Increased gravitational force reveals the mechanical, resonant nature of physiological tremor.

KEY POINTS: Physiological hand tremor has a clear peak between 6 and 12 Hz, which has been attributed to both neural and resonant causes. A reduction in tremor frequency produced by adding an inertial mass to the limb has usually been taken as a method to identify the resonant component. However, adding mass to a limb also inevitably increases the muscular force required to maintain the limb's position against gravity, so ambiguous results have been reported. Here we measure hand tremor at different levels of gravitational field strength using a human centrifuge, thereby increasing the required muscular force to preserve limb position without changing the limb's inertia. By comparing the effect of added mass (inertia + force) versus solely added force upon hand acceleration, we conclude that tremor frequency can be almost completely explained by a resonant mechanical system. ABSTRACT: Human physiological hand tremor has a resonant component. Proof of this is that its frequency can be modified by adding mass. However, adding mass also increases the load which must be supported. The necessary force requires muscular contraction which will change motor output and is likely to increase limb stiffness. The increased stiffness will partly offset the effect of the increased mass and this can lead to the erroneous conclusion that factors other than resonance are involved in determining tremor frequency. Using a human centrifuge to increase head-to-foot gravitational field strength, we were able to control for the increased effort by increasing force without changing mass. This revealed that the peak frequency of human hand tremor is 99% predictable on the basis of a resonant mechanism. We ask what, if anything, the peak frequency of physiological tremor can reveal about the operation of the nervous system.

The effect of trial number on the emergence of the 'broken escalator' locomotor aftereffect.

Walking onto a stationary platform, which had been previously experienced as moving generates a locomotor aftereffect (LAE), which resembles the 'broken escalator' phenomenon. Experimentally, this is achieved by having subjects walk initially onto a stationary sled (BEFORE condition), then onto a moving sled (MOVING condition, or adaptation trials) and then again onto the stationary sled (AFTER condition). Subjects are always appropriately warned of the change in conditions. In this paper, we ask how many adaptation trials are needed to produce such a LAE. Thus, in two experiments, the number of MOVING trials was varied between 20 and 5 (Experiment 1) and between 8 and 1 (Experiment 2). Gait velocity, trunk position, foot contact timing and EMG of the ankle flexor-extensors muscles were measured. In comparison with BEFORE trials all groups in the AFTER trials walked inappropriately fast, experienced a large overshoot of the trunk and showed increased leg EMG, indicating that all groups showed a LAE. In each experiment, and for all variables, no significant difference between the groups (i.e. 20 down to one MOVING trials) was found. The study shows that this LAE, in contrast to other motor aftereffects reported in the literature, can be generated with only one or two adaptation trials and without requiring unexpected 'catch' trials. The fast aftereffect generation observed is likely to depend on two types of mechanisms: (1) the nature of the sensorimotor adaptation process, involving multiple sensory feedbacks (visual, vestibular and proprioceptive), anticipatory control and large initial task errors and (2) the involvement of two phylogenetically old neural mechanisms, namely locomotion and fear. Fear-relevant mechanisms, which are notably resistant to cognitive control, may be recruited during the adaptation trials and contribute to the release of this LAE.

The moving platform aftereffect: limited generalization of a locomotor adaptation.

We have recently described a postural after-effect of walking onto a stationary platform previously experienced as moving, which occurs despite full knowledge that the platform will no longer move. This experiment involves an initial baseline period when the platform is kept stationary (BEFORE condition), followed by a brief adaptation period when subjects learn to walk onto the platform moving at 1.2 m/s (MOVING condition). Subjects are clearly warned that the platform will no longer move and asked to walk onto it again (AFTER condition). Despite the warning, they walk toward the platform with a velocity greater than that observed during the BEFORE condition, and a large forward sway of the trunk is observed once they have landed on the platform. This aftereffect, which disappears within three trials, represents dissociation of knowledge and action. In the current set of experiments, to gain further insight into this phenomenon, we have manipulated three variables, the context, location, and method of the walking task, between the MOVING and AFTER conditions, to determine how far the adaptation will generalize. It was found that when the gait initiation cue was changed from beeps to a flashing light, or vice versa, there was no difference in the magnitude of the aftereffect, either in terms of walking velocity or forward sway of the trunk. Changing the leg with which gait was initiated, however, reduced sway magnitude by approximately 50%. When subjects changed from forward walking to backward walking, the aftereffect was abolished. Similarly, walking in a location other than the mobile platform did not produce any aftereffect. However, in these latter two experiments, the aftereffect reappeared when subjects reverted to the walking pattern used during the MOVING condition. Hence, these results show that a change in abstract context had no influence, whereas any deviation from the way and location in which the moving platform task was originally performed profoundly reduced the size of the aftereffect. Although the moving platform aftereffect is an example of inappropriate generalization by the motor system across time, these results show that this generalization is highly limited to the method and location in which the original adaptation took place.

