Kennisleer word bevestig

Uit teks: Die kennisleer is die leerstuk waarkragtens ’n latere saaklike reghebbende op ’n saak gebonde is aan die regte van ’n vroeëre persoonlike reghebbende op dieselfde saak waarvan die saaklike reghebbende werklik kennis gedra het. As gevolg van die regsfiguur van die kennisleer word beperkte saaklike werking aan ’n vroeëre persoonlike reg verleen. Die kennisleer word aangewend om die belange-konflik tussen ’n persoonlike en ’n saaklike reghebbende op te los

THE EFFECT OF COORDINATION ON VERTICAL JUMPING PERFORMANCE

The purpose of this study was to gain a better understanding of why coordination, or the timing of muscle actions, affects performance in vertical jumping. A forward dynamic simulation model was used, which calculated the motion corresponding to stimulationtime input to the muscles. A maximum-height jump was found by optimizing the stimulation-time input. Jump height amounted to 41 cm. Subsequently, the solution space was constrained by demanding that soleus was switched on 100 ms before any of the other muscles, and the stimulation-time input was again optimized. Jump height in the constrained jump was 9 cm less than that in the maximum-height jump, primarily because glutei and hamstrings produced less work. In the constrained jump, the glutei dissipated energy early in the push-off, and during shortening they produced less energy because their shortening velocity increased too quickly. These undesired effects could be explained by the effects of premature rotation of the foot on the motion in the hip joint. The analysis presented in this paper shows by which mechanisms the timing of muscle actions affects performance in vertical jumping

Reliability and validity of a low-cost portable force platform

BACKGROUND: A small, portable, inexpensive FP is a helpful test instrument in many strength and conditioning settings. OBJECTIVE: To assess the reliability and validity of a portable FP. METHODS: The FP was assessed statically for linearity and regionality using known weights and known weight placements across nine regions. Dynamic assessment was conducted by placing the FP on a laboratory-grade one-dimensional FP and performing static jumps, countermovement, and drop jumps with synchronized data acquisition. Frequency response of the FP was assessed by striking the top surface with a hammer. RESULTS: Excellent static linearity (r> 0.99), trivial differences in regional forces, excellent correlation between FPs in the static, countermovement, and anchored FP for the drop jump (all r> 0.98) were observed. Frequency response from an impact was poor when the FP was not anchored. However, once anchored the FP showed a dominant frequency of more than 10 times the typical jump frequencies and excellent synchrony with the laboratory FP (r> 0.98). CONCLUSION: The FP showed good to excellent characteristics in the static and countermovement jumps and the drop jumps when anchored. The primary limitation of the FP is its small size and light weight

Is the effect of a countermovement on jump height due to active state development?

Purpose: To investigate whether the difference in jump height between countermovement jumps (CMJ) and squat jumps (SJ) could be explained by a difference in active state during propulsion. Methods: Simulations were performed with a model of the human musculoskeletal system comprising four body segments and six muscles. The model's only input was STIM, the stimulation of muscles, which could be switched "off" or "on." After switching "on," STIM increased to its maximum at a fixed rate of change (dSTIM/dt). For various values of dSTIM/dt, stimulation switch times were optimized to produce a maximum height CMJ. From this CMJ, the configuration at the lowest height of the center of gravity (CG) was selected and used as static starting configuration for simulation of SJ. Next, STIM-switch times were optimized to find the maximum height SJ. Results: Simulated CMJ and SJ closely resembled jumps of human subjects. Maximum jump height of the model was greater in CMJ than in SJ, with the difference ranging from 0.4 cm at infinitely high dSTIM/dt to about 2.5 cm at the lowest dSTIM/dt investigated. The greater jump height in CMJ was due to a greater work output of the hip extensor muscles. These muscles could produce more force and work over the first 30% of their shortening range in CMJ, due to the fact that they had a higher active state in CMJ than in SJ. Conclusion: The greater jump height in CMJ than in SJ could be explained by the fact that in CMJ active state developed during the preparatory countermovement, whereas in SJ it inevitably developed during the propulsion phase, so that the muscles could produce more force and work during shortening in CMJ. Copyright © 2005 by the American College of Sports Medicine

Hysteretic magnetoresistance in polymeric diodes

We report on hysteretic organic magnetoresistance (OMAR) in polymeric diodes. We found that magnitude and lineshape of OMAR depends strongly on the scan speed of the magnetic field and on the time delay between two successive measurements. The time-dependent OMAR phenomenon is universal for diodes made with various polymers. However, the width and magnitude of OMAR varied with the polymeric material. The suggestive reason for this hysteretic behavior are trapped carriers, which in presence of a magnetic field changes the ferromagnetic ground-state of the polymer leading to long spin relaxation time. These experimental observations are significant for clarification of the OMAR phenomenon

Contribution of the forelimbs and hindlimbs of the horse to mechanical energy changes in jumping

The purpose of the present study was to gain more insight into the contribution of the forelimbs and hindlimbs of the horse to energy changes during the push-off for a jump. For this purpose, we collected kinematic data at 240 Hz from 23 5-year-old Warmbloods (average mass: 595 kg) performing free jumps over a 1.15 m high fence. From these data, we calculated the changes in mechanical energy and the changes in limb length and joint angles. The force carried by the forelimbs and the amount of energy stored was estimated from the distance between elbow and hoof, assuming that this part of the leg behaved as a linear spring. During the forelimb push, the total energy first decreased by 3.2 J k