Specialized feeding methods evolved repeatedly from a suction-feeding strategy in cichlids. How algae-eaters altered their suction mechanics to transport detached algae efficiently, and how this may hinder capturing larger prey, remains unclear. Here, we study the kinematics and time-resolved volumetrics of a piscivore, an algae picker/nibbler, and an algae scraper from Lake Malawi feeding on attached algae tablets and free pieces of shrimp. Algae specialists lack the common anterior-to-posterior expansion and compression waves of the head, instead exhibiting a synchronous expansion pattern. This alternative pattern may allow algae retention due to low-amplitude dilation of the gill rakers, maximized suction flow speeds for a given local expansion amplitude, and rapid sequences of suction cycles. The trade-off between powerful suction and efficient feeding on algae may explain why algae scrapers' opportunistic switching to suction feeding can only be successful on easy prey, and may have impacted cichlid diversification and trophic niche constraining.

Equid evolution is characterized by a high diversity of extinct species and morphologies, whereas extant equids share a superficially similar, monodactyl morphology. This inferred musculoskeletal similarity of modern equid limbs remains unexplored, and it is often assumed that domestic horse limbs are representative for wild equids (e.g., zebras, onagers, etc.). Our aim was to quantitatively describe the muscle architecture and arrangement of all forelimb muscles in extant wild Equus species to test this assumption, and investigate any differences between the species. We hypothesized that there would be subtle variation linked to locomotion on the different substrates that these species encounter. Gross dissections were performed to record muscle attachment sites and to quantify architectural metrics: muscle-tendon unit (MTU) length, MTU mass, muscle mass, pennation angle, and fascicle length; physiological cross-sectional area (PCSA) of the muscles and tendon cross-sectional area (TCSA) of the distal ligaments were then calculated. Qualitative results confirm common origin and insertion sites of all muscles across all Equus species. When normalized for size, the forelimb muscles across equids generally exhibit comparable muscular architecture and force-generating capacities. However, we observed a trend for higher force-generating potential in the distal limb flexor muscles in two species of zebra naturally found in habitats with inclined or uneven substrates. Although limited by sample size, these results indicate that scaled data for modern wild equids are generally very comparable, which may enable much smoother translation of experimental data from domestic horses into digital simulations of wild equid locomotion, including for extinct equids.

The baubellum (os clitoridis) is a bone found in the clitoris of many female eutherian mammals and is homologous to the baculum in males. In contrast to the baculum, the baubellum has received very little attention regarding its morphological or interspecific diversity, or on hypotheses for its function. The presence of the baubellum in bears (Ursidae) has only been established and mentioned in the literature for the Ursus genus, and not for the other genera of bears. Moreover, no scaled photographs are available for baubella of this clade, and the sizes reported vary between sources. We hereby present and describe the baubellum of a spectacled bear (Tremarctos ornatus), providing a detailed account of baubella in a basal ursid species. The baubellum of Tremarctos is slightly bowed dorsally, with two small prominences at the distal apex. The length of the Tremarctos baubellum in this study is comparable to that of Ursus americanus (American black bear). We infer the specific shape, with longitudinal ridges, of the baubellum in Tremarctos could indicate a discrete function during copulation or sexual arousal. However, future studies, especially regarding the associated soft tissues, will be required to confirm whether this is indeed the case. Our study expands the understanding of baubella within Ursidae, providing new data (including a three-dimensional model) that can be used to further explore the morphological diversity and function of this enigmatic extra-skeletal bone.

