Optimizing Elastic Energy in Sports Footwear for Enhanced Performance

Optimizing the storage and return of elastic energy in sports footwear can significantly improve running economy and athletic performance. By considering various measurable and quantifiable data points, sports scientists and footwear designers can develop innovative solutions to harness the power of elastic energy and provide athletes with a competitive edge. This comprehensive guide delves into the key factors to consider when optimizing elastic energy in sports footwear.

Longitudinal Bending Stiffness: The Cornerstone of Energy Storage and Return

The longitudinal bending stiffness of a shoe, measured in Nm/deg, plays a crucial role in running economy by influencing the storage and return of elastic energy. Studies have shown that an optimal level of longitudinal bending stiffness can enhance energy storage during the loading phase and facilitate efficient energy return during the propulsive phase of the running gait.

To determine the optimal longitudinal bending stiffness, researchers have developed biomechanical models that consider the individual runner’s characteristics, such as body weight, running speed, and running mechanics. These models can be used to calculate the ideal stiffness value that maximizes energy storage and return, ultimately improving running performance.

For example, a study published in the Journal of Biomechanics found that a longitudinal bending stiffness of 34.0 ± 6.8 Nm/deg resulted in the lowest metabolic cost of running for a group of recreational runners. This suggests that sports footwear designed with a longitudinal bending stiffness within this range can potentially enhance running economy and reduce the energetic cost of running.

Midsole Material Compliance and Resilience: Unlocking the Power of Elastic Energy

The compliance and resilience of the shoe midsole material play a significant role in the storage and return of elastic energy during running. Midsole materials with higher compliance and resilience have been shown to reduce the energetic cost of running by approximately 1%.

To quantify the compliance and resilience of midsole materials, researchers often use force-deflection characteristics and energy return properties as key metrics. Force-deflection curves can be obtained through controlled laboratory testing, where the midsole material is subjected to compressive loads, and the resulting deformation is measured.

The energy return properties of midsole materials can be assessed using techniques such as the drop-jump test, where the material is subjected to a controlled impact and the energy returned during the rebound phase is measured. Materials with higher resilience, as indicated by a greater percentage of energy return, are more effective at storing and releasing elastic energy during running.

By selecting midsole materials with optimal compliance and resilience, sports footwear designers can create shoes that efficiently store and return elastic energy, ultimately enhancing running performance and reducing the energetic cost of the activity.

Footwear Design Features (FDFs): Optimizing Biomechanical Factors

Certain footwear design features (FDFs), such as heel flares, midsole longitudinal flexibility/stiffness, and upper modifications, can significantly impact biomechanical running-related factors (BRFs) associated with common running injuries, including medial tibial stress syndrome (MTSS), tibial stress fractures (TSF), and Achilles tendinopathy (AT).

Biomechanical analyses can be used to quantify the impact of these FDFs on BRFs, providing valuable insights for sports footwear optimization. For example, a study published in the International Journal of Sports Nutrition and Exercise Metabolism found that increased midsole longitudinal flexibility was associated with reduced peak tibial acceleration, a risk factor for MTSS and TSF.

By understanding the relationship between FDFs and BRFs, sports footwear designers can make informed decisions to optimize the design of their products. This can lead to the development of shoes that not only enhance elastic energy storage and return but also mitigate the risk of running-related injuries, ultimately improving overall athletic performance and well-being.

Energy Storage and Recovery: Quantifying the Elastic Energy Cycle

To analyze the effect of plantar pressure distributions on the shoe and quantify the energy exchanges occurring during a running step, researchers have developed a multiple-element, non-linear viscoelastic model. This model can help determine the proportion of input energy recovered from the shoe and the local effects on the foot and lower leg.

The key components of this model include:

1. Plantar Pressure Distribution: The plantar pressure distribution during running can be measured using transducer systems like the EMED-SF system, which samples each sensor at a frequency of 70 Hz. This data provides the force-time inputs for the midsole model.

2. Non-linear Viscoelastic Behavior: The midsole materials exhibit non-linear viscoelastic behavior, which can be characterized using a multiple-element model. This model accounts for the complex energy storage and recovery mechanisms within the shoe.

3. Energy Exchanges: By analyzing the energy exchanges within the shoe-foot system, the model can quantify the proportion of input energy that is recovered and returned to the runner, as well as the local effects on the foot and lower leg.

This comprehensive approach to energy storage and recovery provides valuable insights for sports footwear optimization. By understanding the energy flow within the shoe-foot system, designers can make informed decisions to enhance the storage and return of elastic energy, ultimately improving running performance.

Biomechanical Factors: Optimizing the Energetic Cost of Running

Biomechanical factors, such as peak vertical force (Fz), step frequency, and contact time, can significantly affect the energetic cost of running. By measuring and analyzing these factors, sports footwear designers can identify opportunities to optimize the storage and return of elastic energy.

For instance, a study published in the International Journal of Sports Nutrition and Exercise Metabolism found that a higher step frequency was associated with a lower energetic cost of running. This suggests that sports footwear designed to encourage a higher step frequency, potentially through features that promote a more efficient running gait, can contribute to improved running economy.

Similarly, the peak vertical force (Fz) experienced during running can influence the storage and return of elastic energy. By understanding the relationship between Fz and the energetic cost of running, footwear designers can develop midsole and outsole features that effectively manage these forces, enhancing the overall efficiency of the shoe-foot system.

By considering these biomechanical factors and their impact on the energetic cost of running, sports footwear designers can create innovative solutions that optimize the storage and return of elastic energy, ultimately leading to enhanced athletic performance.

Conclusion

Optimizing the storage and return of elastic energy in sports footwear is a multifaceted challenge that requires a comprehensive understanding of various measurable and quantifiable data points. By considering factors such as longitudinal bending stiffness, midsole material compliance and resilience, footwear design features, energy storage and recovery, and biomechanical factors, sports footwear designers can develop innovative solutions that harness the power of elastic energy and provide athletes with a competitive edge.

Through the integration of advanced biomechanical models, laboratory testing, and in-depth data analysis, the optimization of elastic energy in sports footwear can be achieved, leading to improved running economy, reduced energetic cost, and enhanced overall athletic performance.

References:

1. Hoogkamer, W., Kipp, S., Frank, J. H., Farina, E. M., Luo, G., & Kram, R. (2018). A Comparison of the Energetic Cost of Running in Marathon Racing Shoes. Sports Medicine, 48(4), 1009-1019. doi:10.1007/s40279-017-0811-2
2. Nigg, B. M., Stefanyshyn, D., Cole, G., Stergiou, P., & Miller, J. (2003). The effect of material characteristics of shoe soles on muscle activation and energy aspects during running. Journal of Biomechanics, 36(4), 569-575. doi:10.1016/S0021-9290(02)00428-1
3. Sinclair, J., Atkins, S., Taylor, P. J., & Edmundson, C. J. (2015). The influence of footwear kinetic, kinematic and electromyographical parameters on the energy requirements of steady state running. Movement & Sport Sciences, (87), 37-45. doi:10.3917/sm.087.0037
4. Willwacher, S., König, M., Brüggemann, G. P., & Potthast, W. (2013). Does specific footwear facilitate energy storage and return at the metatarsophalangeal joint in running? Journal of Applied Biomechanics, 29(5), 583-592. doi:10.1123/jab.29.5.583
5. Worobets, J., Wannop, J. W., Tomaras, E., & Stefanyshyn, D. (2014). Softer and more resilient running shoe cushioning properties enhance running economy. Footwear Science, 6(3), 147-153. doi:10.1080/19424280.2014.918184