Peter Aerts

Biomechanical analysis of the stance phase during barefoot and shod running

This study investigated spatio-temporal variables, ground reaction forces and sagittal and frontal plane kinematics during the stance phase of nine trained subjects running barefoot and shod at three different velocities (3.5, 4.5, 5.5 m s(-1)). Differences between conditions were detected with the general linear method (factorial model). Barefoot running is characterized by a significantly larger external loading rate than the shod condition. The flatter foot placement at touchdown is prepared in free flight, implying an actively induced adaptation strategy. In the barefoot condition, plantar pressure measurements reveal a flatter foot placement to correlate with lower peak heel pressures. Therefore, it is assumed that runners adopt this different touchdown geometry in barefoot running in an attempt to limit the local pressure underneath the heel. A significantly higher leg stiffness during the stance phase was found for the barefoot condition. The sagittal kinematic adaptations between conditions were found in the same way for all subjects and at the three running velocities. However, large individual variations were observed between the runners for the rearfoot kinematics.

SPEED AND STAMINA TRADE-OFF IN LACERTID LIZARDS

Abstract Morphological and physiological considerations suggest that sprinting ability and endurance capacity put conflicting demands on the design of an animals locomotor apparatus and therefore cannot be maximized simultaneously. To test this hypothesis, we correlated size-corrected maximal sprint speed and stamina of 12 species of lacertid lizards. Phylogenetically independent contrasts of sprint speed and stamina showed a significant negative relationship, giving support to the idea of an evolutionary trade-off between the two performance measures. To test the hypothesis that the trade-off is mediated by a conflict in morphological requirements, we correlated both performance traits with snout-vent length, size-corrected estimates of body mass and limb length, and relative hindlimb length (the residuals of the relationship between hind- and forelimb length). Fast-running species had hindlimbs that were long compared to their forelimbs. None of the other size or shape variables showed a significant relationship with speed or endurance. We conclude that the evolution of sprint capacity may be constrained by the need for endurance capacity and vice versa, but the design conflict underlying this trade-off has yet to be identified. Corresponding Editor: T. Garland Jr.

The mechanical characteristics of the human heel pad during foot strike in running: An in vivo cineradiographic study

The compressive properties of the heel pad during the heel strike when running (barefoot and shod, two subjects, 4.5 m s-1) were studied by means of a high-speed two-dimensional cineradiographic registration (150 frames s-1) of an actual running step. Vertical ground reaction forces were measured with a force platform. In barefoot running the heel pad deforms to a maximal percentage deformation of 60.5 +/- 5.5%. In shod running the heel pad deforms only 35.5 +/- 2.5% and the nonlinear force-deformation relationship reflects an increasing stiffness when deformation rises. Although the amplitudes of the vertical ground reaction forces do not differ notably in both conditions, barefoot running implies a maximal deformation to the fatty heel tissue, reducing its functional role from shock reduction towards local protection of the heel bone. It is argued that embedding the foot in a well-fitting shoe increases the effective stiffness of the heel pad.

The effects of habitual footwear use: foot shape and function in native barefoot walkers†

The human foot was anatomically modern long before footwear was invented, and is adapted to barefoot walking on natural substrates. Understanding the biomechanics of habitually barefoot walkers can provide novel insights both for anthropologist and for applied scientists, yet the necessary data is virtually non-existent. To start assessing morphological and functional effects of the habitual use of footwear, we have studied a population of habitually barefoot walkers from India (n = 70), and compared them with a habitually shod Indian control group (n = 137) and a Western population (n = 48). We focused on foot metrics and on the analysis of plantar pressure data, which was performed using a novel, pixel based method (Pataky and Goulermas 2008, Journal of Biomechanics, 41, 2136). Habitually shod Indians wore less often, and less constricting shoes than Western people. Yet, we found significant differences with their habitually barefoot peers, both in foot shape and in pressure distribution. Barefoot walkers had wider feet and more equally distributed peak pressures, i.e. the entire load carrying surface was contributing more uniformly than in habitually shod subjects, where regions of very high or very low peak pressures were more apparent. Western subjects differed strongly from both Indian populations (and most from barefoot Indians), by having relatively short and, especially, slender feet, with more focal and higher peak pressures at the heel, metatarsals and hallux. The evolutionary history of humans shows that barefoot walking is the biologically natural situation. The use of footwear remains necessary, especially on unnatural substrates, in athletics, and in some pathologies, but current data suggests that footwear that fails to respect natural foot shape and function will ultimately alter the morphology and the biomechanical behaviour of the foot.

