How human runners regulate footsteps on uneven terrain
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eLife assessment
This paper presents findings from a novel experimental study of the dynamics of human overground running on naturalistically uneven terrain. The terrain used in the experiments has mildly stochastic undulating roughness, a condition that closely resembles many natural terrain conditions, such as trail running. The authors present evidence that humans use open-loop intrinsically stable strategies to run on this terrain, and do not visually guided foot placement. The findings make an important contribution toward understanding the context-dependent role of vision in navigating uneven terrain.
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Abstract
Running stably on uneven natural terrain takes skillful control and was critical for human evolution. Even as runners circumnavigate hazardous obstacles such as steep drops, they must contend with uneven ground that is gentler but still destabilizing. We do not know how footsteps are guided based on the uneven topography of the ground and how those choices influence stability. Therefore, we studied human runners on trail-like undulating uneven terrain and measured their energetics, kinematics, ground forces, and stepping patterns. We find that runners do not selectively step on more level ground areas. Instead, the body’s mechanical response, mediated by the control of leg compliance, helps maintain stability without requiring precise regulation of footsteps. Furthermore, their overall kinematics and energy consumption on uneven terrain showed little change from flat ground. These findings may explain how runners remain stable on natural terrain while devoting attention to tasks besides guiding footsteps.
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eLife assessment
This paper presents findings from a novel experimental study of the dynamics of human overground running on naturalistically uneven terrain. The terrain used in the experiments has mildly stochastic undulating roughness, a condition that closely resembles many natural terrain conditions, such as trail running. The authors present evidence that humans use open-loop intrinsically stable strategies to run on this terrain, and do not visually guided foot placement. The findings make an important contribution toward understanding the context-dependent role of vision in navigating uneven terrain.
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Reviewer #1 (Public Review):
This paper addresses an important issue of how humans select foot placement when running on uneven terrain. The authors examine empirical and model-based data to determine whether runners specifically opt for more level locations on such surfaces. The manuscript suggests potential strategies of how humans mitigate fore-aft impulses during foot contact so as to maintain stability in response to changes in terrain.
Overall, the manuscript provides additional insight into lower-limb mechanics of runners on natural surfaces. The model-based analysis of lower limb compliance is especially useful in the context of stability and understanding human motor control. In addition, participant foot placement analysis using empirical and statistical models is very compelling and provides insight into how much planning …
Reviewer #1 (Public Review):
This paper addresses an important issue of how humans select foot placement when running on uneven terrain. The authors examine empirical and model-based data to determine whether runners specifically opt for more level locations on such surfaces. The manuscript suggests potential strategies of how humans mitigate fore-aft impulses during foot contact so as to maintain stability in response to changes in terrain.
Overall, the manuscript provides additional insight into lower-limb mechanics of runners on natural surfaces. The model-based analysis of lower limb compliance is especially useful in the context of stability and understanding human motor control. In addition, participant foot placement analysis using empirical and statistical models is very compelling and provides insight into how much planning occurs during running on uneven terrain. However, there are a few concerns of note:
- A central motivation of the study appears to be that past research did not incorporate height and slope variations when studying gait on uneven terrain. Although some of the past work cited by the authors does focus on step-like terrain, the (Voloshina, Ferris 2015) study had both height and slope variations. In that particular study, the terrain consisted of blocks that were smaller than the dimensions of the average human foot. This means that, during each foot-flat phase, the foot had to span at least two blocks of different heights, placing it at a slope in the fore-aft direction. Similarly, the columns of that terrain layout provided slope variations in the medio-lateral direction. Referring to this surface as "step-like" is inaccurate and potentially misleading. Considering that the terrains in the present study and the study from 2015 likely cause very similar types of perturbations to the runner, the motivation behind the current study is not strongly validated. The authors should consider re-evaluating their results in the context of past studies.
- A primary outcome of the study is the analysis of empirical and model-based fore-aft impulses experienced by runners during foot contact. The authors suggest that this measure is related to stability but do not provide extensive explanations. It would be helpful to include additional background information on how impulse analyses have been used previously and why they are particularly fitting in this context.
- The study evaluates participants running back and forth on a 24m track for up to 10 minutes. This means that participants had to perform many turns during each trial. The authors present metabolic energy expenditure data but do not address how these data may be skewed due to the large instances of directional changes. Measuring metabolic data during such tasks is generally noise-prone, potentially leading to an inaccurate representation of energy expenditure. Considering that this is a comparative study and participants had to perform such turns for each terrain trial, this issue could be minor. However, the authors are encouraged to provide more detail on experimental protocol. Addressing whether or not participants stepped off the terrain to switch directions and providing insight into how this experimental approach could potentially affect outcomes could be particularly helpful.
