Over the years, many achievements have been made in prosthetics.

Recent decades have seen remarkable advancements in prosthetics and orthotics technology. Current devices range from relatively simple but elegant prosthetic hands made inexpensively with a 3D printer, to bionic foot-ankle devices that sense the environment and respond like a biological limb.

As technology facilitates improved prosthetic designs, the focus of rehabilitation for transtibial (i.e., below-knee) amputees is moving from restoring very basic locomotion functions toward enabling individuals to return to healthy active lifestyles.

Approximately 20 years ago, engineers began developing running-specific prosthetic (RSP) devices comprised of a socket and carbon-fiber leaf spring aimed at emulating the spring-like behavior of biological legs. These devices have not only enabled many individuals to participate in activities such as recreational running, but athletes have now been able to use them to sprint fast enough to qualify for the modern-day Olympic Games. Yet despite these miraculous achievements, remarkably little is known about how well these devices emulate biological function or how individuals adapt their neuromuscular system to control them.

Mimicking biology

Research studies recruiting elite-level, Olympic-caliber sprinters have begun to provide us some answers to these questions. For example, we know that RSPs are able to store and return large amounts of elastic energy, similar to the muscles and tendons of biological limbs, but limit how force can be applied to the ground. And for athletes with a single amputation, there are trade-offs between maximizing the output from the prosthetic device and maintaining symmetry between the two legs.

Clearly these passive devices are not capable of reproducing all of the functions of the muscles, tendons and bones lost to amputation. Elastic structures can only store and return energy, whereas muscles are highly dynamic systems that can interchangeably act as motors, brakes or even struts.

Such flexibility in mechanical function enables the same muscles to provide power out of the sprinter’s blocks and then shift to behaving like a spring once top speed is reached. Because of this, the behavior of a leg using a RSP during sprinting is too spring-like compared to the action of a biological leg. And yet through years of hard work and training, athletes have learned how to use these devices to sprint at world-class speeds.

And while most amputees will never reach the Olympics, much can be learned from individuals operating at the extremes of performance. These athletes help us determine what the boundaries of neuromuscular plasticity actually are and provide an enhanced look at how loading patterns may ultimately effect intact muscles and joints. Information from these studies can be applied broadly to the amputee community by using data to set goals for rehabilitation or to know when short-term discomfort might be leading to long-term damage.

A question of control

By understanding what mechanical functions prosthetics can and cannot replace, we are faced with a question of how individuals compensate for functions that are lost. If a device cannot provide the powerful push-off that most of us take for granted during walking, then other muscles must be recruited to power the leg through its swing. The need to understand how individuals adapt to these changes in mechanical requirements goes well beyond trying to improve athletic performance.

A large percentage of people with leg amputation suffer from secondary physical conditions such as osteoarthritis, osteoporosis and chronic back pain that can limit activity and reduce quality of life. But while modern RSP devices can enable individuals to comfortably participate in activities such as running and competitive sports, the forces developed within the residual musculoskeletal system required to control these devices may result in abnormal loading patterns that could lead to painful secondary osteopathic conditions. Understanding the interactions between our biological system and mechanical devices that we design remains a pivotal area in prosthetics and orthotics research. Interdisciplinary teams of biologists, engineers, physiologists and prosthetists are on the forefront of this research. The outcomes of this work are forming the foundation for developing next generation prosthetic designs, rehabilitation strategies and training programs that improve performance and enable lifelong healthy, active lifestyles.

Craig P. McGowan, Ph.D., is an assistant professor in the Department of Biological Sciences and the WWAMI Medical Education Program at the University of Idaho. McGowan’s primary focus and research includes neuromuscular biomechanics, motor control of locomotion, evolution of musculoskeletal design and musculoskeletal modeling and computer simulation.