Why the Human Body Is NOT a Machine: Understanding Adaptation and Davis’ Law

Analogies and metaphors are often used to simplify complex ideas. The human body is frequently compared to a machine, while the brain and nervous system are compared to a computer. Although these comparisons can help create a basic understanding of how certain processes work, they are also limited—and sometimes harmful.

We are not machines.
We are biological organisms, and this distinction fundamentally changes how we should think about health, performance, injury, and longevity.

Machines Wear Out. Humans Adapt.

When a machine is built, it is the strongest it will ever be. From that point forward, it begins to degrade. Parts weaken. Materials break down. Function declines.

But humans don’t operate this way.

There is no such thing as the body “wearing out” in the same mechanical sense. Biological organisms respond to their environment. They adapt. From the moment we are conceived until our final years of life, our bodies constantly remodel in response to stressors.

• If we eat more calories than we burn → we store energy as fat.

• If we lift heavy objects → we build muscle.

• If we’re exposed to UV light → we produce melanin.

This adaptive capacity decreases gradually over time, but it never disappears entirely—as long as we are alive.

Davis’ Law: How Tissues Adapt to Stress

Our musculoskeletal system is always being remodeled. Davis’ Law states that soft tissues—muscles, tendons, ligaments, skin—adapt to the demands placed on them.

Put simply:
When tissues experience stress, they get stronger.
When they lack stress, they weaken.

“Stress” in this context refers to physical demand, not emotional pressure.

Examples of Davis’ Law in Action

• Runners develop stronger Achilles tendons

• Lifters develop denser muscles and more resilient connective tissue

• Sedentary individuals lose strength, stability, and tissue tolerance

Machines wear out when stressed.
Humans adapt when stressed.

This is the foundation of fitness, rehab, and performance training.

Why Stress Is Essential for Health and Performance

If our bodies behaved like machines, the best strategy for longevity would be to avoid stress entirely.

But because we adapt, the opposite is true:

To become strong, resilient, and injury-resistant, we must apply physical stress.

Exercise isn’t optional—it’s essential.

The key is finding the appropriate dosage. Too much stress causes injury. Too little stress causes weakness. The right amount stimulates adaptation.

Stress dosage is influenced by:

• Intensity – how hard the effort is

• Volume – total amount of work

• Frequency – how often the stress occurs

For example, when assessing running stress, we typically consider:

• Runs per week (frequency)

• Speed (intensity)

• Duration/mileage (volume)

When you understand your personal capacity and control these variables, you can build strength, prevent injury, and improve performance.

The Other Half of the Equation: Rest

Stress drives adaptation.
Rest allows adaptation to occur.

After the body experiences an appropriate stimulus, it responds through hormonal and neurological changes that build tissue, improve coordination, and increase resilience.

Without enough rest, adaptation stalls.
Without enough stress, adaptation never begins.

Success in rehab, performance training, and injury prevention requires balancing stress with recovery.

The Bottom Line

A basic understanding of Davis’ Law explains why:

• Rehab requires progressive loading

• Injury prevention requires consistent training

• Performance improves only when stress is applied intentionally

• Avoiding physical stress leads to long-term weakness

• Rest is required—but only after adequate stress

We are not machines.
We are biologically adaptable systems.

And when we apply the right balance of stress + rest, we become stronger, healthier, and more resilient.