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1. Do forelimb shape and peak forces co‐vary in strepsirrhines?

   https://doi.org/10.1002/ajpa.23688  

   Fabre, Anne‐Claire ; Granatosky, Michael C. ; Hanna, Jandy B. ; Schmitt, Daniel ( August 2018 , American Journal of Physical Anthropology)

   In this study, we explore whether ground reaction forces recorded during horizontal walking co‐vary with the shape of the long bones of the forelimb in strepsirrhines. To do so, we quantify (1) the shape of the shaft and articular surfaces of each long bone of the forelimb, (2) the peak vertical, mediolateral, and horizontal ground reaction forces applied by the forelimb during arboreal locomotion, and (3) the relationship between the shape of the forelimb and peak forces.

   Geometric morphometric approaches were used to quantify the shape of the bones. Kinetic data were collected during horizontal arboreal walking in eight species of strepsirrhines that show variation in habitual substrate use and morphology of the forelimb. These data were then used to explore the links between locomotor behavior, morphology, and mechanics using co‐variation analyses in a phylogenetic framework.

   Our results show significant differences between slow quadrupedal climbers (lorises), vertical clinger and leapers (sifaka), and active arboreal quadrupeds (ring‐tailed lemur, ruffed lemur) in both ground reaction forces and the shape of the long bones of the forelimb, with the propulsive and medially directed peak forces having the highest impact on the shape of the humerus. Co‐variation between long bone shape and ground reaction forces was detected in both the humerus and ulna even when accounting for differences in body mass.

   These results demonstrate the importance of considering limb‐loading beyond just peak vertical force, or substrate reaction force. A re‐evaluation of osseous morphology and functional interpretations is necessary in light of these findings.

    

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2. A novel implantable mechanism-based tendon transfer surgery for adult acquired flatfoot deformity: Evaluating feasibility in biomechanical simulation

   https://doi.org/10.1371/journal.pone.0270638  

   Ling, Hantao ; Balasubramanian, Ravi ( September 2022 , PLOS ONE)

   Virdi, Amarjit Singh (Ed.)

   Adult acquired flatfoot deformity becomes permanent with stage III posterior tibialis tendon dysfunction and results in foot pain and difficulty walking and balancing. To prevent progression to stage III posterior tibialis tendon dysfunction when conservative treatment fails, a flexor digitorum longus to posterior tibialis tendon transfer is often conducted. However, since the flexor digitorum longus only has one-third the force-capability of the posterior tibialis, an osteotomy is typically also required. We propose the use of a novel implantable mechanism to replace the direct attachment of the tendon transfer with a sliding pulley to amplify the force transferred from the donor flexor digitorum longus to the foot arch. In this work, we created four OpenSim models of an arched foot, a flatfoot, a flatfoot with traditional tendon transfer, and a flatfoot with implant-modified tendon transfer. Paired with these models, we developed a forward dynamic simulation of the stance phase of gait that reproduces the medial/lateral distribution of vertical ground reaction forces. The simulation couples the use of a fixed tibia, moving ground plane methodology with simultaneous activation of nine extrinsic lower limb muscles. The arched foot and flatfoot models produced vertical ground reaction forces with the characteristic double-peak profile of gait, and the medial/lateral distribution of these forces compared well with the literature. The flatfoot model with implant-modified tendon transfer produced a 94.2% restoration of the medial/lateral distribution of vertical ground reaction forces generated by our arched foot model, which also represents a 2.1X improvement upon our tendon transfer model. This result demonstrates the feasibility of a pulley-like implant to improve functional outcomes for surgical treatment of adult acquired flatfoot deformity with ideal biomechanics in simulation. The real-world efficacy and feasibility of such a device will require further exploration of factors such as surgical variability, soft tissue interactions and healing response. 

