may progress 1
• Modification of exercise intensity may be warranted, particularly avoiding extreme static loads 1
• Medical optimization may include beta-blockers (if hypertrophy is present) or other heart failure medications if systolic dysfunction is documented 1
• Regular monitoring is essential as this can be an early manifestation of evolving cardiomyopathy 3, 2

Common Pitfalls to Avoid

• Do not dismiss symptoms simply because ECGs show normal sinus rhythm—contractility problems are not detected by rhythm analysis alone 6, 3
• Do not assume fitness protects against cardiac dysfunction—decades of heavy static exercise can paradoxically cause pathological remodeling 1, 2
• Do not delay evaluation—early detection of contractile dysfunction allows intervention before irreversible damage occurs 3, 2
• Do not confuse chronotropic competence with contractile function—your heart rate may respond normally while contractility is impaired 1, 3