Karl Zelik is the CSO and Co-Founder of HeroWear. An accomplished scientist in the fields of engineering and biomechanics, Karl pioneers research, development, and technology that augments human performance and health.
A mechanical engineering professor at Vanderbilt University since 2014, Karl holds secondary appointments in the departments of Biomedical Engineering and Physical Medicine & Rehabilitation. He also co-directs the Center for Rehabilitation Engineering & Assistive Technology (CREATE) at Vanderbilt University, which aims to improve health, mobility, and independence for individuals with disabilities and to enhance human capabilities beyond biological limits.
Freelap USA: Wearable IMUs in running are growing in popularity, but they are only as good as the math used. Can you tell me how important the need is to have a development team that understands biology, physics, and coding? Many of the IMU sensors fail because of the sensor quality, but even good hardware fails because of poor engineering. What should professional teams do to help vet this besides expecting a single sport scientist to test it?
Karl Zelik: It takes an interdisciplinary team to make the most of wearable sensors—to ensure reliable data is collected and then processed in a way that provides accurate and actionable insights. In our Center for Rehabilitation Engineering and Assistive Technology at Vanderbilt University, we’ve benefited from close collaborations with biomechanists, roboticists, engineers, data scientists, and clinical specialists (in sports medicine, orthopedics, physical medicine, and rehabilitation).
Wearables for sport are rapidly growing and evolving, which is super exciting! But it also means it’s incredibly important (and not always easy) to match the right monitoring tool with the right use cases, where it can provide value. When this is accomplished, there are significant opportunities to improve scientific understanding, as well as the training, health, and performance of individuals and organizations. However, there are also a myriad number of ways to misuse wearable technology, as it was never intended or validated.
Every system will have pros and cons, and appropriate and inappropriate applications. There’s an interesting balance between exploring new, unprecedented ways to leverage real-world wearable sensor data, and not getting too far ahead of ourselves by over-hyping, over-interpreting, or over-generalizing wearable sensor data and capabilities.I’d love to see more partnerships between private industry (including sports teams and wearable technology manufacturers) and the academic sector. Both have something valuable to offer. Click To Tweet
Sport wearables are an exciting but still fairly nascent field, which I believe will grow best through collaboration and transparency. I would encourage sport organizations exploring new wearable technologies to engage with scientific and clinical experts (both internal and external to their organization) who can help vet systems for intended applications. I’d love to see more partnerships between private industry (including sports teams and wearable technology manufacturers) and the academic sector. Both have something valuable to offer, and I think together they are more than the sum of their parts when it comes to driving innovation, technology translation, trust, and transparency in the field of sports wearables.
Freelap USA: Ground reaction force (GRF) is often used to estimate workload with athlete tracking devices. Can you share how simple force measures may not indicate the true biological strain on a body? Just having an athlete run over a force plate in various sporting actions is a good start but summarizing or calculating their work done isn’t the complete story.
Karl Zelik: You are absolutely right. During running, the peak ground reaction forces (between the shoe and the ground) are typically only 2-3x body weight, whereas the forces on the bones, muscles, and joints of the lower limb can be 6-14x body weight. The ground reaction force alone does not necessarily tell you much about the stress or strain felt by biological tissues like muscles and bones inside the body. In fact, we found in our 2019 study led by Emily Matijevich1that the most commonly used ground reaction force metrics in running research were generally not strongly correlated with peak forces on the tibial bone, a common place for runners to develop a bone stress fracture.
Ground reaction forces are commonly misunderstood, misused, and misinterpreted in an effort to understand physical performance, athlete workload, or musculoskeletal injury risks.2 I often share this analogy: If a mechanical component inside your car engine was breaking down due to repetitive loading, and you were trying to diagnose or prevent the problem from happening in the future, you would not just measure the force between the car tires and the ground to find answers. Likewise, one should not assume that ground reaction forces under your running shoe provide insight on the loading, wear and tear, or workload experienced by musculoskeletal tissues inside your body.One should not assume that ground reaction forces under your running shoe provide insight on the loading, wear and tear, or workload experienced by the musculoskeletal tissues inside your body. Click To Tweet
There was a terrific perspective/review article published recently by Stuart Warden, Brent Edwards, and Richard Willy (2021)3 that I would strongly encourage anyone interested in bone stress injuries or workload metrics to read. It’s accessible as an introduction to these topics, a nice jumping off point to other literature, and it’s also packed with fascinating insights about bone stress injury science and clinical interventions. It really drives home the need to develop tools that can monitor tissue-level musculoskeletal forces (not just ground reaction forces or body segment accelerations), in order to understand microdamage (which can accumulate in tissues to cause overuse injuries), and ultimately to better understand and manage overuse injury risks (due repetitive tissue loading). For extended reading on this topic, Judd Kalkhoven, Mark Watsford, and Franco Impellizzeri provided a nice, detailed conceptual framework in their 2020 publication4.