The broken escalator phenomenon. Aftereffect of walking onto a moving platform.

We investigated the physiological basis of the 'broken escalator phenomenon', namely the sensation that when walking onto an escalator which is stationary one experiences an odd sensation of imbalance, despite full awareness that the escalator is not going to move. The experimental moving surface was provided by a linear motor-powered sled, moving at 1.2 m/s. Sled velocity, trunk position, trunk angular velocity, EMG of the ankle flexors-extensors and foot-contact signals were recorded in 14 normal subjects. The experiments involved, initially, walking onto the stationary sled (condition Before). Then, subjects walked 20 times onto the moving sled (condition Moving), and it was noted that they increased their walking velocity from a baseline of 0.60 m/s to 0.90 m/s. After the moving trials, subjects were unequivocally warned that the platform would no longer move and asked to walk onto the stationary sled again (condition After). It was found that, despite this warning, subjects walked onto the stationary platform inappropriately fast (0.71 m/s), experienced a large overshoot of the trunk and displayed increased leg electromyographic (EMG) activity. Subjects were surprised by their own behaviour and subjectively reported that the 'broken escalator phenomenon', as experienced in urban life, felt similar to the experiment. By the second trial, most movement parameters had returned to baseline values. The findings represent a motor aftereffect of walking onto a moving platform that occurs despite full knowledge of the changing context. As such, it demonstrates dissociation between the declarative and procedural systems in the CNS. Since gait velocity was raised before foot-sled contact, the findings are at least partly explained by open-loop, predictive behaviour. A cautious strategy of limb stiffness was not responsible for the aftereffect, as revealed by no increase in muscle cocontraction. The observed aftereffect is unlike others previously reported in the literature, which occur only after prolonged continuous exposure to a sensory mismatch, large numbers of learning trials or unpredictable catch trials. The relative ease with which the aftereffect was induced suggests that locomotor adaptation may be more impervious to cognitive control than other types of motor learning.

Correlates of the age at natural menopause in Morocco.

PRIMARY OBJECTIVE: To determine the median age of natural menopause and factors associated with the timing of menopause in Morocco. RESEARCH DESIGN: A population-based sample of 299 women from Rabat, Morocco were interviewed using a semi-structured questionnaire. METHODS AND PROCEDURES: Logit analysis and logistic regression were used to estimate the median age and identify factors associated with the age at menopause. MAIN OUTCOMES AND RESULTS: The median age of natural menopause in Morocco is estimated to be 48.4 years (95% CI: 36.9, 70.3). Women who reported the onset of menstruation at age 11 or younger (OR = 2.84, 95% CI: 1.00, 8.10) had an earlier age at menopause than women who started menstruating at age 12 or older. Women who had ever used oral contraceptives were more likely to have a later age at menopause (OR = 0.55, 95% CI: 0.30, 1.00). The length of time a woman used oral contraceptives influenced the timing of the menopause in unadjusted models but after adjusting for the age at menarche the effect was no longer statistically significant. Marital status, parity, education level, and social class were not statistically associated with the age at menopause. CONCLUSIONS: The estimated age at natural menopause in Morocco is 1-3 years earlier than the median ages reported in industrialized countries and some developing countries. Factors that alter the frequency of ovulation or rate of follicular atresia appear to be important in determining the age at menopause.

Discontinuation of postmenopausal hormone therapy in a Massachusetts HMO.

