Understanding Joint Retirement.

Evidence from different sources shows that spouses' retirement decisions are correlated. Retirement policies affecting individuals in couples are therefore also likely to affect behavior of their spouses. It is therefore important to account for joint features in modeling retirement. This paper studies a structural collective model of labor supply and retirement of both partners in a couple with interdependent preferences, imperfect knowledge of preferences of the spouse, and subjective expectations about the future. We propose a novel method to estimate preferences and the intra-household bargaining process, which relies on stated preferences data collected in the Health and Retirement Study. Respondents were asked to choose between hypothetical retirement trajectories describing the retirement ages and replacement rates of both spouses from three perspectives: considering their own preferences only, the preferences of their spouse only, or the most likely decision for the household. With these data, all model parameters are identified and potential sources of joint retirement can be disentangled. Our results suggest that males misperceive their wives' preferences, overestimating their disutility of work. Our estimates correct for this. We find strong positive correlations between preferences for joint leisure (leisure complementarity) of the two partners. Counterfactual simulations with stylized retirement paths suggest that the leisure complementarities explain a substantial part of joint retirement, much more than correlation in unobserved heterogeneity or potential wage rates.

The metabolic cost of in vivo constant muscle force production at zero net mechanical work.

The metabolic cost per unit force is generally thought to increase with the mechanical work done by the muscle fibres. It is currently unclear how the metabolic cost of doing alternating positive and negative muscle fibre mechanical work relates to the metabolic cost of doing zero muscle fibre mechanical work at similar muscle force. The current study aimed to investigate this issue by comparing in vivo metabolic power between a dynamic and an isometric near-constant force production task. In both tasks, participants performed periodic movement about the knee joint in the gravitational field. Therefore, net external mechanical work was constrained to be zero. The tasks mainly differed from each other in average positive knee joint mechanical power, which was 4.3±0.5 W per leg during the dynamic task and 0.1±0.1 W per leg during the isometric task. Knee extension torque was near-constant around 15.2±1.7 N m during the dynamic task and around 15.7±1.7 N m during the isometric task. Owing to near-constant knee extension torque, quadriceps tendon length was presumably nearly constant during both tasks. Therefore, knee joint mechanical work was predominantly done by the muscle fibres in both tasks. Average gross metabolic power was 3.22±0.46 W kg-1 during the dynamic task and 2.13±0.36 W kg-1 during the isometric task. Because tasks differed mainly in the amount of positive muscle fibre mechanical work, these results imply that the metabolic cost of near-constant force production in vivo at zero net mechanical work can be reduced by minimizing positive muscle fibre mechanical work.

The effect of retirement on healthcare utilization: Evidence from China.

We examine the effect of retirement on healthcare utilization in China using longitudinal data. We use a nonparametric fuzzy regression discontinuity design, exploiting the statutory retirement age in urban China as a source of exogenous variation in retirement. In contrast to previous results for developed countries, we find that in China retirement increases healthcare utilization. This increase can be attributed to deteriorating health and in particular to the reduced opportunity cost of time after retirement. For the sample as a whole, income is not a dominating mechanism. People with low education, however, are more likely to forego recommended inpatient care after retirement.

Inverse dynamics of mechanical multibody systems: An improved algorithm that ensures consistency between kinematics and external forces.

Inverse dynamics is a technique in which measured kinematics and, possibly, external forces are used to calculate net joint torques in a rigid body linked segment model. However, kinematics and forces are usually not consistent due to incorrect modelling assumptions and measurement errors. This is commonly resolved by introducing 'residual forces and torques' which compensate for this problem, but do not exist in reality. In this study a constrained optimization algorithm is proposed that finds the kinematics that are mechanically consistent with measured external forces and mimic the measured kinematics as closely as possible. The algorithm was tested on datasets containing planar kinematics and ground reaction forces obtained during human walking at three velocities (0.8 m/s, 1.25 and 1.8 m/s). Before optimization, the residual force and torque were calculated for a typical example. Both showed substantial values, indicating the necessity of developing a mechanically consistent algorithm. The proposed optimization algorithm converged to a solution in which the residual forces and torques were zero, without changing the ground reaction forces and with only minor changes to the measured kinematics. When using a rigid body approach, our algorithm ensures a consistent description of forces and kinematics, thereby improving the validity of calculated net joint torque and power values.