My research team at Vanderbilt has been working on this challenge of noninvasively monitoring tissue-level forces, and just in the last two years we have had a couple breakthroughs in developing practical and accurate wearables for this purpose: one for monitoring tibial bone forces in runners5 and another for monitoring low back loading during manual material handling6. By combining pressure-sensing insoles, an inertial measurement unit (containing accelerometers and gyroscopes), musculoskeletal modeling, and machine learning algorithms, we have been able to demonstrate the feasibility and accuracy of these systems for estimating tissue-level forces and microdamage trends.
There is also some really cool work being done in the labs of Darryl Thelen and Peter Adamczyk at the University of Wisconsin, where they are using a wearable tensiometry system to estimate Achilles tendon forces7,8. As a field, we continue to actively advance biomechanics research and validation on these new wearable systems and algorithms, and I’m optimistic that in the next few years these have the potential to translate into real-world use (commercial devices) that can provide a leap in measurement capability and physiological insight relative to the current generation of wearables.
Freelap USA: Exoskeletons are growing in the labor and construction space. Knee braces are common with ACL injuries as well as armor for the elbow. Where do you see wearable exoskeletons going in the future with sports?
Karl Zelik: I’m highly enthusiastic and optimistic about the trajectory of exoskeletons based on recent progress in the development and adoption of these devices across various sectors (e.g., industrial, medical, recreational). Exoskeletons are wearable devices that assist, support, or enhance movement or posture. Despite the Hollywood imagery that this term evokes, exoskeletons are becoming increasingly practical, accessible, and affordable. The field of exoskeletons turned a corner several years ago when it stopped trying to do everything—when it stopped trying to make Iron Man—and started listening deeply to end users and designing devices that match specific user needs.
I’ve been involved in the research and validation of these exo technologies (as a scientist and engineering professor at Vanderbilt University), in the commercialization and implementation of exoskeletons (as co-founder and Chief Scientist at HeroWear, an exosuit manufacturer and spin-off company from my lab), and also in industrial standards development (as a member of the ASTM F48 standards committee on exoskeletons and exosuits). I can tell you there have been huge advances in the exoskeleton field in the last decade—not just in the effectiveness and usability of the technology itself, but also in the scientific validation, the industrial standards developed, and the early successes with end users and organizations (e.g., with occupational shoulder exoskeletons at Toyota factories).
There have been some interesting exoskeletons developed for skiing and other recreational activities. And I expect there will be many opportunities to develop targeted exoskeleton solutions that support specific aspects of sports; for instance, to improve training, facilitate return to play, rehabilitate injuries, or even enhance physical performance.
One sub-class of exoskeletons—called soft exosuits—may be of particular interest and relevance to the future of sport. These are exoskeletons made primarily from soft materials like textiles and elastomers, which allows these devices to be worn more form-fitting like performance apparel or a small hydration pack. I’ve been researching and developing exosuits for the last five years, primarily for the occupational sector. For instance, our R&D on back-assist exosuits has been highly visible over the last few years9, and it led to the creation of HeroWear, which commercialized this technology into the Apex exosuit used by warehouse workers and others in construction, manufacturing, etc.
Somewhat less known is that our team at Vanderbilt also created a similar, clothing-like exosuit to assist the calf muscles. In our Yandell et al. 2019 study10, we showed that this 1-pound, passive-elastic (i.e., unmotorized) exosuit—which looks a bit like a calf sleeve with a custom insole—reduced soleus (calf) muscle strain by up to 17% (assessed via muscle activity). I could envision exosuits like this being adapted to support return-to-play applications or even being used to enhance physical performance capabilities (e.g., by reducing muscle fatigue during running). These are just examples, but more broadly I see a lot of opportunity for innovative new exo technologies to be created for targeted sport applications or just as recreational tools/accessories to help keep people physically active and doing the things they love.Exosuits have the potential to enhance sport or physical performance, and I fully expect conversations about allowable exo technology & the unfair advantage they provide in the not-too-distant future. Click To Tweet
It wasn’t too long ago that advances in swimsuits were shaking up that sport. And new running shoe designs have been capturing headlines recently over performance advantages. Exosuits have similar potential to enhance sport or physical performance, and I fully expect conversations about allowable exo technology and the unfair advantage they provide in the not-too-distant future. I was a track and field athlete (long and triple jumper) for many years but am no longer at a stage where I’m trying to maximize my performance. Nevertheless, when I’m a bit older, I do fully expect to have a hiking exosuit so I can more easily keep up with my kids on the trails, and perhaps a weekend warrior exosuit to save my back during gardening and home improvement projects.
Freelap USA: Metabolic demand increases with external loading, such as weight vests. Could you get into how the location of wearable loads matters, as there is a big difference between external loading prominently and distally?
Karl Zelik: There have been a number of great studies demonstrating that it is more energetically expensive to carry additional weight distally (e.g., on the lower legs or feet) as compared to near one’s pelvis or core11. There’s a pretty big metabolic penalty for carrying weight distally. For instance, Franz, Wierzbinski, and Kram12 found that oxygen consumption increased by about 1% during running for every 100 grams added to the foot.