Objective: Altered mechanical loading is a known risk factor for osteoarthritis. Destabilization of the medial meniscus (DMM) is a preclinical gold standard model for post-traumatic osteoarthritis and is thought to induce instability and locally increased loading. However, the joint- and tissue-level mechanical environment underlying cartilage degeneration remains poorly documented. Design: Using a custom multiscale modeling approach, we assessed joint and tissue biomechanics in rats undergoing sham surgery and DMM. High-fidelity experimental gait data were collected in a setup combining biplanar fluoroscopy and a ground reaction force plate. Knee poses and joint-level loading were estimated through musculoskeletal modeling, using bony landmarks, semi-automatically tracked via deep learning on fluoroscopic images, and ground reaction forces. A musculoskeletal model of the rat hindlimb was adapted to represent knee flexion-extension, valgus-varus, and internal-external rotation. The tissue-level cartilage mechanical environment was then spatially estimated, using the musculoskeletal modeling parameters as inputs into a dedicated finite element (FE) model of the rat knee, comprising cartilage and meniscal tissues. Experimental gait data and modeling workflows, including musculoskeletal models and FE meshes, are openly shared through a data repository. Results: In rats with DMM, the frontal plane knee pose was altered, yet there was no indication of joint-level overloading. Tissue-level mechanical cues typically linked with cartilage degeneration were not increased in the medial tibial cartilage, despite evidence of tissue structural changes. Conclusion: DMM did not increase joint and tissue mechanical responses in the knee medial compartment, suggesting that mechanical loading alone does not explain the observed osteoarthritis-like structural changes. ### Competing Interest Statement The authors have declared no competing interest.

The adaptive reason(s) as to why some tetrapods walk or run on only two legs is far from straightforward. Compared to a quadruped, maintaining balance is obviously challenging for a biped, since the number of ground contacts per cycle is reduced. Consequently, cyclic limb loading can also be expected to be higher. In association with these mechanical constraints, some species show clear adaptations that enable them to continuously walk and/or run bipedally. Others, however, can only perform bipedally for short bouts after which they proceed further on all fours. Apparently, in the latter case, an optimal functional anatomical context (the so-called 'evolutionary spandrel') favours occasional bipedal performance. In this context, it is possible that the morphological features in the extant species that routinely practice bipedal walking and/or running (i.e., birds and humans) are convergent. Indeed, a similar (analogous) adaptive trait might have evolved independently in these 'bipedal' taxa as a result of similar selection pressures. Similarly, since occasional bipedalism should be a mandatory intermediate evolutionary stage leading to habitual, or obligate, bipedalism, the evolutionary pathways leading to bipedal adaptations can be explored in extant animals practicing different levels of bipedal performance. In the present chapter, we discuss different functional and evolutionary contexts that have led species with different Bauplans to use bipedal walking and/or running (for short or longer behavioural bouts). Firstly, we consider the involuntary but dynamically imposed bipedal running observed in extant lizards. Secondly, the voluntary but anatomically constrained bipedal walking behaviour of extant non-human primates is discussed. Thirdly, the use of the bipedal posture in birds and humans is compared (i.e. for the species for which this mode of progression is anatomically imposed and constrained). Finally, in an attempt to link the mechanical constraints and the potential evolutionary pathways related to occasional, habitual and obligate bipedalism, we argue that bipedal tetrapods should converge upon the same functional anatomical outcomes. Indeed, based on a virtual limb that would connect the body's centre of mass to the foot, angular impulses about the body's centre of mass over a (half-) stance time of the (vertical) ground reaction forces should cancel each other out. This might leave only two adaptive pathways open that could lead to a more habitual use of bipedalism: (1) make the bipedal functional/anatomical limb coincide with the virtual limb, or (2) make the virtual limb coincide with the anatomical (quadrupedal) limb.

The evolution of the limbs of horses has fascinated scholars and laymen alike for generations. From the late 1800s until the present day, different ideas have been proposed to explain the evolution of the monodactyl (one-toed) condition in modern horse limbs. Theses such as maximal speed, stability, body mass, inertial load, and distance transport propose different selective pressures which drove the evolution of monodactyly in Equus-line equids. We posit that previous equid researchers have approached the question of monodactyly from too narrow an angle, focusing on one aspect of the story rather than treating monodactyly as the result of multiple factors. Our Equal Strength Synthesis treats intrinsic bone strength as the starting point for the evolution of monodactyly in equids and combines the biomechanical and habitual aspects of previous theses. Assuming equids have never been on the brink of failure, and that bone strength was equal in tridactyl and monodactyl equids, we demonstrate significant reductions in distal limb mass would have been possible by reducing the digit number to one. This synthesis presents a rigorous biomechanical revision to an age-old conundrum, bringing us one step closer to truly understanding one of the great transitions in vertebrate evolution.

