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Biomechanics of the transition from symmetrical to asymmetrical gaits in humans

Pieter Fiers (UGent)
(2014)
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Abstract
Background and purpose: In humans walking and running are the preferred and most commonly used gaits, however alternative options to move about exist. One option is unilateral skipping, which is also known as bipedal galloping. In contrast to walk and run, bipedal gallop is an asymmetrical locomotion pattern, both in the temporal and spatial dimension, which means that the successive foot falls are not evenly spaced in time and that one leg remains positioned in front of the other. As such, a trailing and leading leg, each with specific functions, can be distinguished. In gallop, the trailing foot is the foot that contacts the ground first. The leading foot strikes the ground, generally when the trailing foot is still in contact with the ground. This generally results in two single support phases separated by a short period of double support. Following push-off of the leading leg, a relative long flight phase emerges. While gallop is the preferred gait at higher speeds in many animals, human bipedal gallop does not occur spontaneously except in very specific conditions such as during the rapid descent of steep inclines. The aforementioned observations raise two questions: 1) Why do humans not use this gait for fast locomotion on level-ground, and 2) Why do humans use this gait for the rapid descent of steep inclines? This thesis addresses these questions. In doing so, insights into the execution and control of human asymmetrical locomotion patterns and, more generally, of human gait are expanded. Methods: To address the first question, subjects were instructed to gallop and run at a self-selected speed over an instrumented runway. Measurements of the ground reaction forces together with the segments' kinematics enabled an in-depth comparison of the mechanics of both gaits. In a separate treadmill experiment, the metabolic costs of running and galloping were determined. To address the second question, two experiments were conducted. The first experiment focused on running and galloping down steep inclines, whereas the second experiment involved transitions to these gaits. In the first experiment, subjects ran and galloped down an instrumented, steep incline at a given speed. Again, ground reaction forces and segmental kinematics were recorded, allowing a thorough comparison of both gaits when descending. In the second experiment, transitions from walk to run and from walk to gallop on the level were investigated by analyzing the segments' kinematics during these transitions. Differences between these transitions on level-ground provide useful insights to explain the spontaneous transitions that occur during steep downhill locomotion. Results and discussion: Question 1: "Why do humans not use bipedal gallop for fast locomotion on level-ground?" The asymmetrical nature of gallop involves distinct hip actions and foot placement, which give the legs different functions compared to run. The trailing leg is initially placed almost under the body and therefore it decelerates the body in the vertical direction, while accelerating it in the direction of movement. The leading leg is placed more in front of the body and acts in the opposite way to the trailing leg: it decelerates the body in the direction of movement, while accelerating it vertically, propelling it into the flight phase. These specific leg functions, which are mainly governed by actions of the hips, involve more energy dissipation and generation compared with running, resulting in a higher metabolic cost and higher levels of muscular stress at the hips. These reasons explain why running is favoured over galloping for fast locomotion on level-ground. However, it is also possible that other reasons, such as challenged gaze stability - not investigated in this thesis - may explain why gallop is not spontaneously used in humans. Question 2: "Why do humans use bipedal gallop for the rapid descent of steep inclines? During the descent of steep inclines, the tangential component of the gravity constantly tends to accelerate the body in the direction of movement. In order to descend at a constant speed, the legs have to compensate this accelerating force by absorbing mechanical energy. Compared to downhill running, downhill galloping involves shorter strides in which relatively more time is spent during foot contacts. Moreover, the specific orientation of the galloping legs enables a functional distinction between the legs, resulting in different amounts of work absorbed by the joints of each leg. Furthermore, the more vertical initial contact position of the trailing leg during galloping facilitates an initial forefoot contact, which is adopted by half of the subjects. Regardless of the initial foot contact pattern of the trailing galloping leg, the maximal absorption powers in and the amount of absorbed work by the knee extensors of the leading leg are lower compared with those of a running leg. Additionally, the maximal absorption powers in the trailing galloping knee extensors are lower than in running. However, the amount of absorbed work by these muscles is only lower when the trailing leg initially contacts the ground with the forefoot (because of a shift in power from the knee to the ankle). Since maximal absorption powers and large amounts of absorbed work are associated with muscle soreness, galloping probably results in less soreness in the quadriceps compared to running after rapidly descending steep inclines. In addition to the advantages associated with the galloping pattern itself, the characteristics of the two-step walk-to-gallop transition may also explain the spontaneous use of bipedal gallop when descending. This two-step transition involves: 1) a lowering and acceleration of the body in the direction of movement by the trailing leg, and 2) a pivoting action of the leading leg that decelerates the body in the direction of movement and propels it in a first, high flight. Although humans may not spontaneously do a transition from walk to gallop on level-ground, we argue that this transition is easier to perform when descending based on its mechanics as found during level transitions. Moreover, the walk-to-gallop transition during descents may even have benefits (e.g. easier to control locomotion speed) over the walk-to-run transition, again provided that this walk-to-run transition is performed in a mechanically similar way as on level-ground. In conclusion, whereas bipedal gallop does not spontaneously emerge on level-ground because it has a higher cost of transport and involves higher muscular stress, bipedal gallop is spontaneously used when descending steep inclines. In this dissertation, possible reasons associated with the galloping gait itself as well as with the transition to it are identified explaining the spontaneous use of gallop during the descent of steep inclines.