This understanding has been important in informing the design of athletic footwear as well as wearable devices like exoskeletons, and it underlies the common goal of minimizing weight added distally to the limbs. In addition to the location of load carriage, it is also important to consider how the load attaches to the human body, as this can affect muscular and metabolic demands of load carriage.In addition to the location of load carriage, it is also important to consider how the load attaches to the human body, as this can affect muscular and metabolic demands of load carriage. Click To Tweet
As a brief aside, there is some really fascinating research on how Nepalese porters13 and African women14 carrying head-supported loads achieve surprisingly economical locomotion. These and other studies suggest that there may be clever ways to use passive dynamics (e.g., elastic oscillations) or other biomechanical insights15 to help minimize the metabolic demand increases due to load carriage.
Freelap USA: The foot acts differently during walking, running, and sprinting. With an increase in velocity, the foot will adapt to the functions and demands imposed on it. Can you share how velocity and elastic energy change as speed increases? Why is this important for footwear?
Karl Zelik: The human body is amazing at adapting its dynamics for a wide variety of movements (walking, running, sprinting, jumping, cutting, etc.). A deep understanding of movement biomechanics helps us design technologies (from footwear to wearables to exoskeletons) that can support a targeted subset of tasks, for instance by reducing physical demand or enhancing performance. There are clever ways to use elastic energy storage and return to augment human movement.
Sutrisno and Braun16 recently presented a great, forward-looking example. They created a theoretical model showing how human augmentation exoskeletons have the potential to increase top running speed by more than 50% by modulating elastic stiffness, even without providing external energy. In addition to exo technologies, there continues to be a lot of ingenuity and exploration in how to optimize footwear for different sports and activities.
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1. Matijevich, E. S., Branscombe, L. M., Scott, L. R., and Zelik, K. E. “Ground reaction force metrics are not strongly correlated with tibial bone load when running across speeds and slopes: Implications for science, sport and wearable tech.” PloS One. 2019;14(1):e02100002. Vigotsky, A. D., Zelik, K. E., Lake, J., and Hinrichs, R. N. “Mechanical Misconceptions: Have we lost the “mechanics” in “sports biomechanics”?” Journal of Biomechanics. 2019;93:1-5.
2. Warden, S. J., Edwards, W. B., and Willy, R. W. “Preventing Bone Stress Injuries in Runners with Optimal Workload.” Current Osteoporosis Reports. 2021:1-10.
3. Kalkhoven, Judd T., Mark L. Watsford, and Franco M. Impellizzeri. “A conceptual model and detailed framework for stress-related, strain-related, and overuse athletic injury.” Journal of Science and Medicine in Sport. 2020;23(8):726-734.
4. Matijevich, E. S., Scott, L. R., Volgyesi, P., Derry, K. H., and Zelik, K. E. “Combining wearable sensor signals, machine learning and biomechanics to estimate tibial bone force and damage during running.” Human Movement Science. 2020;74(2):102690.
5. Matijevich, E. S., Volgyesi, P., and Zelik, K. E. “A Promising Wearable Solution for the Practical and Accurate Monitoring of Low Back Loading in Manual Material Handling.” Sensors. 2021;21(2):340.
6. Martin, J. A., Brandon, S. C., Keuler, E. M., et al. “Gauging force by tapping tendons.” Nature Communications. 2018;9(1):1-9.
7. Harper, S. E., Roembke, R. A., Zunker, J. D., Thelen, D. G., and Adamczyk, P. G. “Wearable Tendon Kinetics.” Sensors. 2020;20(17):4805.
8. Lamers, E. P., Soltys, J. C., Scherpereel, K. L., Yang, A. J., and Zelik, K. E. “Low-profile elastic exosuit reduces back muscle fatigue.” Scientific Reports. 2020;10(1):1-16.
9. Yandell, M. B., Tacca, J. R., and Zelik, K. E. “Design of a low profile, unpowered ankle exoskeleton that fits under clothes: overcoming practical barriers to widespread societal adoption.” IEEE Transactions on Neural Systems and Rehabilitation Engineering. 2019;27(4): 712-723.
10. Browning, R. C., Modica, J. R., Kram, R., and Goswami, A. “The effects of adding mass to the legs on the energetics and biomechanics of walking.” Medicine & Science in Sports & Exercise. 2007;39(3):515-525.
11. Franz, J. R., Wierzbinski, C. M., and Kram, R. “Metabolic cost of running barefoot versus shod: is lighter better?”. Medicine & Science in Sports & Exercise. 2012;44(8):1519-1525.
12. Bastien, G. J., Schepens, B., Willems, P. A., and Heglund, N. C. “Energetics of load carrying in Nepalese porters.” Science. 2005;308(5729):1755-1755.
13. Heglund, N. C., Willems, P. A., Penta, M., and Cavagna, G. A. “Energy-saving gait mechanics with head-supported loads.” Nature. 1995;375(6526):52-54.
14. Kuo, A. D. “Harvesting energy by improving the economy of human walking.” Science. 2005;309(5741):1686-1687.
15. Sutrisno, A. and Braun, D. J. “How to run 50% faster without external energy.” Science Advances. 2020;6(13):eaay1950.