Synopsis Intrauterine undernutrition in humans typically results in low birth weight ([small for gestational age] SGA) and delayed postnatal neuromotor maturation. Since SGA and intrauterine growth retardation are also common in domestic pigs, piglets are premised as models to study delayed motor development. Applied to the locomotor paradigm, however, questions emerge: (i) how to map the developmental time scale of the precocial model onto the altricial target species and (ii) how to distinguish size from maturation effects? Gait data were collected at self-selected voluntary walking speed during early development (0-96 hours postpartum; pp) for SGA- and normal ([appropriate for gestational age] AGA) piglets. Dimensionless spatiotemporal gait characteristics (according to dynamic similarity) become invariant already after 4 hours pp, suggesting rapid postnatal neuromotor maturation. Moreover, dimensionless gait data are largely identical for SGA- and AGA-siblings, indicating that primarily size effects explain absolute locomotor differences. This is further supported by (i) normalized force-generating capacity of limb muscles, (ii) joint kinematics (<10 hours pp), and (iii) normalized ground reaction forces (<5 days pp) being indifferent between SGA- and AGA- piglets. Furthermore, predictive modeling based on limb joint kinematics is unable to discern the majority of SGA- from AGA-piglets (<10 hours pp). All this leads to the conclusion that, although smaller than the AGA piglets in absolute terms, SGA-piglets mature (neuromechanically speaking) just like, and equally fast as their AGA littermates. Yet, it remains a fact that early SGA piglets are reported to be less mobile, less vital, and less competitive than their AGA siblings (even often die before day 3 pp). This conspicuous difference likely results from the energy level (blood glucose and glycogen) and its mobilization being considerably different between the piglet categories during early development.

Altered mechanical loading is a widely suggested, but poorly understood potential cause of cartilage degeneration in osteoarthritis. In rodents, osteoarthritis is induced following destabilization of the medial meniscus (DMM). This study estimates knee kinematics and contact forces in rats with DMM to gain better insight into the specific mechanisms underlying disease development in this widely-used model. Unilateral knee surgery was performed in adult male Sprague-Dawley rats (n=5 with DMM, n=5 with sham surgery). Radio-opaque beads were implanted on their femur and tibia. 8 weeks following knee surgery, rat gait was recorded using the 3D²YMOX setup (Sanctorum et al. 2019, simultaneous acquisition of biplanar XRay videos and ground reaction forces). 10 trials (1 per rat) were calibrated and processed in XMALab (Knörlein et al. 2016). Hindlimb bony landmarks were labeled on the XRay videos using transfer learning (Deeplabcut, Mathis et al. 2019; Laurence-Chasen et al. 2020). A generic OpenSim musculoskeletal model of the rat hindlimb (Johnson et al. 2008) was adapted to include a 3-degree-of-freedom knee. Inverse kinematics, inverse dynamics, static optimization of muscle forces, and joint reaction analysis were performed. In rats with DMM, knee adduction was lower compared to sham surgery. Ground reaction forces were less variable with DMM, resulting in less variability in joint external moments. The mediolateral ground reaction force was lower, resulting in lower hip adduction moment, thus less force was produced by the rectus femoris. Rats with DMM tended to break rather than propel, resulting in lower hip flexion moment, thus less force was produced by the semimembranosus. These results are consistent with lower knee contact forces in the anteroposterior and axial directions. These preliminary data indicate no overloading of the knee joint in rats with DMM, compared with sham surgery. We are currently expanding our workflow to finite element analysis, to examine mechanical cues in the cartilage of these rats (Fig1G).

