James Robert Stirling

Does exercise training during pregnancy affect gestational age? A randomised controlled trial

Background: Some controversy exists over the possibility that exercise during pregnancy might increase the risk of preterm delivery. Objective: This study aimed to determine the possible cause–effect relationship between regular exercise performed during the second and third trimesters of pregnancy by previously sedentary, healthy gravidae and gestational age at the moment of delivery. Methods: Caucasian (Spanish) women with singleton gestation were assigned to either a training (n = 72) or a control (n = 70) group. The supervised training programme focused mainly on very light resistance and toning exercises and included ∼80 sessions (three times/week, 35 min/session from weeks 12–13 to weeks 38–39 of pregnancy). Results: No significant differences were found (p>0.05) between the groups in those maternal characteristics (age, smoking habits, number of hours standing or prior parity history) that could potentially influence gestational age. The mean gestational age did not differ (p = 0.745) between the training (39 weeks,3 days (SD 1 day)) and the control group (39 weeks,4 days (SD 1 day)). Conclusions: Previously sedentary, healthy gravidae with singleton gestation can safely engage in moderate, supervised exercise programmes until the end of gestation as this would not affect gestational age.

Type of delivery is not affected by light resistance and toning exercise training during pregnancy: a randomized controlled trial

OBJECTIVE We examined the effect of light-intensity resistance exercise training that is performed during the second and third trimester of pregnancy by previously sedentary and healthy women on the type of delivery and on the dilation, expulsion, and childbirth time. STUDY DESIGN We randomly assigned 160 sedentary women to either a training (n=80) or a control (n=80) group. We recorded several maternal and newborn characteristics, the type of delivery (normal, instrumental, or cesarean), and dilation, expulsion, and childbirth time. RESULTS The percentage of women who had normal, instrumental, or cesarean delivery was similar in the training (70.8%, 13.9%, and 15.3%, respectively) and control (71.4%, 12.9%, and 15.7%, respectively) groups. The mean dilation, expulsion, and childbirth time did not differ between groups. CONCLUSION Light-intensity resistance training that is performed over the second and third trimester of pregnancy does not affect the type of delivery.

A Model of Heart Rate Kinetics in Response to Exercise

Abstract We present a mathematical model, in the form of two coupled ordinary differential equations, for the heart rate kinetics in response to exercise. Our heart rate model is an adaptation of the model of oxygen uptake kinetics of Stirling et al. [21]; a physiological justification for this adaptation, as well as the physiological basis of our heart rate model is provided. We also present the optimal fit of the heart rate model to a set of raw un averaged data for multiple constant intensity exercises for an individual at a particular level of fitness.

Counterpoint: The kinetics of oxygen uptake during muscular exercise do not manifest time-delayed phases

The existence of time-delayed phases ([1][1]) is not supported by oxygen uptake kinetics data. Despite many attempts for a number of years, no convincing physiological mechanism for such behavior has been proven to exist. The reason is that these time-delayed phases are a figment of the incorrect

Stochastic optimization for the calculation of the time dependency of the physiological demand during exercise and recovery

The stochastic optimization method ALOPEX IV is successfully applied to the problem of estimating the time dependency of the physiological demand in response to exercise. This is a fundamental and unsolved problem in the area of exercise physiology, where the lack of appropriate tools and techniques forces the assumption and the use of a constant demand during exercise. By the use of an appropriate partition of the physiological time series and by means of stochastic optimization, the time dependency of the physiological demand during heavy intensity exercise and its subsequent recovery is, for the first time, revealed.

Modeling the basin of attraction as a two-dimensional manifold from experimental data: applications to balance in humans.

We present a method of modeling the basin of attraction as a three-dimensional function describing a two-dimensional manifold on which the dynamics of the system evolves from experimental time series data. Our method is based on the density of the data set and uses numerical optimization and data modeling tools. We also show how to obtain analytic curves that describe both the contours and the boundary of the basin. Our method is applied to the problem of regaining balance after perturbation from quiet vertical stance using data of an elite athlete. Our method goes beyond the statistical description of the experimental data, providing a function that describes the shape of the basin of attraction. To test its robustness, our method has also been applied to two different data sets of a second subject and no significant differences were found between the contours of the calculated basin of attraction for the different data sets. The proposed method has many uses in a wide variety of areas, not just human balance for which there are many applications in medicine, rehabilitation, and sport.

The Point of Maximum Curvature as a Marker for Physiological Time Series

Abstract We present a geometric analysis of the model of Stirling et al. [14]. In particular we analyze the curvature of a heart rate time series in response to a step like increment in the exercise intensity. We present solutions for the point of maximum curvature which can be used as a marker of physiological interest. This marker defines the point after which the heart rate no longer continues to rapidly rise and instead follows either a steady state or slow rise. These methods are then applied to find analytic solutions for a mono exponential model which is commonly used in the literature to model the response to a moderate exercise intensity. Numerical solutions are then found for the full model and parameter values presented in Stirling et al. [14].

Stochastic optimization for the calculation of the optimal critical curve from experimental data in a model of the process of regaining balance after perturbation from quiet stance

We demonstrate the successful application of ALOPEX stochastic optimization to the problem of calculating the optimal critical curve in a dynamical systems model of the process of regaining balance after perturbation from quiet stance. Experimental data provide the time series of angles for which the subjects were able to regain balance after an initial perturbation. The optimal critical curve encloses all data points and has a minimum distance from the border points of the data set. We demonstrate the results of the optimization firstly using the traditional cost function of chi-square distance. We then successfully introduce a modified cost function that fits the model to the experimental data by taking into account the specific requirements of the model. By use of the proposed cost function, combined with the efficiency of our optimization method, an optimal critical curve is calculated even in the cases of very asymmetric data sets that lie within the capabilities of the existing model.

Last Word on Point:Counterpoint: The kinetics of oxygen uptake during muscular exercise do/do not manifest time-delayed phases

TO THE EDITOR: Not one of the proposed physiological mechanisms support the existence of time-delayed instantaneous stepwise increments in oxygen demand that mark the beginning of each time-delayed phase and are the very foundation of the time-delayed three-phase exponential model. The chance of finding physiological mechanisms to support such behavior is nonexistent, due mainly to the instantaneous nature of the increment. Not one of the proposed physiological mechanisms contradicts the model (2, 3), as even without time-delayed phases it produces features that only become apparent after a period of time. As shown in (5), this model can incorporate a far more realistic oxygen demand (i.e., one that changes smoothly with intensity and time to account for inertia and changing underlying physiological processes) than the obviously erroneous instantaneous increments of the three-phase model.

Geometry and transport in a model of two coupled quadratic nonlinear waveguides.

This paper applies geometric methods developed to understand chaos and transport in Hamiltonian systems to the study of power distribution in nonlinear waveguide arrays. The specific case of two linearly coupled chi((2)) waveguides is modeled and analyzed in terms of transport and geometry in the phase space. This gives us a transport problem in the phase space resulting from the coupling of the two Hamiltonian systems for each waveguide. In particular, the effect of the presence of partial and complete barriers in the phase space on the transfer of intensity between the waveguides is studied, given a specific input and range of material properties. We show how these barriers break down as the coupling between the waveguides is increased and what the role of resonances in the phase space has in this. We also show how an increase in the coupling can lead to chaos and global transport and what effect this has on the intensity.