What they say and what they do: comparing physical activity across the USA, England and the Netherlands.

BACKGROUND: Physical activity (PA) is important for maintaining health, but there are fundamental unanswered questions on how best it should be measured. METHODS: We measured PA in the Netherlands (n=748), the USA (n=540) and England (n=254), both by a 7 day wrist-worn accelerometer and by self-reports. The self-reports included a global self-report on PA and a report on the frequency of vigorous, moderate and mild activity. RESULTS: The self-reported data showed only minor differences across countries and across groups within countries (such as different age groups or working vs non-working respondents). The accelerometer data, however, showed large differences; the Dutch and English appeared to be much more physically active than Americans h (For instance, among respondents aged 50 years or older 38% of Americans are in the lowest activity quintile of the Dutch distribution). In addition, accelerometer data showed a sharp decline of PA with age, while no such pattern was observed in self-reports. The differences between objective measures and self-reports occurred for both types of self-reports. CONCLUSION: It is clear that self-reports and objective measures tell vastly different stories, suggesting that across countries people use different response scales when answering questions about how physically active they are.

Relationships between infant mortality, birth spacing and fertility in Matlab, Bangladesh.

Although research on the fertility response to childhood mortality is widespread in demographic literature, very few studies focused on the two-way causal relationships between infant mortality and fertility. Understanding the nature of such relationships is important in order to design effective policies to reduce child mortality and improve family planning. In this study, we use dynamic panel data techniques to analyse the causal effects of infant mortality on birth intervals and fertility, as well as the causal effects of birth intervals on mortality in rural Bangladesh, accounting for unobserved heterogeneity and reverse causality. Simulations based upon the estimated model show whether (and to what extent) mortality and fertility can be reduced by breaking the causal links between short birth intervals and infant mortality. We find a replacement effect of infant mortality on total fertility of about 0.54 children for each infant death in the comparison area with standard health services. Eliminating the replacement effect would lengthen birth intervals and reduce the total number of births, resulting in a fall in mortality by 2.45 children per 1000 live births. These effects are much smaller in the treatment area with extensive health services and information on family planning, where infant mortality is smaller, birth intervals are longer, and total fertility is lower. In both areas, we find evidence of boy preference in family planning.

Informal Caregiving, Employment Status and Work Hours of the 50+ Population in Europe.

Using panel data on the age group 50-70 in 15 European countries, we analyze the effects of providing informal care to parents, parents-in-law, stepparents, and grandparents on employment status and work hours. We account for fixed individual effects and test for endogeneity of caregiving using moments exploiting standard instruments (e.g., parental death) as well as higher-order moment conditions (Lewbel instruments). Specification tests suggest that informal care provision and daily caregiving can be treated as exogenous variables. We find a significant and negative effect of daily caregiving on employment status and work hours. This effect is particularly strong for women. On the other hand, providing care at a weekly (or less than weekly) frequency does not significantly affect paid work. We do not find evidence of heterogeneous effects of caregiving on paid work across European regions.

Optimizing the Distribution of Leg Muscles for Vertical Jumping.