We investigated how baboons transition from quadrupedal to bipedal walking without any significant interruption in their forward movement (i.e. transition 'on the fly'). Building on basic mechanical principles (momentum only changes when external forces/moments act on the body), insights into possible strategies for such a dynamical mode transition are provided and applied first to the recorded planar kinematics of an example walking sequence (including several continuous quadrupedal, transition and subsequent bipedal steps). Body dynamics are calculated from the kinematics. The strategy used in this worked example boils down to: crouch the hind parts and sprint them underneath the rising body centre of mass. Forward accelerations are not in play. Key characteristics of this transition strategy were extracted: progression speed, hip height, step duration (frequency), foot positioning at touchdown with respect to the hip and the body centre of mass (BCoM), and congruity between the moments of the ground reaction force about the BCoM and the rate of change of the total angular moment. Statistical analyses across the full sample (15 transitions of 10 individuals) confirm this strategy is always used and is shared across individuals. Finally, the costs (in J kg(-1) m(-1)) linked to on the fly transitions were estimated. The costs are approximately double those of both the preceding quadrupedal and subsequent bipedal walking. Given the short duration of the transition as such (<1 s), it is argued that the energetic costs to change walking posture on the fly are negligible when considered in the context of the locomotor repertoire.

Locomotor kinematics have been challenging inputs for automated diagnostic screening of livestock. Locomotion is a highly variable behavior, and influenced by subject characteristics (e.g., body mass, size, age, disease). We assemble a set of methods from different scientific disciplines, composing an automatic, high through-put workflow which can disentangle behavioral complexity and generate precise individual indicators of non-normal behavior for application in diagnostics and research. For this study, piglets (Sus domesticus) were filmed from lateral perspective during their first 10 h of life, an age at which maturation is quick and body mass and size have major consequences for survival. We then apply deep learning methods for point digitization, calculate joint angle profiles, and apply information-preserving transformations to retrieve a multivariate kinematic data set. We train probabilistic models to infer subject characteristics from kinematics. Model accuracy was validated for strides from piglets of normal birth weight (i.e., the category it was trained on), but the models infer the body mass and size of low birth weight (LBW) piglets (which were left out of training, out-of-sample inference) to be “normal.” The age of some (but not all) low birth weight individuals was underestimated, indicating developmental delay. Such individuals could be identified automatically, inspected, and treated accordingly. This workflow has potential for automatic, precise screening in livestock management.

ABSTRACT Purpose Running with increased duty factors (DF) has been shown to effectively reduce external forces during running. In this study, we investigated whether running with increased DF (INCR) also reduces internal musculoskeletal loading measures, defined as peak muscle forces, muscle force impulses, and peak joint contact forces compared with a runners’ preferred running pattern (PREF). Method Ten subjects were instructed to run with increased DF at 2.1 m·s−1. Ground reaction forces and three-dimensional kinematics were simultaneously measured. A musculoskeletal model was used to estimate muscle forces based on a dynamic optimization approach, which in turn were used to calculate muscle force impulses and (resultant and three-dimensional) joint contact forces of the ankle, knee, and hip joint during stance. Results Runners successfully increased their DF from 40.6% to 49.2% on average. This reduced peak muscle forces of muscles that contribute to support during running, i.e., the ankle plantar flexors (−19%), knee extensors (−18%), and hip extensors (−15%). As a consequence, peak joint contact forces of the ankle, knee, and hip joint reduced in the INCR condition. However, several hip flexors generated higher peak muscle forces near the end of stance. Conclusions Running with increased DF lowers internal loading measures related to support during stance. Although some swing-related muscles generated higher forces near the end of stance, running with increased DF can be considered as a preventive strategy to reduce the occurrence of running-related injuries, especially in running populations that are prone to overuse injuries.

