Reading time: 2 min
A thought experiment. Imagine you’re running fast on the road. Then, suddenly, the road ends and you’re running fast on a sandy beach. Your running surface goes from hard to soft in an instant.
What happens? Most people will feel their legs trip up under them, maybe even fall. Why? On soft surfaces, more energy “leaks” into the ground compared to a hard surface. Hence, your legs need to “stiffen” to prevent you from sinking down. Until your stiffness adjusts, you’ll feel like you are losing your balance.
“Stiffness” in this context refers to the combination of the bend (or lack thereof) in the hip, knee, ankles and feet plus the tension in the muscles, tendons and connective tissues.
In general, hard surfaces require less stiffness and soft surfaces more stiffness.
Now, think about this: What if you’re always running on a hard surface (like a road) with the stiffness usually reserved for a soft surface (like grass)? Or vice-versa?
At least, there will be a decrease in performance. At worst, there will be certain types of injuries that will recur. Too much stiffness is associated with bony injuries, while too little stiffness is associated with soft tissue injuries.
Optimal Stiffness
Based on anatomical factors like limb length and muscle fibre distribution, and biomechanical factors like your running style, you have an optimal stiffness.
However, if you do not give your body the opportunity to explore different surfaces in your training, you may get locked in on a stiffness that is inappropriate for you.
If you are prone to soft tissue injuries like muscle strains or tendonitis, you are likely running with insufficient stiffness. There is too much give in the system. Muscle and tendon are overloaded controlling your foot strike.
If on the other hand bone injuries like stress fractures plague your training, your stiffness is too high. Your rigidity on foot strike doesn’t allow the soft tissues to absorb the strain. Bony structures take up the load.
Running Surface
The best way to teach your body to naturally adjust it’s stiffness to an “optimal” level is to run on varying surfaces often. Changing surfaces is like taking a tuning fork to your kinetic chain. Let me explain.
Your body’s internal feedback loop aims to maintain your center of mass (CoM) moving in a consistent pattern. When the surface changes, the motion of your CoM is disrupted. For example, when going from a hard to a soft surface your CoM will lower if your stiffness remains unchanged. However, your movement sensors immediately detect the drop in your CoM and inform your nervous system. Your stiffness is then tweaked until the motion of your CoM is back to your normal.
Imagine each new surface as a tweak to a set of guitar strings to get the pitch just right. And, just like an experienced guitarist that can tune his instrument by ear, your CNS slowly learns to dial into the appropriate Kinetic Chain stiffness for each surface.
Knowing this, now think about your neighbourhood. The roads, the parks, the trails nearby. The long stretches of grass or compact dirt or gravel. The concrete sideways. The sandy playgrounds or beaches. So many surfaces to choose from. These are gifts from the Running Gods. Use them all!
If you found this interesting, take a moment to read my article on BASE Running. I discuss running over variable terrains among other interesting ideas you can incorporate as foundational elements of your run training.