Comparative Biomechanics

Birds provide an interesting opportunity to study the relationships between body size, limb morphology and bipedal locomotor function. Birds are ecologically diverse and span a large range of body size and limb proportions, yet all use their hindlimbs for bipedal terrestrial locomotion, for at least some part of their life history. Here, we review the scaling of avian striding bipedal gaits to explore how body mass and leg morphology influence walking and running. We collate literature data from 21 species, spanning a 2500× range in body mass from painted quail to ostriches. Using dynamic similarity theory to interpret scaling trends, we find evidence for independent effects of body mass, leg length and leg posture on gait. We find no evidence for scaling of duty factor with body size, suggesting that vertical forces scale with dynamic similarity. However, at dynamically similar speeds, large birds use relatively shorter stride lengths and higher stride frequencies compared with small birds. We also find that birds with long legs for their mass, such as the white stork and red-legged seriema, use longer strides and lower swing frequencies, consistent with the influence of high limb inertia on gait. We discuss the observed scaling of avian bipedal gait in relation to mechanical demands for force, work and power relative to muscle actuator capacity, muscle activation costs related to leg cycling frequency, and considerations of stability and agility. Many opportunities remain for future work to investigate how morphology influences gait dynamics among birds specialized for different habitats and locomotor behaviors.

Dam walls are like open laboratories useful to study the gradient limits of locomotion. Two dam walls, where Alpine ibex (Capra ibex) use to climb searching for the salty exuded, were filmed at 0.2 fps. The straight slope of the walls ranged from 123% to almost vertical. In total 54 animals were filmed and their body mass estimated as medium size, small size and kids. No large males were observed moving on the walls. The overall weighted average incline of their paths was 37% uphill and 46% downhill. They used to climb on zigzag routes and run down on more linear tracks. The gaits employed by the animals were walk and gallop. The steepest paths travelled by kids were 155% up and 157% down, the maximum height was 49 m, while their maximum estimated speeds were 2.6 ms-1 uphill and-4.2 ms-1 downhill. Medium: +143% and-157%; 49 m; +1.1 and-4.1 ms-1. Large: +102% and-123%; 32 m; +0.7 and –1.0 ms-1. The climbing performance of Alpine ibex, in term of speed and inclination, appeared to be negatively influenced by body mass, while the friction coefficient between their hooves and the dam walls was in a range higher than rubber on concrete surfaces. Protection against toppling depends on the slope and the ratio between the basal width and body centre of mass (bCOM) height. We propose a safety factor index (Fst), similar to that used in geology, defined as the ratio between the major distance from a downstream to an upstream leg and the centre of mass height, all divided by the tangent of the slope. An index value of " 1 " is the discriminant between unsafe and relative safe positions. Animals with shorter legs and lower bCOM, like females and kids, can negotiate steeper paths with a higher safety factor.

Birds are a diverse and agile lineage of vertebrates that all use bipedal locomotion for at least part of their life. Thus birds provide a valuable opportunity to investigate how biomechanics and sensorimotor control are integrated for agile bipedal locomotion. This review summarizes recent work using terrain perturbations to reveal neuromechanical control strategies used by ground birds to achieve robust, stable and agile running. Early experiments in running guinea fowl aimed to reveal the immediate intrinsic mechanical response to an unexpected drop (‘pothole’) in terrain. When navigating the pothole, guinea fowl experience large changes in leg posture in the perturbed step, which correlates strongly with leg loading and perturbation recovery. Analysis of simple theoretical models of running has further confirmed the crucial role of swing-leg trajectory control for regulating foot contact timing and leg loading in uneven terrain. Coupling between body and leg dynamics results in an inherent trade-off in swing leg retraction rate for fall avoidance versus injury avoidance. Fast leg retraction minimizes injury risk, but slow leg retraction minimizes fall risk. Subsequent experiments have investigated how birds optimize their control strategies depending on the type of perturbation (pothole, step, obstacle), visibility of terrain, and with ample practice negotiating terrain features. Birds use several control strategies consistently across terrain contexts: 1) independent control of leg angular cycling and leg length actuation, which facilitates dynamic stability through simple control mechanisms, 2) feedforward regulation of leg cycling rate, which tunes foot-contact timing to maintain consistent leg loading in uneven terrain (minimizing fall and injury risks), 3) load-dependent muscle actuation, which rapidly adjusts stance push-off and stabilizes body mechanical energy, and 4) multi-step recovery strategies that allow body dynamics to transiently vary while tightly regulating leg loading to minimize risks of fall and injury. In future work, it will be interesting to investigate the learning and adaptation processes that allow animals to adjust neuromechanical control mechanisms over short and long timescales.

Abstract It is generally thought that elephants do not run 1, 2, 3, 4, 5, but there is confusion about how fast they can move across open terrain and what gait they use at top speed. Here we use video analysis to show that Asian elephants (Elephas maximus L.) can move at surprisingly high speeds of up to 6.8 ms− 1 (25 km h− 1) and that, although their gait might seem to be a walk even at this speed, some features of their locomotion conform to definitions of running.

This study describes a unique assessment
of primate intrinsic foot joint kinematics based upon
bone pin rigid cluster tracking. It challenges the assumption
that human evolution resulted in a reduction of midfoot
flexibility, which has been identified in other primates
as the “midtarsal break.” Rigid cluster pins were inserted
into the foot bones of human, chimpanzee, baboon, and
macaque cadavers. The positions of these bone pins were
monitored during a plantarflexion-dorsiflexion movement
cycle. Analysis resolved flexion-extension movement patterns
and the associated orientation of rotational axes for
the talonavicular, calcaneocuboid, and lateral cubometatarsal
joints. Results show that midfoot flexibility occurs
primarily at the talonavicular and cubometatarsal joints.
The rotational magnitudes are roughly similar between
humans and chimps. There is also a similarity among
evaluated primates in the observed rotations of the lateral
cubometatarsal joint, but there was much greater rotation
observed for the talonavicular joint, which may serve to
differentiate monkeys from the hominines. It appears that
the capability for a midtarsal break is present within the
human foot. A consideration of the joint axes shows that
the medial and lateral joints have opposing orientations,
which has been associated with a rigid locking mechanism
in the human foot. However, the potential for this same
mechanism also appears in the chimpanzee foot. These
findings demonstrate a functional similarity within the
midfoot of the hominines. Therefore, the kinematic capabilities
and restrictions for the skeletal linkages of the
human foot may not be as unique as has been previously
suggested.