[Work environment. When things go wrong].

Children in the first weeks of independent locomotion display a wide variety of walking forms. The walking forms differ in mechanical strategy and concern with balance. Three extreme walking forms are presented: the Twister, who uses trunk twist, the Faller, who uses gravity, and the Stepper, who remains balanced as much as possible. Each walking form is presented as a "d-space", a mathematical format combining continuous and discrete aspects, developed to express the sequence and pattern of a movement without the inappropriate precision of a physical trajectory. The three d-spaces represent analyses of three extreme modes of early walking. They are used to generate the variety of early walking forms and to predict mixtures of mechanical strategies as children mature and converge to more similar walking forms over the first few months of independent locomotion.

Biomechanical Phenomena↗

PubMed1995

Characterization of the unique mechanism mediating the shear-dependent binding of soluble von Willebrand factor to platelets.

We have studied the mechanism of interaction between soluble von Willebrand factor (vWF), labeled with fluorescein isothiocyanate (FITC), and platelets exposed to shear in a cone-and-plate viscometer. A flow cytometer calibrated with fluorescent bead standards was used to calculate the number of molecules associated with each platelet in suspension. To validate the methods and reagents used, binding of the same labeled vWF was assessed in the presence of ristocetin or alpha-thrombin and found to be saturable, with a narrow and symmetric distribution on > 90% of the platelets. As expected, essentially all bound ligand interacted exclusively with platelet membrane glycoprotein (GP) Ib alpha in the presence of ristocetin and with GP IIb-IIIa after stimulation with alpha-thrombin. In contrast, only a minor proportion (< 20%) of the platelets exposed to shear were found to bind vWF, with no evidence for saturation and markedly decreased interaction when the platelet count was below 100,000 microliters. Moreover, shear-induced vWF binding was blocked equally effectively by selected monoclonal antibodies against either GP Ib alpha or GP IIb-IIIa or against the respective binding sites in vWF. Thus, both receptors are involved in the process, possibly through initial transient interactions mediated by GP Ib alpha that lead to platelet activation and subsequent irreversible binding supported by GP IIb-IIIa. While the levels of shear stress theoretically applied to platelets in these experiments are above those thought to occur in the normal circulation, our findings demonstrate a unique vWF binding mechanism that is not mimicked by other known modulators and correlates with platelet aggregation. Similar processes may occur in response to lower shear stress when platelets are exposed to thrombogenic surfaces and agonists generated at sites of vascular injury during thrombus formation.

Biomechanical Phenomena↗