Running: Understanding Temporary Height Reduction and Spinal Health

Why am I shorter after running?

It is a common physiological phenomenon to experience a slight, temporary reduction in height after running, primarily due to the compression and temporary dehydration of the intervertebral discs in your spine caused by repetitive axial loading.

Understanding Your Spine: The Body's Central Pillar

To grasp why you might feel shorter after a run, it's essential to understand the intricate structure of your spine. Your vertebral column, or spine, is a marvel of engineering, providing both stability and flexibility. It's composed of 33 individual bones called vertebrae, stacked one upon the other. Between most of these vertebrae lie specialized structures known as intervertebral discs.

The Vital Role of Intervertebral Discs

Intervertebral discs are crucial for spinal health and function. These jelly-like shock absorbers are composed of two main parts:

• Annulus Fibrosus: The tough, fibrous outer ring that provides structural integrity.
• Nucleus Pulposus: The soft, gelatinous inner core that acts like a ball bearing, distributing pressure evenly.

These discs are highly hydrated, particularly the nucleus pulposus, which can be up to 80% water in young, healthy individuals. Their primary functions include:

• Shock Absorption: Cushioning the impact forces on the spine during movement.
• Flexibility and Movement: Allowing the spine to bend, twist, and flex.
• Space Creation: Maintaining the necessary space between vertebrae for nerves to exit the spinal cord.

Running's Impact: Axial Loading and Compression

Running, by its very nature, is an activity that places significant axial loading on your spine. With each stride, your body experiences ground reaction forces that travel up through your kinetic chain, directly compressing your vertebral column. Even if your form is perfect, this repetitive, rhythmic compression puts pressure on your intervertebral discs.

Think of the discs as sponges. When you apply pressure to a sponge, water is squeezed out. Similarly, the repetitive impact of running causes the nucleus pulposus to temporarily lose some of its fluid content. This fluid expulsion leads to a slight decrease in the disc's thickness.

Disc Dehydration and Fluid Loss

The intervertebral discs are avascular, meaning they don't have a direct blood supply. Instead, they rely on a process called imbibition (absorption of fluid) to receive nutrients and maintain their hydration. During activities like running, the sustained compression exceeds the rate at which fluid can be drawn in, leading to a net loss of water from the discs.

This temporary dehydration results in:

• Reduced Disc Height: As fluid is expelled, the discs become slightly thinner.
• Decreased Spinal Length: The cumulative effect of many slightly thinner discs results in a measurable, albeit small, reduction in overall spinal length and thus, your height.

Temporary vs. Permanent Changes

It's crucial to understand that this height reduction is temporary. Your discs are designed to rehydrate. During periods of rest, especially when you are lying down or sleeping, the axial load on your spine is significantly reduced. This allows the discs to re-imbibe fluid, swelling back to their original thickness. This is why most people are slightly taller in the morning than in the evening. Running simply accelerates the natural diurnal (daily) process of disc compression.

Factors Influencing the Effect

Several factors can influence how noticeable this height change is:

• Duration and Intensity of Run: Longer and more intense runs will lead to greater disc compression and fluid loss.
• Individual Disc Health: Healthier, well-hydrated discs may experience less relative compression.
• Hydration Status: Being adequately hydrated supports overall bodily fluid balance, including disc hydration.
• Time of Day: If you run in the evening, you're already experiencing some natural diurnal height loss, so the effect might be more pronounced.
• Age: As we age, the water content of our discs naturally decreases, making them less resilient and potentially more susceptible to compression.

Is It Harmful? Understanding the Implications

For the vast majority of individuals, experiencing a temporary height reduction after running is a normal physiological response and is not indicative of harm or spinal damage. It demonstrates that your discs are performing their shock-absorbing function effectively.

However, prolonged or excessive compression without adequate recovery can contribute to disc degeneration over many years, particularly if coupled with poor posture, weak core muscles, or pre-existing spinal conditions. The key is proper recovery and supporting spinal health.

Strategies to Mitigate the Effect (and Support Spinal Health)

While you can't entirely prevent this natural phenomenon, you can support your spinal health and potentially lessen the perceived effect:

• Stay Hydrated: Adequate water intake is fundamental for disc hydration and overall bodily function.
• Strengthen Your Core: A strong core (abdominal and back muscles) helps stabilize the spine, reducing excessive movement and distributing forces more effectively during running. Focus on exercises like planks, bird-dog, and dead bugs.
• Practice Good Running Form: An efficient running gait minimizes unnecessary impact and optimizes shock absorption through the legs and hips, reducing direct spinal loading.
• Incorporate Post-Run Stretching and Mobility: Gentle spinal decompression exercises (e.g., hanging from a pull-up bar, cat-cow stretch) can help encourage disc rehydration and improve spinal flexibility.
• Prioritize Rest and Recovery: Allowing sufficient time for your discs to rehydrate, especially through quality sleep, is crucial.
• Vary Your Training: Incorporate cross-training activities like swimming or cycling, which are non-impact and allow the spine to decompress.

When to Be Concerned

While temporary height loss is normal, persistent back pain, radiating pain down the legs, numbness, tingling, or weakness after running are not. These symptoms could indicate a more serious underlying issue, such as a disc herniation or nerve impingement, and warrant a consultation with a healthcare professional, such as a physical therapist or sports medicine doctor.

Conclusion

The sensation of being shorter after running is a fascinating, yet benign, example of your body's intelligent physiological response to mechanical stress. It's a testament to the dynamic nature of your intervertebral discs, which temporarily compress under axial load and then rehydrate during rest. By understanding this process and implementing strategies to support spinal health, you can continue to enjoy the benefits of running while maintaining the long-term well-being of your vertebral column.