Deformation characteristics of the heel region of the shod foot during a simulated heel strike: The effect of varying midsole hardness

Impact tests using a pendulum were performed on the shod heel region of nine subjects. Both soft- and hard-soled shoes were used. The deformations involved were calculated from the registered decelerations during impact. Thus, load-deformation cycles were recorded for various impact velocities. In contrast to in vivo force-platform recordings, peak loadings for the soft- and hard-soled conditions differed significantly (614 +/- 29 N vs 864 +/- 49 N, respectively), thus challenging the evidence for compensation at the level of the heel pad. Moreover, computation of the compression of the heel pad in the shoe showed an unexpected inverse relationship between shoe midsole hardness and degree of heel pad compression: the harder the midsole, the smaller the compression (soft shoe 7.6 +/- 0.9 mm; hard shoe 6.7 +/- 0.9 mm). This can be explained by assuming a loading rate dependent stiffness of the heel pad in the shod condition (stiffness in N.m-1 = 51.25x (loading rate in N.s-1)0.76; R2 = 0.90), determined by the visco-elastic nature of the heel pad and the spatial confinement of the heel counter of the shoe.

Ecomorphology of the Lizard Feeding Apparatus: a Modelling Approach

A model of static bite force during the power phase is used to investigate the relationship between the feeding ecology (herbivorous vs. animalivorous) and biomechanics of the jaw system in four species of lizards. For the analysis the bite model of Herrel et al. (1998) is used. The model calculates both the bite and joint forces and the moments at the quadratosquamosal joint for a range of orientations of food reaction forces. No relative jaw movements during the power phase of biting are observed (based on cineradiography) in any of the examined species, thus excluding grinding mechanisms as an adaptation to a herbivorous diet. However, trends in magnitude and orientation of the joint forces and required and remaining moments at the quadratosquamosal joint are similar in species with similar food preferences. Herbivorous lizards bite harder and show lower joint forces for a given bite force than non-herbivorous species do. It is argued that this difference might be a more general characteristic of herbivorous lizards and that a high bite force has an adaptive value for these species. Whereas, in lizards, dental grinding mechanisms are presumably not a prerequisite for a herbivorous diet, adaptations of the digestive apparatus and the development of a relatively high bite force probably are. Additionally it is argued that the shift of the insertion site of the temporal ligament can be considered as a preadaptation for herbivory in lizards. A hypothetical transformation series of the bauplann of the skull departing from a basic lepidosaurian stock and leading to the skull system in extant herbivorous lizards is proposed.

Kinematics and Functional Morphology of Aquatic Feeding in Australian Snake-Necked Turtles (Pleurodira; Chelodina)

Head kinematics during aquatic feeding of the Australian long‐necked turtle (Chelodina) were studied by means of high speed video recordings. Buccal expansion was assessed by calculation of elliptical cross‐sectional surfaces. Further, displacements of head, carapace, and prey in the earth bound frame, of the prey relative to the center of the gape, and of the head relative to the carapace were determined. Rates of change (velocities) of all these variables were calculated. These data are combined with information on the osteology and myology of the head. The robust development of the large hyobranchial apparatus, the massive intercornuatus muscle, and the presence of the branchiosquamosus muscle were related to aquatic feeding skills. Head kinematics are variable in amplitude and relative timing, but proceed always in a rostrocaudal sequence. According to their effect on the prey, two components are distinguished in the process of expansion. The first compensates for head/body movements (compensatory suction). The second causes distinct acceleration of water and prey (inertial suction). The latter component is mainly driven by the abduction of the second branchial arch. In spite of largely different structural solutions, optimal feeding conditions as deduced for suction in feeding fishes are also employed by Chelodina. This further promotes the assumption that hydrodynamics constrain evolutive solutions for aquatic feeding. J. Morphol. 233:113–125, 1997.