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3. Granular Jamming Feet Enable Improved Foot-Ground Interactions for Robot Mobility on Deformable Ground

   Chopra, Shivam Tolley ( April 2020 , IEEE robotics automation letters)

   Recent studies on dynamic legged locomotion have focused on incorporating passive compliant elements into robot legs which can help with energy efficiency and stability, enabling them to work in wide range of environments. In this work, we present the design and testing of a soft robotic foot capable of active stiffness control using granular jamming. This foot is designed and tested to be used on soft, flowable ground such as sand. Granular jamming feet enable passive foot shape change when in contact with the ground for adaptability to uneven surfaces, and can also actively change stiffness for the ability to apply sufficient propulsion forces. We seek to study the role of shape change and stiffness change in foot-ground interactions during foot-fall impact and shear. We have measured the acceleration during impact, surface traction force, and the force to pull the foot out of the medium for different states of the foot. We have demonstrated that the control of foot stiffness and shape using the proposed foot design leads to improved locomotion, specifically an approximately 52% reduced foot deceleration at the joints after impact, an approximately 63% reduced depth of penetration in the sand on impact, higher shear force capabilities for a constant depth above the ground, and an approximately 98% reduced pullout force compared to a rigid foot. 

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4. Red muscle activity in bluegill sunfish Lepomis macrochirus during forward accelerations

   https://doi.org/10.1038/s41598-019-44409-7  

   Schwalbe, Margot A. B. ; Boden, Alexandra L. ; Wise, Tyler N. ; Tytell, Eric D. ( May 2019 , Scientific Reports)

   Fishes generate force to swim by activating muscles on either side of their flexible bodies. To accelerate, they must produce higher muscle forces, which leads to higher reaction forces back on their bodies from the environment. If their bodies are too flexible, the forces during acceleration could not be transmitted effectively to the environment, but fish can potentially use their muscles to increase the effective stiffness of their body. Here, we quantified red muscle activity during acceleration and steady swimming, looking for patterns that would be consistent with the hypothesis of body stiffening. We used high-speed video, electromyographic recordings, and a new digital inertial measurement unit to quantify body kinematics, red muscle activity, and 3D orientation and centre of mass acceleration during forward accelerations and steady swimming over several speeds. During acceleration, fish co-activated anterior muscle on the left and right side, and activated all muscle sooner and kept it active for a larger fraction of the tail beat cycle. These activity patterns are both known to increase effective stiffness for muscle tissuein vitro, which is consistent with our hypothesis that fish use their red muscle to stiffen their bodies during acceleration. We suggest that during impulsive movements, flexible organisms like fishes can use their muscles not only to generate propulsive power but to tune the effective mechanical properties of their bodies, increasing performance during rapid movements and maintaining flexibility for slow, steady movements.

    

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5. Evaluation of force feedback in walking using joint torques as 'naturalistic' stimuli

   https://doi.org/10.1152/jn.00120.2021  

   Zill, Sasha N. ; Dallmann, Chris J ; Szczecinski, Nicholas ; Büschges, Ansgar ; Schmitz, Josef ( June 2021 , Journal of Neurophysiology)

   null (Ed.)

   Control of adaptive walking requires the integration of sensory signals of muscle force and load. We have studied how mechanoreceptors (tibial campaniform sensilla) encode 'naturalistic' stimuli derived from joint torques of stick insects walking on a horizontal substrate. Previous studies showed that forces applied to the legs using the mean torque profiles of a proximal joint were highly effective in eliciting motor activities. However, substantial variations in torque direction and magnitude occurred at the more distal femoro-tibial joint, which can generate braking or propulsive forces and provide lateral stability. To determine how these forces are encoded, we utilized torque waveforms of individual steps that had maximum values in stance in the directions of flexion or extension. Analysis of kinematic data showed that the torques in different directions tended to occur in different ranges of joint angles. Variations within stance were not accompanied by comparable changes in joint angle but often reflected vertical ground reaction forces and leg support of body load. Application of torque waveforms elicited sensory discharges with variations in firing frequency similar to those seen in freely walking insects. All sensilla directionally encoded the dynamics of force increases and showed hysteresis to transient force decreases. Smaller receptors exhibited more tonic firing. Our findings suggest that dynamic sensitivity in force feedback can modulate ongoing muscle activities to stabilize distal joints when large forces are generated at proximal joints. Further, use of 'naturalistic' stimuli can reproduce characteristics seen in freely moving animals that are absent in conventional restrained preparations. 

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