Finger stability in precision grips
Stable precision grips using the fingertips are a cornerstone of human hand dex- terity. Occasionally, however, our fingers become unstable and snap into a hyper- extended posture. This is because multi-link mechanisms, like our fingers, can buckle under tip forces. Suppressing this instability is crucial for hand dexterity, but how the neuromuscular system does so is unknown. Here we show that finger stability is due to the stiffness from muscle contraction. We recorded maximal force applica- tion with the index finger and found that most buckling events lasted less than 50 ms, too fast for sensorimotor feedback to act. However, a biomechanical model of the finger predicted that muscle-induced stiffness is also insufficient for stability at maxi- mal force unless we add springs to stiffen the joints. We tested this prediction in 23 volunteers. Upon adding stiffness, maximal force increased by 31$\pm$3%, and muscle electromyography readings were 22$\pm$3% higher for the finger flexors (mean$\pm$standard error). Hence, people refrain from applying truly maximal force unless an external stabilizing stiffness allows their muscles to apply higher force without losing stability. Using our finger model, we found that there is leeway for muscles to co-contract and stabilize the finger at sub-maximal but not maximal force. Our experiment reveals that human fingers are indeed significantly co-contracted. More stiffness helps stability but would affect everyday hand usage because we need compliant fingers to adapt to com- plex object geometries and regulate force precisely. Thus, our results show how hand function arises from neurally tuned muscle stiffness that balances finger stability with compliance.
Size and shape of terrestrial animals
Small land animals tend to have a crouched or sprawled posture, whereas larger animals are generally more upright. Legged locomotion biomechanics is considered central to the evolution of such size-dependent body shape in birds and mammals. But debates continue about how body shape affects locomotion and how the natural environment affects locomotion. These debates have left out a crucial factor for survival, namely, locomotion stability. In particular, lateral stability when stand- ing or moving is profoundly affected by the frontal aspect ratio of the stance width to center of mass height. The wider an animal, the more stable it is. But for natural terrain, the further apart two ground support points are, the higher the variance in height difference. An aspect ratio scaling law emerges from this competition between lateral stability and terrain unevenness. Here we show that the scaling law arising from the need for stability on natural terrain correctly predicts the frontal aspect ratio scaling across 335 terrestrial vertebrates and inver- tebrates, spanning eight orders of magnitude in mass from 28mg to 22,000kg, so that smaller animals have a wider aspect ratio. Phylogenetic comparative analysis reveals that the trend does not simply arise by gradual changes in traits over time due to phylogenetic relatedness. Thus, we propose that adaptation to stability demands on natural terrain likely drove the macroevolution of body aspect ratio across multiple clades of terrestrial animals with diverse body plans, gait styles, and limb morphologies. Neural limb control and niche-specific morphological specializations further modulate the baseline frontal aspect ratio defined by the frontal profile.
Active viscoelasticity in sarcomeres
The perturbation response of muscle is important for the versatile, stable and agile control capabilities of animals. Muscle resists being stretched by developing forces in the passive tissues and in the active crossbridges. This review focuses on the active perturbation response of the sarcomere. The active response exhibits typical stress relaxation, and thus approximated by a Maxwell material that has a spring and dashpot arranged in series. The ratio of damping to stiffness in this approximation defines the relaxation timescale for dissipating stresses that are developed in the crossbridges due to external perturbations. Current understanding of sarcomeres suggests that stiffness varies nearly linearly with neural excitation, but not much is known about damping. But if both stiffness and damping have the same functional (linear or not) dependence on neural excitation, then the stress relaxation timescale cannot be varied depending on the demands of the task. This implies an unavoidable and biologically unrealistic trade-off between how freely the crossbridges can yield and dissipate stresses when stretched (injury avoidance in agile motions) vs. how long they can maintain perturbation-induced stresses and behave like a solid material (stiffness maintenance for stability). We hypothesize that muscle circumvents this trade-off by varying damping in a nonlinear manner with neural excitation, unlike stiffness that varies linearly. Testing this hypothesis requires new experimental and mathematical characterization of muscle mechanics, and also identifies new design goals for robotic actuators.
Morphology of joints in animals
Flexural joints are becoming increasingly prevalent in monolithic robotic limbs. A flexural joint is made up of a soft elastic interconnecting piece, called the flexure, between two stiffer segments. When the joint is flexed by an external force, most of the flexion is concentrated within the soft flexural element. While several detailed analyses exist for specific geometries of flexures, an all encapsulating theory for flexure function is missing. We use simple models to specifically characterize how the stiffness of flexure scales with size. We find that stable flexures severely limit the maximal rotation of actuated flexural joints, with typical values of rotation being less than 0.2 radians. This limit is independent of the size of the joint. In calculating this bound, we optimistically assume that the joints are actuated by motors whose torque density is as high as biological muscle. These results call into question the feasibility of flexural joints, especially for mobile or lightweight applications that use weaker actuators. We find the scaling relationships for the range of rotation of an actuated flexural joint as a function of the flexure’s material properties, cross-sectional geometry, and the torque density of the actuator. These results provide guidelines for the design and development of scalable flexural robotic limbs.
Ph.D. dissertation: On the role of stability in animal morphology and neural control