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Understanding leg and joint stiffness adjustment during maximum hopping may provide important information for developing more effective training methods. It has been reported that ankle stiffness has major influence on stable spring-mass dynamics during submaximal hopping, and that knee stiffness is a major determinant for hopping performance during maximal hopping task. Furthermore, there are no reports on how the height of the previous hop could affect overall stiffness modulation of the subsequent maximum one. The purpose of the present study was to determine whether and how the jump height of the previous hop affects leg and joint stiffness for subsequent maximum hop. Ten participants completed trials in which they repeatedly hopped as high as possible (MX task) and trials in which they were instructed to perform several maximum hops with 3 preferred (optimal) height hops between each of them (P3MX task). Both hopping tasks were performed at 2.2 Hz hopping frequency and at the participant's preferred (freely chosen) frequency as well. By comparing results of those hopping tasks, we found that ankle stiffness at 2.2 Hz (p=0.041) and knee stiffness at preferred frequency (p=0.045) was significantly greater for MX versus P3MX tasks. Leg stiffness for 2.2 Hz hopping is greater than for the preferred frequency. Ankle stiffness is greater for 2.2 Hz than for preferred frequencies; opposite stands for knee stiffness. The results of this study suggest that preparatory hop height can be considered as an important factor for modulation of maximum hop.
Content available Modeling of human tissue for medical purposes
The paper describes the possibilities offered for medicine by modeling of human tissue using virtual and augmented reality. It also presents three proposals of breast modeling for the use in clinical practice. These proposals are the result of arrangements of medical and computer scientists team (the authors) and will be pursued and implemented in the near future. There is included also a brief description of the most popular methods for modeling of human tissue: spring-mass model and finite element method. Moreover the paper attempts to estimate the benefits of the developed models.
Content available remote Rigid body assembly impact models for adiabatic cutoff equipments
This paper is concerned with systems consisting of components colliding with each other. In particular, a high velocity adiabatic impact cutoff machinę is investigated. For generał understanding of the impact dynamics (affected by a large number of parameters), the mech-anisms are modelled in a simplified and accurate manner. Two simple models are developed: the energy-balance model and the spring-mass model. The energy-balance model is based on the principle of total energy conservation. It provides only the punch minimum kinetic energy reąuired for efEcient cutting. Concerning the spring-mass model, the different components are represented by rigid masses and their deformations are modelled by springs (linear or non-linear in the case of contact stiffness). The resulting non-linear eąuations are solved using the Newmark numerical teclmiąue. The impact force, velocity, displacement and acceleration histories are calculated what makes possible a fine description of the cutoff cycle steps. The two models are helpful for both the design and tuning of the mechanisms involving impacts between their components.
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