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A Ruina

Publications and source records attributed to A Ruina.

8 recordsLinked to original sources
PubMed2001

Prediction of stable walking for a toy that cannot stand.

Previous experiments [M. J. Coleman and A. Ruina, Phys. Rev. Lett. 80, 3658 (1998)] showed that a gravity-powered toy with no control and that has no statically stable near-standing configurations can walk stably. We show here that a simple rigid-body statically unstable mathematical model based loosely on the physical toy can predict stable limit-cycle walking motions. These calculations add to the repertoire of rigid-body mechanism behaviors as well as further implicating passive dynamics as a possible contributor to stability of animal motions.

Animals↗
PubMed2001

Multiple walking speed-frequency relations are predicted by constrained optimization.

A person constrained to walk at a given speed v on a treadmill, chooses a particular step frequency f and step length d=v/f. Testing over a range of speeds generates a speed-frequency (v-f) relationship. This relationship is commonly posited as a basic feature of human gait. It is often further posited that this curve follows from minimum energy cost strategy. We observed that individuals walking under different constraint circumstances--walking to a range of fixed metronome frequencies (fixed f) or over a range of spaced markers (fixed d)--produce speed-frequency relations distinct from the constrained v relation. We show here that three distinct speed-frequency curves, similar to those observed, are predicted by the assumption that a walking person optimizes an underlying objective function F (v, f) that has a minimum at the preferred gait. Further, the metabolic cost of transport is a reasonable approximate candidate for the function F.

Energy Metabolism↗
PubMed1999

A point-mass model of gibbon locomotion.

In brachiation, an animal uses alternating bimanual support to move beneath an overhead support. Past brachiation models have been based on the oscillations of a simple pendulum over half of a full cycle of oscillation. These models have been unsatisfying because the natural behavior of gibbons and siamangs appears to be far less restricted than so predicted. Cursorial mammals use an inverted pendulum-like energy exchange in walking, but switch to a spring-based energy exchange in running as velocity increases. Brachiating apes do not possess the anatomical springs characteristic of the limbs of terrestrial runners and do not appear to be using a spring-based gait. How do these animals move so easily within the branches of the forest canopy? Are there fundamental mechanical factors responsible for the transition from a continuous-contact gait where at least one hand is on a hand hold at a time, to a ricochetal gait where the animal vaults between hand holds? We present a simple model of ricochetal locomotion based on a combination of parabolic free flight and simple circular pendulum motion of a single point mass on a massless arm. In this simple brachiation model, energy losses due to inelastic collisions of the animal with the support are avoided, either because the collisions occur at zero velocity (continuous-contact brachiation) or by a smooth matching of the circular and parabolic trajectories at the point of contact (ricochetal brachiation). This model predicts that brachiation is possible over a large range of speeds, handhold spacings and gait frequencies with (theoretically) no mechanical energy cost. We then add the further assumption that a brachiator minimizes either its total energy or, equivalently, its peak arm tension, or a peak tension-related measure of muscle contraction metabolic cost. However, near the optimum the model is still rather unrestrictive. We present some comparisons with gibbon brachiation showing that the simple dynamic model presented has predictive value. However, natural gibbon motion is even smoother than the smoothest motions predicted by this primitive model.

Animals↗
PubMed1998

The simplest walking model: stability, complexity, and scaling.

We demonstrate that an irreducibly simple, uncontrolled, two-dimensional, two-link model, vaguely resembling human legs, can walk down a shallow slope, powered only by gravity. This model is the simplest special case of the passive-dynamic models pioneered by McGeer (1990a). It has two rigid massless legs hinged at the hip, a point-mass at the hip, and infinitesimal point-masses at the feet. The feet have plastic (no-slip, no-bounce) collisions with the slope surface, except during forward swinging, when geometric interference (foot scuffing) is ignored. After nondimensionalizing the governing equations, the model has only one free parameter, the ramp slope gamma. This model shows stable walking modes similar to more elaborate models, but allows some use of analytic methods to study its dynamics. The analytic calculations find initial conditions and stability estimates for period-one gait limit cycles. The model exhibits two period-one gait cycles, one of which is stable when 0 < gamma < 0.015 rad. With increasing gamma, stable cycles of higher periods appear, and the walking-like motions apparently become chaotic through a sequence of period doublings. Scaling laws for the model predict that walking speed is proportional to stance angle, stance angle is proportional to gamma 1/3, and that the gravitational power used is proportional to v4 where v is the velocity along the slope.

Algorithms↗
PubMed1997

A dynamic force and moment analysis system for brachiation.

We describe a transducer system and analysis strategy that allows the determination of dynamic forces and moments applied by an arm-swinging animal during locomotion. We have employed readily available technology and analysis procedures to produce a low-cost but effective system. The solutions to several problems in the design of the system are provided, and the functional characteristics of the system are demonstrated using both an inert pendulum and an actively brachiating gibbon (Hylobates lar).

Animals↗
PubMed1990

[Picotamide does not interfere with the anticoagulant activity of warfarin in patients wearing heart valve prostheses].

Picotamide, a new antiplatelet drug which has only slight effects on bleeding time, could be useful, in combination with oral anticoagulants, for the prevention of thromboembolic complications in patients with heart valve prostheses. We have evaluated in a randomized, controlled, double-blind, cross-over study, the effect of picotamide on the anticoagulant activity of warfarin. Administration of 300 mg t.i.d. for 10 days to 10 patients with aortic or mitral valve prostheses did not modify significantly either the level of anticoagulation or the mean daily dosage of warfarin. We observed a trend towards a reduction of plasma levels of beta-thromboglobulin. In conclusion the results of this study show that picotamide does not interfere with the anticoagulant activity of warfarin.

Aged↗
PubMed1990

A mechanics model of the compression of cells with finite initial contact area.

The effect of intercellular bonding on the stress-strain behavior of soft plant tissue is considered. In our mechanical model, a conglomerate of identical cells is arranged in a regular array. Each cell is pressurized and bonded across flat contact areas with adjacent cells in the direction of the applied load. The cell wall is a finitely-deformed mechanical membrane bounding an incompressible fluid (the cytoplasm). A nonlinear elastic constitutive law is presented that describes data for apple parenchyma. Results show that intercellular bonding has a strong effect on the macroscopic properties of the whole tissue. A larger intercellular contact area increases tissue stiffness and magnifies the effect of initial turgor pressure on tissue stiffness.

Biomechanical Phenomena↗