How do bioengineers think about running?

From tissue engineering to computer simulations, and from protein design to biomechanical work, bioengineers seem to have a finger in every delicious pie. With every engineering practice comes a need for a common way to measure and communicate parts of whatever it is you're working with. For bioengineers working with biomechanics, this set of standards crystalized most coherently in 1998 when orthopedic surgeon Tom Novacheck published his review titled "The biomechanics of running" (1). This paper consolidated knowledge related to the biomechanics of walking and sprinting while also producing a standard for analyzing human gait, allowing engineers to work toward improving humans' sprinting pace to outrun those tornados. 

Novacheck begins with a description of the generic gait cycle displayed below (1):

There are many phases listed above, but the important information is really just that there are specific states that occur during a gait cycle (meaning one step from starting and ending on the same foot). The specifics related to each of these phases can be found in the original paper (1), but the important overview is the differentiation between the swing state, in which a leg is lifted off of the ground, and the stance state, in which a leg is flexibly planted on the ground supporting the body weight. The most useful definition of these terms is in the ability to describe a gait based on the time spent in each phase for each limb (1):

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In the figure about, each horizontal bar represent the movement of one leg over time, with pairs of bars representing pairs of legs verses . Stance phase (shaded boxes), and swing phase (white boxes), can be observed in alternating patterns, as is expected in a moving individual. Interestingly, the fraction of time spent in stance phase decreases as running speed increases, indicating that less time is spent in contact with the ground while running. With this insight alone, it is possible to define an individual's gait and to measure performance of any engineered sprinting system to a standard. Further, the increase in moving pace corresponds to a transition from overlapping stance phases (the first pair of bars) to overlapping swing phases (the second through fourth pairs of bars), allowing for a technically standard definition of running. 

Novacheck next describes methods for analyzing the muscle control of running movements, primarily the the timing of muscle activity (1):

In this figure, the activity of a specific muscle (y-axis) is represented as a binary value with the black bars representing activated muscles as a function of time (x-axis). Dashed lines mark the transitions between stance and swing phase. The muscle activities were measured using electromyography (EMG), which essentially means sticking an electric sensor on top of someone's skin above a specific muscle and deciding a muscle is active or inactive based on the amount of electrical activity measured (more electricity indicating more muscle activity). Now with this understanding, it is possible to notice the huge lack of muscle activity during the swing phase. During the very end of swing phase and most of stance, all leg muscles measured here show measurable activity. However, during a majority of the swing phase, only one or two muscles show any activity at all. This indicates that the swing phase is almost an entirely passive process, with the only activity existing in the rectus (to pull the leg forward), and the anterior tibial (to keep the toe off the ground). Just by adding this simple measurement of muscle activity, bioengineers gain great insight into the mechanics of running. Understanding that energy is required for transition from swing to stance, but essentially nothing is required for the swing phase itself, provides valuable insight into bioengineers ability to work with methods for improving gait.

Later in the paper, Novacheck reviews current understanding of muscular kinetics (how muscles move and accomplish goals). Though this portion of the paper goes into great technical details, the beginning mentions the four main tasks muscles perform, generating a concrete list of performance tasks that may be measured and engineered for improving gait. These tasks are (1):

1. Shock absorption
2. Balance and posture control
3. Energy generation forward and upward
4. Controlling direction changes of the body

Following this list includes specific, tangible methods for quantifying performance in these tasks. While these tasks describe muscle necessities, they also provide a foundation for aspects of performance that may be engineered to improve gait. For example, realizing that muscles act as shock absorbers could lead to inspiration for engineering design in several ways. Possibly someone could focus on the use of rubber bands or hydraulics as shock absorbers in a foot stilt-like device, allowing for improved shock absorption, and allowing for possibly improved gait. Thinking of muscles as sources of energy generation could lead to easy quantification of mechanical prosthetic performance, maybe comparing the output energy of a device to the typical output energy of standard muscles. Overall, this paper provides with great detail methods for measuring and analyzing gait while also describing knowledge related to the field. This paper has been cited more than five-hundred times already, indicating that their now exists a vast field of knowledge using these fundamental measurements and techniques. As this knowledge base increases, the ability to bioengineer gate will continue to grow.

Citations:

1:

Novacheck, T. F. (1998). The biomechanics of running. Gait & Posture, 7(1), 77–95. https://doi.org/10.1016/S0966-6362(97)00038-6