Isometric impulse frequencies associated with active tremor and force regulation were examined in 10 patients with idiopathic Parkinson's disease (PD) and in 10 older adults (OAs) who performed an isometric tracing task. The authors decoupled and analyzed the data to determine whether PD-related tremor in the thumb and in the index finger during isometric force control are related and whether PD impairs the performance of volitional force control beyond the errors contributed by tremor. After decoupling, there were clear and robust differences in PD patients' control of isometric force that could not be attributed to action-tremor error. Those errors, which occurred in the absence of movement, suggest impairment in coordinated recruitment and derecruitment of motor units during a fine-motor task.
Studies of the biology of aging (both experimental and evolutionary) frequently involve the estimation of parameters arising in various multi-parameter survival models such as the Gompertz or Weibull distribution. Standard parameter estimation methodologies, such as maximum likelihood estimation (MLE) or nonlinear regression (NLR), require knowledge of the actual life spans or their explicit algebraic equivalents in order to provide reliable parameter estimates. Many fundamental biological discussions and conclusions are highly dependent upon accurate estimates of these survival parameters (this has historically been the case in the study of genetic and environmental effects on longevity and the evolutionary biology of aging). In this article, we examine some of the issues arising in the estimation of gerontologic survival model parameters. We not only address issues of accuracy when the original life-span data are unknown, we consider the accuracy of the estimates even when the exact life spans are known. We examine these issues as applied to known experimental data on diet restriction and we fit the frequently used, two-parameter Gompertzian survival distribution to these experimental data. Consequences of methodological misuse are demonstrated and subsequently related to the values of the final parameter estimates and their associated errors. These results generalize to other multiparametric distributions such as the Weibull, Makeham, and logistic survival distributions.
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