Purpose: Rats are widely used to study tissue mechano-adaptation in the context of knee homeostasis and degeneration. In particular, surgical interventions are often used in rats to destabilize the knee, and to induce osteoarthritis. To characterize local mechanical stimuli in knee tissues and examine their mechano-adaptive response, loading (i.e. contact forces) must be estimated at the joint level. The purpose of this study is to estimate knee contact forces in the knee of intact rats using musculoskeletal modeling. Ground reaction forces and torques, as well as accurate hindlimb motion using 3D X-Ray Reconstruction of Moving Morphology (XROMM) (Brainerd et al., 2010) were used as model inputs. Methods: Two male Sprague-Dawley rats were implanted with radio-opaque markers on their right tibia and femur at age 12 weeks (lead-free tin, 0.35 mm diameter, 3-4 markers for the tibia and 2-3 markers for the femur). Rats were then trained to cross a walkway from age 14-16 weeks with a consistent gait (trotting without galloping) (Webb et al., 2011). At 16 weeks of age (391g), rat gaits were recorded using the 3D2YMOX setup (University of Antwerp) (Sanctorum et al., 2019). Rat motion was captured dynamically with a calibrated biplanar X-Ray video setup, while the rats trotted through an instrumented walkway (X-Ray sources: 65 and 71kV, 40mA, 300ms, record rate 500 fps). For each rat, 5 trials with contact of the right hindlimb on the force plate were acquired. Rats were then sacrificed, and right hindlimbs were imaged using micro-computed tomography to locate the radio-opaque markers. Synchronously, force plate data were acquired using a piezo force plate embedded in the walkway (Kistler force plate, acquisition rate of 20000 Hz). Force plate outputs were amplified and acquired with a DAQ. To increase chances of only the right hindlimb hitting the force plate, two contact sockets were mounted above the force plate (see Figure 1A) and the force plate was calibrated correspondingly. DAQ channels were filtered with a band-pass filter (0.2, 2000 Hz) to remove noise, voltage drift and mechanical oscillations. Forces, torques and center of pressure were then computed, and data during the stance phase were further smoothed using a Savitzky-Golay filter. Biplanar X-Ray images were processed in XMALab (Knörlein et al. 2016). Images were calibrated using calibration cube images, and undistorted using distortion grid images. The implanted radio-opaque markers as well as bony landmarks were tracked manually on the right foot, tibia, femur, pelvis and on the spine (Figure 1B). A generic musculoskeletal rat model of the rat hindlimb (Johnson et al., 2008) was scaled in OpenSim (Delp et al., 2007) (Figure 1C). This model has 3 degrees of freedom at the ankle and hip (ab/adduction, int/external, flexion), and 1 degree of freedom at the knee (flexion). Joint contact forces in the knee were estimated following inverse kinematics, inverse dynamics, and static optimization of the muscles. Results: Ground reaction forces were highest in the vertical direction, and peaked at 85% and 80% of the rat body weight on average for rat 1 and 2 respectively (Figure 2A, grey line). The anterior-posterior component of the ground reaction force reached 12% and 17% body weight in rats 1 and 2 respectively (Figure 2A, green line). The medial-lateral component reached about 12% body weight in both rats (Figure 2A, purple line). Knee contact forces were lowest in the medial-lateral component (Figure 2C, red line). Knee compressive forces peaked at 70% and 37% body weight on average for rat 1 and 2 respectively (Figure 2C, yellow line). The highest component was in the posterior-anterior direction (peaking at 99% and 157% body weight on average for rats 1 and 2 respectively, Figure 2C, blue line). The higher contact forces in rat 2 are in agreement with the deeper knee flexion (by about 25 degrees, Figure 2B, magenta line), combined with higher hip and ankle flexion compared to rat 1 (Figure 2B, blue and green lines). In comparison, knee contact forces during stance phase in humans are highest in the direction longitudinal to the tibia (high compressive forces, Figure 2D, yellow line, adapted with permission from Van Rossom et al. 2018). This is explained by the erect position assumed by humans when they walk, while rats have a more crouched gait. Conclusions: Musculoskeletal modeling of the rat knee reveals that compressive contact forces are relatively lower in rats than in humans. This emphasizes that the mechanics of rodent models is different from humans. Such contact forces can be used as boundary conditions in finite element models to determine local mechanical stimuli in articular cartilage and subchondral bone.View Large Image Figure ViewerDownload Hi-res image Download (PPT)