Long-term postmenopausal hormone therapy alleviates menopausal symptoms, decreases women's risk of osteoporosis and has been shown to reduce cardiovascular morbidity and mortality in more than 30 observational studies. However, nearly half of all women initiating hormone therapy discontinue within the first year. This study was conducted to determine the rate and predictors of hormone therapy discontinuation in a clinical practice setting. We identified 992 women aged 45-59 who began hormone therapy between 1993 and 1995 in a Massachusetts health maintenance organization. Women were followed 2 years from the day they filled a prescription for estrogen. 53% discontinued hormone therapy by the end of the first year and one-fifth stopped after the first prescription. A prescription from a gynecologist (RR = 0.82, 95% CI: 0.68, 0.99) and a mammogram a year prior to initiation (RR = 0.75, 95% CI: 0.63, 0.89) were associated with a lower rate of discontinuation. Women who were using monoamine oxidase inhibitors (MAOI) or selective serotonin reuptake inhibitors (SSRI) antidepressants 3 months prior to initiation (RR = 2.07, 95% CI: 1.26, 3.39) or who had been enrolled in the health plan for less than 3 years (RR = 1.33, 95% CI: 1.10, 1.62) had an increased risk of discontinuing hormone therapy. The year a woman entered the cohort was also associated with a higher rate of discontinuation (RR = 1.40, 95% CI: 1.14, 1.74 for 1994 and RR = 1.95, 95% CI: 1.52, 2.50 for 1995). The results indicate that long-term hormone therapy use is uncommon in clinical practice, particularly when a woman or her physician attempts to use hormone therapy as an alternative to antidepressant therapy, and that the rates of discontinuation of hormone therapy were rising rapidly in the mid-1990s.

Side effects and sociobehavioral factors associated with the discontinuation of hormone therapy in a Massachusetts health maintenance organization.

OBJECTIVE: To identify sociobehavioral factors and side effects associated with the discontinuation of postmenopausal hormone therapy in a clinical practice setting DESIGN: A total of 816 women aged 45-59 who began hormone therapy between July 1993 and June 1995 in a Massachusetts health maintenance organization were followed for 2 years from the day they received a prescription for estrogen. This cohort has been previously studied for health, treatment, and demographic determinants of hormone therapy discontinuation. In March 1999, these women were mailed a questionnaire containing closed and open-ended questions. A total of 449 women (55%) completed the survey. Discrete-time hazards models were used to identify determinants of discontinuation, controlling for medical predictors of survey nonresponse. RESULTS: Women separated from their partners when they initiated hormone therapy (relative risk [RR] of discontinuation = 3.42; 95% confidence interval [CI] = 1.09, 10.73) and women with a body mass index greater than 29.0 (RR = 1.62; 95% CI = 1.18, 2.23) were more likely to discontinue. Women who had ever used oral contraceptives were less likely to discontinue hormone therapy (RR = 0.70; 95% CI = 0.51, 0.98). After women began using hormone therapy, those who experienced irregular bleeding (RR = 1.58; 95% CI = 1.08, 2.31), edema (RR = 2.18; 95% CI = 1.42, 3.34), or abdominal cramps and pelvic pain (RR = 2.42; 95% CI = 1.46, 4.02) while using hormones were more likely to discontinue. The effect of edema and abdominal cramps on the rate of discontinuation was greatest during the first 6 months of use. Women who adjusted their progestin schedule on their own were four times more likely than other women to discontinue hormones (RR = 4.18; 95% CI = 2.20, 7.94). The use of alternative therapies was not statistically associated with discontinuation. CONCLUSIONS: Women who report therapeutic benefits from hormone therapy are more likely to continue using hormones long-term. The experience of certain side effects, especially during the first few months of hormone use, strongly affects whether women continue using hormone therapy.

Age at natural menopause in Beirut, Lebanon: the role of reproductive and lifestyle factors.

PRIMARY OBJECTIVE: To determine the median age of natural menopause and factors associated with the timing of menopause in Beirut, Lebanon. RESEARCH DESGIN: A population-based sample of 298 women were interviewed using a semi-structured questionnaire. METHODS AND PROCEDURES: Logit analysis and logistic regression were used to estimate the median age and identify correlates of menopausal status. MAIN OUTCOMES AND RESULTS: The median age of natural menopause in Beirut, Lebanon is estimated to be 49.3 years. Not being currently married (odds ratio (OR)=4.19, 95% CI: 1.53, 11.51) and smoking (OR = 1.02 for each year of smoking, 95% CI: 1.00, 1.05) are significantly associated with an earlier age at menopause. Women with menstrual bleeding longer than 5 days have later ages at menopause (OR = 0.50, 95% CI: 0.26, 0.97). Age at menarche, parity, education and socio-economic status are not statistically associated with the age at menopause. CONCLUSIONS: The age at natural menopause in Lebanon is comparable to median ages reported for women in industrialized countries (49.3-51.4). Our results emphasize the role of smoking in determining an earlier age at menopause and indicate that reproductive characteristics affect the timing of menopause.