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Reviewer #2 (Public Review):
The authors' paper extends their earlier work based on a 2D model of running stability while negotiating sloped terrain of random variable height, extending from a traditional point mass-spring model (SLIP) but with a moment of inertia about the CoM ([19], Dhawale et al. Roy Soc Open Sci 2019). In this study the authors carry out an experimental study of human subjects running over an experimentally-created undulating terrain surface (0.6 m wide x 24 m long) with a known 3D topography, in which they combine a 3D kinematics analysis of foot movement trajectory and placement relative to the terrain topography and in relation to body CoM (hip) movement; with measurements of ground reaction forces to estimate foot-substrate impulses over a subregion of the terrain, and measurements of the runners' metabolic …
Reviewer #2 (Public Review):
The authors' paper extends their earlier work based on a 2D model of running stability while negotiating sloped terrain of random variable height, extending from a traditional point mass-spring model (SLIP) but with a moment of inertia about the CoM ([19], Dhawale et al. Roy Soc Open Sci 2019). In this study the authors carry out an experimental study of human subjects running over an experimentally-created undulating terrain surface (0.6 m wide x 24 m long) with a known 3D topography, in which they combine a 3D kinematics analysis of foot movement trajectory and placement relative to the terrain topography and in relation to body CoM (hip) movement; with measurements of ground reaction forces to estimate foot-substrate impulses over a subregion of the terrain, and measurements of the runners' metabolic energetics via a portable runner-carried gas analyzer system.
The authors' findings are generally supported by their results, showing that runners do not appear to rely on visual guidance to select foot placement on undulating terrain (this based on computational Monte Carlo simulations of foot placement probabilities favoring level terrain surfaces) and likely achieve stability while running largely by means of limb joint compliance that passively adjusts to variable foot-ground impulses (based on ground reaction force estimates and a collisional multi-segment limb joint model for which joint compliance was varied). As a result, the authors found no significant increase in the metabolic cost of uneven terrain versus level surface running.
However, whereas the authors motivate their study by its relevance to the evolution of human running ability and persistence hunting, which requires running over uneven natural terrain, a weakness is that their in-depth analysis is heavily focused on the mechanics and resulting energetics of running over undulating terrain in the context of foot placement strategies for maintaining stability and whether this depends on visual guidance of foot placement relative to the terrain. The authors claim surprise (Discussion, l.191-192) that the runners do not appear to rely on visual information about unevenness to guide their footsteps. However, based on the nature of their sloped undulating surface, their results were unsurprising to this reviewer.
The authors' study was also motivated to examine the effect of sloped surfaces on running biomechanics, as previous studies have examined step-like terrain comprised if piecewise level blocks or step height transitions, which the authors (correctly) note represent obstacle negotiation rather than how runners may be challenged by undulating sloping terrain. The authors argue (l. 5-6) that a combination of height and slope variations like a natural undulating terrain will be more challenging than one that involves only step height transitions. However, the basis for this statement is not clear. And, indeed, the results the authors find for humans running over a sloped, undulating terrain (height range ~ 40 mm) shows that a sloped, undulating terrain does not actually present a significant challenge, given that it appears to require little or no visual guidance of foot placement and no significant increase in metabolic energy use. To the contrary, this reviewer would argue that obstacle avoidance is the more challenging feature of natural terrains that must be successfully negotiated, which is a common experience for trail runners. The reviewer, therefore, fully agrees with the authors' conclusion (l. 259-261) "Our data thus suggests that terrain-guided foot placement strategies are not required for stability on gently undulating terrain [compared with obstacle avoidance on more complex terrain]".
The principal novelty and value of the authors' study is the analysis of fore-aft impulse and the role of limb joint compliance for adjusting to changes in fore-aft impulse to favor running stability. The authors' paper suffers from overstating the broader relevance of its findings and by merging methods and discussion with the results that it reports. The methods, themselves, are detailed and thorough in their description, and the authors' modeling approaches appear sound, sophisticated and appropriate for the analyses of foot placement strategies and limb compliance in relation to collisional impulse.
Repeatedly in the Results section, however, these methods are summarized when reporting a result (based on the method) and discussion points are mentioned. Specifically:
l. 66-96 This starting section does not present results per se, but a summary description of experimental methods an analytical approach. Actual results findings are not presented until l. 97.
l. 111-117: This summarizes analytical methods; not results per se.
l. 135-145: Summary of methods/analytical approach continues to be blended in with results in these sections.
l. 169 - Comparison of limb retraction rate on uneven vs level terrain of human subjects here with running birds is fine for discussion but not results per se.
l. 170-171: This is a discussion point, not a result.
l. 187-189: Again, discussion not a result.
A final concern is whether and how the requirement that runners repeatedly decelerate, turn and reaccelerate to run back and forth over the 24 m long uneven and level terrains at 3 m/s affects the metabolic measurements? Running at 3 m/s indicates 8 sec to traverse the runway length and, if adding another second for turning to reverse direction and run back = 9 s, this would indicate for a 8 to 10 min metabolic running trial ~53 to 67 turns per trial. Presumably, these would have an effect on running cost.
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