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Citation

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MLA
Fiers, Pieter. Biomechanics of the Transition from Symmetrical to Asymmetrical Gaits in Humans. University of Antwerp. Faculty of Science ; Ghent University. Faculty of Medicine and Health Sciences, 2014.
APA
Fiers, P. (2014). Biomechanics of the transition from symmetrical to asymmetrical gaits in humans. University of Antwerp. Faculty of Science ; Ghent University. Faculty of Medicine and Health Sciences, Antwerp ; Ghent, Belgium.
Chicago author-date
Fiers, Pieter. 2014. “Biomechanics of the Transition from Symmetrical to Asymmetrical Gaits in Humans.” Antwerp ; Ghent, Belgium: University of Antwerp. Faculty of Science ; Ghent University. Faculty of Medicine and Health Sciences.
Chicago author-date (all authors)
Fiers, Pieter. 2014. “Biomechanics of the Transition from Symmetrical to Asymmetrical Gaits in Humans.” Antwerp ; Ghent, Belgium: University of Antwerp. Faculty of Science ; Ghent University. Faculty of Medicine and Health Sciences.
Vancouver
1.
Fiers P. Biomechanics of the transition from symmetrical to asymmetrical gaits in humans. [Antwerp ; Ghent, Belgium]: University of Antwerp. Faculty of Science ; Ghent University. Faculty of Medicine and Health Sciences; 2014.
IEEE
[1]
P. Fiers, “Biomechanics of the transition from symmetrical to asymmetrical gaits in humans,” University of Antwerp. Faculty of Science ; Ghent University. Faculty of Medicine and Health Sciences, Antwerp ; Ghent, Belgium, 2014.
@phdthesis{5332767,
  abstract     = {{Background and purpose: In humans walking and running are the preferred and most commonly used gaits, however alternative options to move about exist. One option is unilateral skipping, which is also known as bipedal galloping. In contrast to walk and run, bipedal gallop is an asymmetrical locomotion pattern, both in the temporal and spatial dimension, which means that the successive foot falls are not evenly spaced in time and that one leg remains positioned in front of the other. As such, a trailing and leading leg, each with specific functions, can be distinguished. In gallop, the trailing foot is the foot that contacts the ground first. The leading foot strikes the ground, generally when the trailing foot is still in contact with the ground. This generally results in two single support phases separated by a short period of double support. Following push-off of the leading leg, a relative long flight phase emerges. While gallop is the preferred gait at higher speeds in many animals, human bipedal gallop does not occur spontaneously except in very specific conditions such as during the rapid descent of steep inclines.
The aforementioned observations raise two questions: 1) Why do humans not use this gait for fast locomotion on level-ground, and 2) Why do humans use this gait for the rapid descent of steep inclines? This thesis addresses these questions. In doing so, insights into the execution and control of human asymmetrical locomotion patterns and, more generally, of human gait are expanded.
Methods: To address the first question, subjects were instructed to gallop and run at a self-selected speed over an instrumented runway. Measurements of the ground reaction forces together with the segments' kinematics enabled an in-depth comparison of the mechanics of both gaits. In a separate treadmill experiment, the metabolic costs of running and galloping were determined.
