Why Scientists Can't Agree on What Our Bodies Feel Like
Imagine if every carpenter in the world used different rulers to measure wood—some using inches, others centimeters, some a unique personal system. The result would be chaos in construction, with pieces that never fit together properly. This is surprisingly similar to the current challenge in tissue biomechanics, the field dedicated to understanding the mechanical properties of biological tissues.
Scientists reviewed decades of research on tissue elasticity and found something puzzling: the reported stiffness of gray matter in the brain was sometimes described as softer than white matter, sometimes stiffer. The ratio of their elasticity swung wildly from study to study.
These aren't minor differences—they represent fundamental disagreements about the very nature of our biological building blocks. This article explores why these discrepancies exist and how scientists are working to solve this mystery that has profound implications for medicine, tissue engineering, and our understanding of human health.
To understand why measuring tissues is so tricky, we first need to appreciate what they're made of. Your body isn't a homogeneous blob—it's an intricate composite material refined over millions of years of evolution.
At the most fundamental level, soft fibrous tissues contain three key players working together:
Often called "the steel of biological materials," collagen acts as the primary load-bearing element, providing stiffness and strength. Think of it as the steel rebar in concrete.
This is the elastic component that allows tissues to stretch and recoil, similar to natural rubber. When hydrated, it enables incredible flexibility.
These negatively charged molecules absorb water like tiny sponges (up to 1000 times their volume!), providing shock absorption and viscoelasticity 1 .
Different tissues mix these components in varying proportions, creating a spectacular range of mechanical behaviors.
| Tissue | Collagen (%) | Elastin (%) | Proteoglycans & GAGs (%) |
|---|---|---|---|
| Tendon | 75-85 | <3 | 1-5 |
| Ligament | 70-80 | 10-15 | 4-7 |
| Skin | 60-80 | 2-4 | ~20 |
| Aorta | 25-35 | 40-50 | 2-5 |
| Meniscus | 70-80 | <1 | 17 |
Data compiled from multiple sources 1
This architectural complexity is just the beginning. Tissues also display what material scientists call hierarchy—organization across multiple scales from nano to macro. Collagen fibers form fibrils, which assemble into fibers, which then organize into larger bundles with specific patterns like crimps or waves. Add to this the constant presence of water creating complex fluid-solid interactions, and you begin to see why measuring a single "property" is so challenging 1 .
When it comes to actually measuring tissue properties, researchers face a veritable maze of challenges. The path taken through this maze dramatically influences the final destination—the reported numbers.
How you probe a tissue dramatically affects what it "feels" like:
Uses tiny nano-sized probes to press on minute tissue areas
Uses sound waves to observe how mechanical vibrations travel through tissue
Stretches tissue samples between two grips to measure resistance to pulling
Each method interacts with the tissue's structure differently. A technique applying rapid deformation might engage the collagen network immediately, while a slow method might allow fluid redistribution that changes the response. As one study noted, "increasing measurement force and speed increased the measured elasticity of both gray and white matter" in spinal cord tissue 5 .
Experimental conditions introduce another layer of variability:
Both gray and white matter elasticity decrease at similar rates as temperature rises from 20°C to 38°C 5 .
Since water content so dramatically affects tissue behavior through proteoglycans, drying samples even slightly can radically change measurements.
Tissues begin altering their properties immediately after death. One remarkable study showed that gray matter elasticity decreases about 3 hours postmortem until plateauing after 6 hours, while white matter changes differently 5 .
Perhaps the most surprising source of discrepancy comes from simple misuse of scientific language. In one review of medical literature, scientists found 13 different articles that consistently reported "tissue stiffness" using units of meters per second (m/s)—which are actually units of speed, not stiffness 2 .
This confusion stems from a fundamental misunderstanding: stiffness (a property of an object, measured in force per displacement) versus elastic modulus (a property of a material). As the authors explained, "if two cylinders of the same material with equal height but different diameters are pushed with the same force... the one with the larger diameter will appear stiffer even though both cylinders are made with material having the same modulus" 2 .
| Term | Definition | Correct Units | Common Misconceptions |
|---|---|---|---|
| Stiffness | Resistance of an object to deformation | N/m (Newton/meter) | Often confused with modulus |
| Elastic (Young's) Modulus | Intrinsic material property | Pa (Pascals) | Depends only on material composition |
| Shear Modulus | Resistance to shearing deformation | Pa (Pascals) | Related to Young's Modulus (E ≈ 3μ) |
| Shear Wave Speed | Speed of wave propagation | m/s (meters/second) | Sometimes mislabeled as "stiffness" |
To see these challenges in action, let's examine a clever experiment designed specifically to untangle why gray and white matter measurements vary so dramatically between studies.