During the first half of the 20th century, extraordinary high jumping performances of East-African athletes were observed. These athletes used a specific native jumping style called Gusimbuka Urukiramende. Eye-witnesses believed that these performances could have been world-records and that these athletes could have competed at the Olympics. However, these athletes never participated in international competitions and there is no other proof to support these performance claims. We have analysed historical photos and cine sequences of these jumps, documented the movement analysis of this technique, quantified performance and compared it to contemporaneous elite performances. Our analyses demonstrate that Gusimbuka Urukiramende athletes did not jump as high as the world record. Nevertheless, even though they used a suboptimal jump technique (because they had to lift their bodies higher to cross the bar) they could cross bar heights of 188 cm or 106% body height and as such their performance still was worthy of participation to the Olympics.

Primates exhibit unusual quadrupedal features (e.g. diagonal gaits, compliant walk) compared with other quadrupedal mammals. Their origin and diversification in arboreal habitats have certainly shaped the mechanics of their walking pattern to meet the functional requirements necessary for balance control in unstable and discontinuous environments. In turn, the requirements for mechanical stability probably conflict with mechanical energy exchange. In order to investigate these aspects, we conducted an integrative study on quadrupedal walking in the olive baboon (Papio anubis) at the Primatology station of the CNRS in France. Based on kinematics, we describe the centre of mass mechanics of the normal quadrupedal gait performed on the ground, as well as in different gait and substrate contexts. In addition, we studied the muscular activity of six hindlimb muscles using non-invasive surface probes. Our results show that baboons can rely on an inverted pendulum-like exchange of energy (57% on average, with a maximal observed value of 84%) when walking slowly (<0.9 m s-1) with a tight limb phase (∼55%) on the ground using diagonal sequence gaits. In this context, the muscular activity is similar to that of other quadrupedal mammals, thus reflecting the primary functions of the muscles for limb movement and support. In contrast, walking on a suspended branch generates kinematic and muscular adjustments to ensure better control and to maintain stability. Finally, walking using the lateral sequence gait increases muscular effort and reduces the potential for high recovery rates. The present exploratory study thus supports the assumption that primates are able to make use of an inverted pendulum mechanism on the ground using a diagonal walking gait, yet a different footfall pattern and substrate appear to influence muscular effort and efficiency.

Objectives Recreational runners show a large interindividual variation in spatiotemporal characteristics. This research focused on slow runners and intended: (1) to document the variance in duty factor (DF) between runners in a real-life running setting and (2) examine whether the interindividual variation in DF and stride frequency (SF) relates to differences in external loading parameters between runners. Methods Spatiotemporal characteristics of 23 slow runners (ie, <2.6 m/s) were determined during a 5.2 km running event. To relate the interindividual variation in DF and SF to differences in external forces between runners (maximal vertical ground reaction force (FzMax), peak braking force (PBF) and vertical instantaneous loading rate (VILR)), 14 of them were invited to the lab. They ran at 1.9 m/s on a treadmill while ground reaction forces were recorded. A multiple linear regression analysis was conducted to investigate the effect of DF and SF on external force measures. Results DF between slow runners varied from 42.50% to 56.49% in a recreational running event. DF was found to be a significant predictor of FzMax (R²=0.755) and PBF (R²=0.430). SF only improved the model for PBF, but to a smaller extent than DF (R² change=0.191). For VILR, neither DF nor SF were significant predictors. Conclusion External forces are lower in recreational runners that run with higher DFs and slightly lower SFs. These findings may be important for injury prevention purposes, especially directed to recreational runners that are more prone to overuse injuries.

La marche bipede humaine est particulierement raffinee et efficace. Les primates non-humains (PNHs), quant a eux, utilisent la bipedie occasionnellement au sein d’un repertoire posturo-locomoteur souvent varie. Dans le contexte de l’evolution des modes locomoteurs chez les primates (incluant les hominines), une hypothese suggere l’existence d’un mecanisme de controle basique et similaire en bipedie et en quadrupedie. La tester necessite une observation directe de l’activite musculaire liee a ...