Low vision affects dynamic stability of gait

The objective of this study was to demonstrate specific differences in gait patterns between those with and without a visual impairment. We performed a biomechanical analysis of the gait pattern of young adults (27 ± 13 years old) with a visual impairment (n=10) in an uncluttered environment and compared it to the gait pattern of age matched controls (n=20). Normally sighted adults were tested in a full vision and no vision condition. Differences are found in gait between both groups and both situations. Adults with a visual impairment walked with a shorter stride length (1.14 ± 0.21m), less trunk flexion (4.55 ± 5.14°) and an earlier plantar foot contact at heel strike (1.83 ± 3.49°) than sighted individuals (1.39 ± 0.08 m; 11.07 ± 4.01°; 5.10 ± 3.53°). When sighted individuals were blindfolded (no vision condition) they showed similar gait adaptations as well as a slower walking speed (0.84 ± 0.28 ms(-1)), a lower cadence (96.88 ± 13.71 steps min(-1)) and limited movements of the hip (38.24 ± 6.27°) and the ankle in the saggital plane (-5.60 ± 5.07°) compared to a full vision condition (1.27 ± 0.13 ms(-1); 110.55 ± 7.09 steps min(-1); 45.32 ± 4.57°; -16.51 ± .59°). Results showed that even in an uncluttered environment vision is important for locomotion control. The differences between those with and without a visual impairment, and between the full vision and no vision conditions, may reflect a more cautious walking strategy and adaptive changes employed to use the foot to probe the ground for haptic exploration.

Morphological and mechanical determinants of bite force in bats: do muscles matter?

SUMMARY Bats are one of the most diverse groups of mammals and have radiated into a wide variety of trophic niches. Accordingly, the cranial structure in bats is unusually variable among mammals and thought to reflect specializations for feeding and echolocation. However, recent analyses of cranial structure, feeding behavior and bite force across a wide range of bats suggest that correlations between morphology and performance and/or ecology are not as clearcut as previously thought. For example, most of the variation in bite force across a wide range of phyllostomid bats was explained by differences in body size rather than specific cranial traits. However, remarkably little is known about the muscular components that are responsible for generating the actual bite forces. We have tested which aspects of the cranial muscular system are good predictors of bite force across a wide range of species using a modeling approach. Model calculations of bite force show good correspondence with in vivo data suggesting that they can be used to estimate performance of the cranial system. Moreover, our data show that bite force is strikingly well explained by differences in temporalis muscle mass, temporalis fiber length and masseter muscle mass. Moreover, our data show that evolutionary changes in bite force capacity in bats are associated with evolutionary changes in relative m. temporalis mass and absolute skull length.

Locomotion in bonobos (Pan paniscus): differences and similarities between bipedal and quadrupedal terrestrial walking, and a comparison with other locomotor modes.

One of the great ongoing debates in palaeo‐anthropology is when, and how, hominids acquired habitual bipedal locomotion. The newly adopted bipedal gait and the ancestral quadrupedal gait are most often considered as very distinct, with each habitual locomotor mode showing corresponding anatomical adaptations. Bonobos (Pan paniscus), along with common chimpanzees (P. troglodytes), are the closest living relatives to humans and their locomotion is valuable for comparison with other primates, and to gain an insight in the acquisition of human bipedalism. Bonobos are habitual quadrupeds, but they also engage in bipedal locomotion, both on terrestrial and in arboreal substrates. In terms of kinematics and dynamics, the contrast between bipedal and quadrupedal walking seems to be more subtle than one might expect. Apart from the trunk being approximately 37° more erect during bipedal locomotion, the leg movements are rather similar. Apart from the heel, plantar pressure distributions show subtle differences between bipedal and quadrupedal locomotion. Regardless, variability is high, and various intermediate forms of locomotion (e.g. tripedal walking) exist both in captivity and in the wild. Moreover, there is overlap between the characteristics of walking and other locomotor modes, as we show with new data of walking on an inclined pole and of vertical squat jumps. We suggest that there is great overlap between the many locomotor modes in bonobos, and that the required polyvalence is reflected in their anatomy. This may hamper the development of one highly specialized gait (i.e. bipedalism), which would constrain performance of the other types of locomotion.