Researchers used atomic force microscopy to systematically test how measurement force, speed, postmortem time, and temperature affect the reported elasticity of spinal cord tissue 5 . Their approach was comprehensive:
This methodical variation of parameters allowed them to see how each factor influenced the final results—something rarely done in single-focused studies.
The findings revealed why the literature contained such contradictions:
The gray-to-white matter stiffness ratio (Kg/Kw) swung dramatically from ~5 at low forces and speeds down to ~1 at high forces and speeds.
Postmortem time significantly affected measurements, with gray and white matter changing at different rates and patterns.
The anatomical direction of measurement mattered tremendously—Kg/Kw was much higher in transverse sections than longitudinal ones.
Temperature changes affected both tissues similarly, thus not altering their ratio.
This single study demonstrated that you could get almost any ratio you might expect simply by choosing different measurement parameters! The apparent contradictions in literature weren't necessarily errors—they were measurements taken under different conditions that genuinely engaged different aspects of the tissue's complex mechanical behavior 5 .
| Factor | Effect on Kg/Kw | Practical Significance |
|---|---|---|
| Measurement Force | Decreases from ~5 to ~1 as force increases | Low forces may better reflect in vivo conditions |
| Measurement Speed | Decreases with increasing speed | Different techniques use different loading rates |
| Postmortem Interval | Increases until ~6 hours, then stabilizes | Timing of experiments critical for consistency |
| Anatomical Plane | Higher in transverse vs. longitudinal sections | Direction of measurement changes results |
| Temperature | Minimal effect | Not a major contributor to discrepancies |
Based on findings from 5
Beyond understanding the challenges, it's helpful to know the key tools and materials researchers use to probe tissue properties:
| Tool/Reagent | Function | Key Applications |
|---|---|---|
| Atomic Force Microscope | Measures nano-scale forces and topography | Mapping tissue microstructure at cellular level |
| Shear Wave Elastography Systems | Tracks shear wave propagation speed | Clinical assessment of tissue stiffness |
| Single-cell RNA Sequencing | Profiles gene expression of individual cells | Understanding cellular heterogeneity in tissues |
| Laboratory Information Systems | Stores large datasets of patient results | Big Data analysis of biological variation |
| Reference Phantoms | Provides standardized materials for calibration | Ensuring consistency across instruments and labs |
The field is now developing solutions to these challenges. The Radiological Society of North America has established a Quantitative Imaging Biomarkers Alliance to standardize measurements 2 . Meanwhile, data science approaches are helping researchers extract more reliable information from existing datasets 3 6 .
Organizations are working to establish standardized protocols and reference materials for consistent tissue measurement.
Advanced computational methods help extract meaningful patterns from complex, heterogeneous tissue data.
Innovative methods like angle-resolved ultrasound elastography provide more comprehensive tissue characterization 9 .
The recognition that "mechanical biocompatibility"—the ability of implanted materials to match the load-bearing characteristics of native tissue—is as important as biological compatibility drives improved testing 1 . Novel techniques like angle-resolved ultrasound elastography can now simultaneously measure both tensile and shear properties, recognizing that tissues have different characteristics in different directions 9 .
Perhaps the most important shift is the growing recognition that we cannot reduce living tissues to simple numbers. As one review noted, "the mechanical behavior of soft fibrous tissues offers an optimal combination of stiffness, strength, and toughness—properties essential for performance under demanding, long-term physiological conditions" 1 . It's this exquisite combination—not any single property—that evolution has optimized and that scientists seek to understand.
The very complexity that makes tissue measurement so challenging is what enables our bodies to function so remarkably. Those conflicting measurements aren't just noise—they're clues pointing to a deeper truth about the sophisticated, multi-functional, and hierarchical nature of the materials that make us who we are.