A goal of biomechanics and motor control is to understand the design of the human musculoskeletal system. Here we investigated human functional morphology by making predictions about the muscle volume distribution that is optimal for a specific motor task. We examined a well-studied and relatively simple human movement, vertical jumping. We investigated how high a human could jump if muscle volume were optimized for jumping, and determined how the optimal parameters improve performance. We used a four-link inverted pendulum model of human vertical jumping actuated by Hill-type muscles, that well-approximates skilled human performance. We optimized muscle volume by allowing the cross-sectional area and muscle fiber optimum length to be changed for each muscle, while maintaining constant total muscle volume. We observed, perhaps surprisingly, that the reference model, based on human anthropometric data, is relatively good for vertical jumping; it achieves 90% of the jump height predicted by a model with muscles designed specifically for jumping. Alteration of cross-sectional areas-which determine the maximum force deliverable by the muscles-constitutes the majority of improvement to jump height. The optimal distribution results in large vastus, gastrocnemius and hamstrings muscles that deliver more work, while producing a kinematic pattern essentially identical to the reference model. Work output is increased by removing muscle from rectus femoris, which cannot do work on the skeleton given its moment arm at the hip and the joint excursions during push-off. The gluteus composes a disproportionate amount of muscle volume and jump height is improved by moving it to other muscles. This approach represents a way to test hypotheses about optimal human functional morphology. Future studies may extend this approach to address other morphological questions in ethological tasks such as locomotion, and feature other sets of parameters such as properties of the skeletal segments.

The Relationship between Pedal Force and Crank Angular Velocity in Sprint Cycling.

PURPOSE: Relationships between tangential pedal force and crank angular velocity in sprint cycling tend to be linear. We set out to understand why they are not hyperbolic, like the intrinsic force-velocity relationship of muscles. METHODS: We simulated isokinetic sprint cycling at crank angular velocities ranging from 30 to 150 rpm with a forward dynamic model of the human musculoskeletal system actuated by eight lower extremity muscle groups. The input of the model was muscle stimulation over time, which we optimized to maximize average power output over a cycle. RESULTS: Peak tangential pedal force was found to drop more with crank angular velocity than expected based on intrinsic muscle properties. This linearizing effect was not due to segmental dynamics but rather due to active state dynamics. Maximizing average power in cycling requires muscles to bring their active state from as high as possible during shortening to as low as possible during lengthening. Reducing the active state is a relatively slow process, and hence must be initiated a certain amount of time before lengthening starts. As crank angular velocity goes up, this amount of time corresponds to a greater angular displacement, so the instant of switching off extensor muscle stimulation must occur earlier relative to the angle at which pedal force was extracted for the force-velocity relationship. CONCLUSION: Relationships between pedal force and crank angular velocity in sprint cycling do not reflect solely the intrinsic force-velocity relationship of muscles but also the consequences of activation dynamics.

Cause-specific neonatal deaths in rural Bangladesh, 1987-2005: levels, trends, and determinants.

This paper reports on an analysis of neonatal mortality from communicable and non-communicable diseases in Bangladesh. The competing-risks model employed incorporated both observed and unobserved heterogeneity and allowed the two heterogeneity terms to be correlated. The data used came from the Health and Demographic Surveillance System, Matlab. The results confirm the conclusions of previous studies about the levels, trends, and causes of neonatal death in the Matlab area: the education of the mother helps protect her children from death from both communicable and non-communicable diseases; the children of a father in a low-status occupation are particularly vulnerable to death from communicable diseases; and children born to mothers aged less than 20 face a particularly high risk of dying from a non-communicable disease. The risks of dying from a communicable disease and from a non-communicable disease were both found to fall significantly as the distance to the nearest health centre decreased.

Contraceptive use, birth spacing, and child survival in Matlab, Bangladesh.

To reduce infant mortality through improved family planning, a better understanding of the factors driving contraceptive use and how this decision affects infant survival is needed. Using dynamic panel-data models of infant deaths, birth intervals, and contraceptive use, this study analyzes the causal effects of birth spacing on subsequent infant mortality and of infant mortality on the use of contraceptives and the length of the next birth interval. Data are drawn from the Health and Demographic Surveillance System in Matlab, Bangladesh, where almost 32,000 births have been observed from 1982 to 2005. Our main finding is that complete contraceptive use could reduce infant mortality of birth order two and higher by 7.9 percent. The net effect of complete contraceptive use on the total infant mortality rate is small (2.9 percent), however, because the favorable effect on higher order births is partly offset by the rise in the proportion of high-risk first births.