To address the second question, two experiments were conducted. The first experiment focused on running and galloping down steep inclines, whereas the second experiment involved transitions to these gaits. In the first experiment, subjects ran and galloped down an instrumented, steep incline at a given speed. Again, ground reaction forces and segmental kinematics were recorded, allowing a thorough comparison of both gaits when descending. In the second experiment, transitions from walk to run and from walk to gallop on the level were investigated by analyzing the segments' kinematics during these transitions. Differences between these transitions on level-ground provide useful insights to explain the spontaneous transitions that occur during steep downhill locomotion.
Results and discussion: Question 1: "Why do humans not use bipedal gallop for fast locomotion on level-ground?"
The asymmetrical nature of gallop involves distinct hip actions and foot placement, which give the legs different functions compared to run. The trailing leg is initially placed almost under the body and therefore it decelerates the body in the vertical direction, while accelerating it in the direction of movement. The leading leg is placed more in front of the body and acts in the opposite way to the trailing leg: it decelerates the body in the direction of movement, while accelerating it vertically, propelling it into the flight phase. These specific leg functions, which are mainly governed by actions of the hips, involve more energy dissipation and generation compared with running, resulting in a higher metabolic cost and higher levels of muscular stress at the hips. These reasons explain why running is favoured over galloping for fast locomotion on level-ground. However, it is also possible that other reasons, such as challenged gaze stability - not investigated in this thesis - may explain why gallop is not spontaneously used in humans.
Question 2: "Why do humans use bipedal gallop for the rapid descent of steep inclines?
During the descent of steep inclines, the tangential component of the gravity constantly tends to accelerate the body in the direction of movement. In order to descend at a constant speed, the legs have to compensate this accelerating force by absorbing mechanical energy. Compared to downhill running, downhill galloping involves shorter strides in which relatively more time is spent during foot contacts. Moreover, the specific orientation of the galloping legs enables a functional distinction between the legs, resulting in different amounts of work absorbed by the joints of each leg. Furthermore, the more vertical initial contact position of the trailing leg during galloping facilitates an initial forefoot contact, which is adopted by half of the subjects. Regardless of the initial foot contact pattern of the trailing galloping leg, the maximal absorption powers in and the amount of absorbed work by the knee extensors of the leading leg are lower compared with those of a running leg. Additionally, the maximal absorption powers in the trailing galloping knee extensors are lower than in running. However, the amount of absorbed work by these muscles is only lower when the trailing leg initially contacts the ground with the forefoot (because of a shift in power from the knee to the ankle). Since maximal absorption powers and large amounts of absorbed work are associated with muscle soreness, galloping probably results in less soreness in the quadriceps compared to running after rapidly descending steep inclines.
In addition to the advantages associated with the galloping pattern itself, the characteristics of the two-step walk-to-gallop transition may also explain the spontaneous use of bipedal gallop when descending. This two-step transition involves: 1) a lowering and acceleration of the body in the direction of movement by the trailing leg, and 2) a pivoting action of the leading leg that decelerates the body in the direction of movement and propels it in a first, high flight. Although humans may not spontaneously do a transition from walk to gallop on level-ground, we argue that this transition is easier to perform when descending based on its mechanics as found during level transitions. Moreover, the walk-to-gallop transition during descents may even have benefits (e.g. easier to control locomotion speed) over the walk-to-run transition, again provided that this walk-to-run transition is performed in a mechanically similar way as on level-ground.
In conclusion, whereas bipedal gallop does not spontaneously emerge on level-ground because it has a higher cost of transport and involves higher muscular stress, bipedal gallop is spontaneously used when descending steep inclines. In this dissertation, possible reasons associated with the galloping gait itself as well as with the transition to it are identified explaining the spontaneous use of gallop during the descent of steep inclines.}},
  author       = {{Fiers, Pieter}},
  isbn         = {{9789057284564}},
  language     = {{eng}},
  pages        = {{188}},
  publisher    = {{University of Antwerp. Faculty of Science ; Ghent University. Faculty of Medicine and Health Sciences}},
  school       = {{Ghent University}},
  title        = {{Biomechanics of the transition from symmetrical to asymmetrical gaits in humans}},
  year         = {{2014}},
}