Beneath the Surface: A Look at the Impact of Surface on Running by Jamie Hershfang

[This is a guest blog from Jamie Hershfang. Jamie is an exercise and movement science major at Lewis University in Chicago, IL. With a background in cross country and track and field, she is currently a sport performance coach intern at Athletic Lab.]

As any runner knows, volume and intensity of training are the most critical elements to success. However, other key factors are often overlooked. When deciding where to run, it is important to consider the properties of each surface for optimal training adaptations and performance. Runners tend to be creatures of habit, often running the same routes, at the same pace, at the same time of the day, every day. With the repetitive nature of distance running itself and the monotony of training, biomechanical imperfections are constantly reinforced. Different surfaces allow for various running techniques and loading patterns, incorporating a variety of tissue stresses and reducing the risk of overuse injuries. As more body parts are engaged through variations in training, physiological adaptations are more significant, improving long term performance even further.

There is a constant debate about running on softer surfaces for injury prevention. While there has been research showing an increase in the force on the legs while running on harder surfaces, there are no studies that connect that to increased rates of injury or a change in performance. A possible explanation for this comes from understanding leg stiffness and how the leg interacts with different surfaces while running.

During impact with the ground, muscles and tendons in the leg function as a spring, absorbing and releasing energy during the gait cycle. Similarly, your shoes and the surface you run on also function as springs. Leg stiffness is pre-tuned before impact for each individual foot strike. Combining feedback from the previous stride and information stored in the brain, the body anticipates the stiffness of the surface you?re running on and adjusts how strongly the leg muscles contract before impact (3).?Interestingly, if you are running on a hard surface, your legs are less stiff than they are on a softer surface. Studies have found that your legs bend more to absorb the shock on a stiff surface, but your legs stay stiffer on a softer surface (4). To put this into perspective, think of the hard ground as a stiff spring and the soft ground as a loose spring. This can help better understand the effects of the various training surfaces. The following sections will cover how different surfaces can influence the impact forces and various parts of the body during a run.

There is a common misconception that harder surfaces allow for the greatest speeds. Considering the stable ground for push off, it seems logical to think you can push off harder and propel yourself forward with more force. However, asphalt is actually so dense that it dampens the ability of the legs to gain energy return. Because of this damping effect, the asphalt creates such a large impact that it overpowers the elastic energy return to the legs (2). This creates a shock to the muscles as they try to absorb the impact, but are not readily prepared to receive it. Due to the nature of this surface, the body tries to protect itself by extending the landing phase leading to longer ground contact time. However, these longer contact times result in slower speeds, the opposite of what we initially thought to be true. ???????????

Getting off the beaten path, running on grass provides many benefits, returning elastic energy to the legs without the hard impact like the road. It can be used to build leg strength and more efficient turnover with a lower risk of injury. However, it may take some time for the body to adapt to the grass. Some runners often incorporate barefoot running to strengthen the ankles, feet, and improve balance. Starting with a relatively soft and flat surface such as turf, and transitioning to more uneven grass allows the body to adapt to the new surface. Runners should start with 15 to 20 minutes of easy grass running 1 to 2 times per week, allowing up to 3 weeks for a full transition period (5). When used for speed work, it is critical to allow this time to prevent injuries stemming from instability and soft tissue injuries. Starting with strides, and building up to longer intervals, training on grass can be useful for all runners. Regardless of fitness levels, gradually increasing the volume and intensity of workouts on the grass will provide the greatest long term benefits.

Running on softer surfaces generally comes hand in hand with uneven ground. Irregular surfaces, such as on a trail, creates variation with each step. Compared to running countless miles on the road, uneven terrain changes muscle activation and distributes more work to the knee joint. When the surface isn?t flat, runners generally focus more on foot placement, causing the hips and knees to work harder. By incorporating a variety of movements to achieve the ideal foot placement, more stabilizer muscles are recruited, which also increases the demand for oxygen (4). This is often why trail running feels harder than most other surfaces. To maintain the same pace and effort, runners must react quickly and increase leg turnover depending on the technicality of the trail. This can help with speed development and stride efficiency.

For people who suffer from chronic shin splints, it may also be beneficial to run on uneven surfaces at least once a week to activate the shin muscles differently. For those who suffer from plantar fasciitis, the risk of over pronating on this soft, uneven terrain increases due to the extra torque on the plantar fascia ligament that runs along the bottom of the foot (2). With any type of ankle instability, ligaments may be more susceptible to repeated sprains, especially on technical trails with a lot of roots or rocks that cause the feet to land at an angle.

With a very compliant surface such as sand, it is important to consider the damping effects and history of injury. Ground contact time and step length have been shown to increase with sand running (1). Deep sand running provides the benefit of resistance training with minimal impact on the joints. However, from a performance perspective, it is best to stay away from sand unless it is hard-packed. Due to its relatively unstable surface, it generates a greater impact on the hips, knees, and ankles. With a history of Achilles tendon issues or ankle sprains, it may be best to avoid running barefoot in sand or avoid it completely. Due to the high rate of muscle fatigue associated with sand running, it may provide the greatest benefit for shorter distance runs if incorporated into training.

One of the greatest surfaces for competitive events, tracks contain the ideal amount of stiffness to produce the greatest energy return while also balancing impact forces, generating peak speeds among athletes. These track surfaces have evolved over time and many competitive tracks are now made of a rubber compound. It has been seen that synthetic tracks made from such material produced improvements in running speed by two to three percent compared to other surfaces (4). It provides a hard surface similar to asphalt, but generates an increased energy return which benefits the runner. However, when doing interval training on a track, it is important to understand that there is an unequal torque on the feet and legs due to the consistent left turns. Therefore, unless the athlete performs exclusively on a track, it may be beneficial to alternate directions between intervals to balance the torque.

For beginners or athletes looking for a controlled setting, treadmills are often a popular choice. Some studies have shown that the plantar forces of the foot were lowest with treadmill running, a common assessment of injury risk. The cushioning of the belt may also reduce stress on the back, knees, hips, and feet. It is a relatively even and consistent surface, however, the compliance may vary between machines. Due to the moving belt, runners can propel themselves forward with less force at greater speeds. However, the repetitive stress on the same joints, created either by inefficient biomechanics or simply the extended periods of time training on the treadmill, may cause muscle imbalances and generate a limited range of motion. Compared to over ground running, treadmills do not take environmental factors into consideration. If incorporated into training, runners should adjust the setting of the treadmill to an appropriate grade to reflect the energetic cost of outdoor running. While the accuracy of pace varies on machines, treadmills can be a relatively useful tool to run specific workouts in specific conditions if desired.

When seeking an answer to identifying the best running surface, it is really a matter of variety in training. Varying surfaces stress different parts of the body, reducing injuries from the repetitive nature of running. Before you begin changing your running surface every day, it is important to emphasize that injuries can also come from drastic change. While variety helps strengthen leg muscles, it?s imperative to ease into running on new surfaces. Complexity should gradually increase as you progress, similar to volume and intensity of workouts. When training for a specific race, training should be more focused on the same surface as the upcoming race to prepare the body for the physical demands on that day. While it is crucial to vary your training to reduce injury risks, the surface you choose depends primarily on experience and training priorities.

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