Control of position and movement is simplified by combined muscle spindle and Golgi tendon organ feedback.

Whereas muscle spindles play a prominent role in current theories of human motor control, Golgi tendon organs (GTO) and their associated tendons are often neglected. This is surprising since there is ample evidence that both tendons and GTOs contribute importantly to neuromusculoskeletal dynamics. Using detailed musculoskeletal models, we provide evidence that simple feedback using muscle spindles alone results in very poor control of joint position and movement since muscle spindles cannot sense changes in tendon length that occur with changes in muscle force. We propose that a combination of spindle and GTO afferents can provide an estimate of muscle-tendon complex length, which can be effectively used for low-level feedback during both postural and movement tasks. The feasibility of the proposed scheme was tested using detailed musculoskeletal models of the human arm. Responses to transient and static perturbations were simulated using a 1-degree-of-freedom (DOF) model of the arm and showed that the combined feedback enabled the system to respond faster, reach steady state faster, and achieve smaller static position errors. Finally, we incorporated the proposed scheme in an optimally controlled 2-DOF model of the arm for fast point-to-point shoulder and elbow movements. Simulations showed that the proposed feedback could be easily incorporated in the optimal control framework without complicating the computation of the optimal control solution, yet greatly enhancing the system's response to perturbations. The theoretical analyses in this study might furthermore provide insight about the strong physiological couplings found between muscle spindle and GTO afferents in the human nervous system.

Conclusions on motor control depend on the type of model used to represent the periphery.

Within the field of motor control, there is no consensus on which kinematic and kinetic aspects of movements are planned or controlled. Perturbing goal-directed movements is a frequently used tool to answer this question. To be able to draw conclusions about motor control from kinematic responses to perturbations, a model of the periphery (i.e., the skeleton, muscle-tendon complexes, and spinal reflex circuitry) is required. The purpose of the present study was to determine to what extent such conclusions depend on the level of simplification with which the dynamical properties of the periphery are modeled. For this purpose, we simulated fast goal-directed single-joint movement with four existing types of models. We tested how three types of perturbations affected movement trajectory if motor commands remained unchanged. We found that the four types of models of the periphery showed different robustness to the perturbations, leading to different predictions on how accurate motor commands need to be, i.e., how accurate the knowledge of external conditions needs to be. This means that when interpreting kinematic responses obtained in perturbation experiments the level of error correction attributed to adaptation of motor commands depends on the type of model used to describe the periphery.

Satisfaction with Social Contacts of Older Europeans.

This paper analyses the determinants of an important component of well-being among individuals aged 50 years or older in eleven European countries: satisfaction with social contacts. We use data from the Survey of Health, Ageing and Retirement in Europe and anchoring vignettes to correct for potential differences in responses scales across countries and socio-demographic groups. On average, older Europeans report being satisfied with their social contacts, but there exist substantial differences across countries: respondents from Northern countries tend to be more satisfied than individuals from Central or Mediterranean countries. Our analysis shows that correcting for response scale differentials alters the country ranking for of satisfaction with social contacts, while it has much less effect on the estimates of what drives within country determinants.

Do we use a priori knowledge of gravity when making elbow rotations?

In this study, we aim to investigate whether motor commands, emanating from movement planning, are customized to movement orientation relative to gravity from the first trial on. Participants made fast point-to-point elbow flexions and extensions in the transverse plane. We compared movements that had been practiced in reclined orientation either against or with gravity with the same movement relative to the body axis made in the upright orientation (neutral compared to gravity). For each movement type, five rotations from reclined to upright orientation were made. For each rotation, we analyzed the first trial in upright orientation and the directly preceding trial in reclined orientation. Additionally, we analyzed the last five trials of a 30-trial block in upright position and compared these trials with the first trials in upright orientation. Although participants moved fast, gravitational torques were substantial. The change in body orientation affected movement planning: we found a decrease in peak angular velocity and a decrease in amplitude for the first trials made in the upright orientation, regardless of whether the previous movements in reclined orientation were made against or with gravity. We found that these decreases disappeared after participants familiarized themselves with moving in upright position in a 30-trial block. These results indicate that participants used a general strategy, corresponding to the strategy observed in situations with unreliable or limited information on external conditions. From this, we conclude that during movement planning, a priori knowledge of gravity was not used to specifically customize motor commands for the neutral gravity condition.

