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The Kinesthetic Ability Trainer (KAT 2000) is a balance platform designed for training and functional testing of the neuromuscular control system. Forty healthy and sports-active persons were tested and 1 month later retested to investigate the reliability of the KAT 2000 testing "one-leg static balance" and "two-leg dynamic balance". A significant improvement at retesting on the same day was seen in both tests; furthermore the dynamic test result improved significantly with retesting 1 month later. The data obtained made it possible to calculate the 95% confidence limits for an unchanged test result for a single person and a group of persons. The results show a clear learning effect when the persons are retested, especially in the dynamic test. The KAT 2000 can be used as a tool for testing groups of persons both in short- and long-term studies, but it cannot be used for testing single persons due to the great variance in the test results. Further investigations involving injured persons are needed to determine the range of improvement after intervention.

New measurements and Monte Carlo simulations of the total coefficients eta + for the backscattering of positrons from elemental solids are reported as a function of atomic number Z between 13 and 82, incident energies E from 1 to 50 keV, and incident angles between 0 degrees and 65 degrees . The measurements and simulations show generally good agreement with each other and with the recent measurements of Massoumi et al (1992) and Makinen et al (1992). Both experiment and simulations suggest that the monotonic increase of eta + with Z seen at high E is not observed for E below 10 keV. Where possible, the new results are compared with earlier measurements of electron coefficients.

A theoretical treatment of atomic structure and positron states in noble-gas bubbles in metals is presented. The Al-He and Cu-Kr systems are considered as specific examples. For large bubbles (radii above a few tens of angstroms) a calculational scheme is developed combining moleculardynamics results for the metalnoble-gas interface with positron calculations. It is demonstrated that a positron is trapped at the surface of a noble-gas bubble, i.e. , at the metal-gas interface. The annihilation rate with metal electrons is similar to that at a clean surface, while simultaneously there is a significant annihilation rate with gas-atom electrons. This enables relationships between the gas density and the positron lifetime to be obtained for the systems considered. Experimental evidence supports the theoretical relations. In the molecular-dynamics simulations a trend towards close-packed layer-by-layer ordering of the gas atoms close to the metal-gas interface is found even in the cases where the bulk gas is in a Quid phase. The positron-state calculations also cover the case of adsorbed noble-gas layers at surfaces. For vacancynoble-gas clusters containing up to 13 vacancies complementary positron results obtained with the calculational method developed by Puska and Nieminen are presented.Calculations for positrons trapped in small vacancy gas clusters will also be presented, supplementing the interface results applicable to large bubbles.This paper is organized as follows: Section II contains the results for the Al-He system and presents the general calculational approach applied. The results for the Cu-Kr system are presented separately in Sec. III. The re-36 8219 1987 The American Physical Society KJELD O. JENSEN AND R. M. NIEMINEN suits are discussed in Sec. IV. Section V provides a summary and conclusion.

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