Infant death clustering in families: magnitude, causes, and the influence of better health services, Bangladesh 1982-2005.

This analysis of infant mortality in Bangladesh focuses on explaining death clustering within families, using prospective data from a rural region in Bangladesh, split into areas with and without extensive health services (the area covered by the International Centre for Diarrhoeal Disease Research and the comparison area, respectively). The modelling framework distinguishes between two explanations of death clustering: (observed and unobserved) heterogeneity across families and a causal 'scarring' effect of the death of one infant on the survival chances of the next to be born. Keeping observed and unobserved characteristics constant, we find scarring in the comparison area only. There the likelihood of infant death is about 29 per cent greater if the previous sibling died in infancy than otherwise. This effect mainly works through birth intervals: infant deaths are followed by shorter birth intervals, which increases the risk of infant death for the next child.

Validating the Use of Anchoring Vignettes for the Correction of Response Scale Differences in Subjctive Questions.

Comparing self-assessed indicators of subjective outcomes such as health, work disability, political efficacy, job satisfaction, etc. across countries or socio-economic groups is often hampered by the fact that different groups use different response scales. This paper develops an integrated framework in which objective measurements are used to validate vignette-based corrections. The framework is applied to objective and subjective self-assessments of drinking behavior by students in Ireland. Model comparisons using the Akaike information criterion favor a specification with response consistency and vignette corrected response scales. Put differently, vignette based corrections appear quite effective in bringing objective and subjective measures closer together.

The puzzle of Muslim advantage in child survival in India.

The socioeconomic status of Indian Muslims is, on average, considerably lower than that of upper-caste Hindus. Muslims nevertheless exhibit substantially higher child survival rates, and have done for decades. This paper analyses this seeming puzzle. A decomposition of the survival differential confirms that some compositional effects favour Muslims but that, overall, differences in characteristics and especially the Muslim deficit in parental education predict a Muslim disadvantage. The results of this study contribute to a recent literature that debates the importance of socioeconomic status (SES) in determining health and survival. They augment a growing literature on the role of religion or culture as encapsulating important unobservable behaviours or endowments that influence health, indeed, enough to reverse the SES gradient that is commonly observed.

Gross efficiency during rowing is not affected by stroke rate.

PURPOSE: It has been suggested that the optimal stroke rate in rowing is partly determined by the stroke-rate dependence of internal power losses. This should be reflected in a stroke-rate dependency of gross efficiency (e(gross)). The purpose of this study was to investigate if e(gross) is affected by stroke rate. A second aim was to determine whether internal power losses can be estimated by the negative power output during the stroke cycle (P(negative)). METHODS: Seventeen well-trained female rowers participated in this study. They rowed three trials on a modified rowing ergometer on slides at a submaximal intensity, with a respiratory exchange ratio of 1 or close to 1. Stroke rates were 28, 34, and 40 strokes per minute. The trials were fully randomized. Power transfer to the flywheel was kept constant whereas e(gross) was determined during each trial. RESULTS: No significant differences in e(gross) were found between conditions. This finding suggests that in rowing internal power losses are not influenced by stroke rate. Furthermore, although P(negative) increased at increasing stroke rate (P < 0.001), no relationship was found with e(gross). This suggests that P(negative) is not a reliable measure to estimate internal power losses. CONCLUSION: This study shows that within the range of stroke rates applied in competitive rowing, internal power losses are unrelated to rowing cycle frequency.