摘要:

Breast cancer is one of the leading causes of cancer and cancer mortality among Canadian women. Based on the current incidence rates, the National Cancer Institute of Canada has estimated that one in 10 women will develop breast cancer during her lifetime. In an effort to control this disease, various prevention strategies have been proposed. One promising strategy involves the promotion of exercise in healthy women. It has been hypothesized that exercise may protect against breast cancer by influencing regularity of ovulatory cycles, by changing body fat composition, or by enhancing natural immunity. In this paper both the epidemiological and experimental evidence that speaks to a protective effect of exercise is reviewed. Although there are several biologically plausible mechanisms for the association, the existing epidemiological and experimental data are inconclusive due to the small number of studies and their methodological insufficiencies. However, given the enormous potential benefit of even a small protective effect of exercise, further studies designed to address the relationship between exercise and breast cancer risk are warranted.Key words: physical activity, mammary carcinogenesis, epidemiology

ISSN:1066-7814    

DOI:10.1139/h94-020

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Eight men and 8 women were tested for voluntary and isometric evoked contractions of the dorsiflexors. Maximal voluntary contractions were conducted isometrically (30° plantarflexion) and isokinetically. Men produced greater peak torques at all but the highest isokinetic velocity. They exceeded women in measures of absolute voluntary isometric (33.8%), tetanic (46.3%), and twitch (37.9%) peak torque. Men demonstrated a higher tetanus/MVC ratio (17.8%). This ratio disparity would indicate that gender differences in peak torque in this study were more dependent upon muscle mechanisms. Although men exceeded women in measures of absolute voluntary maximum rate of torque development (MRTD) (38.7%), tetanic MRTD (39.1%), twitch MRTD (27.9%), voluntary maximum rate of torque relaxation (MRTR) (46.9%), and tetanic MRTR (41.9%), there were no significant gender differences in percentage rates of torque development or relaxation with voluntary or evoked contractions. Therefore the central and peripheral mechanisms controlling MRTD and MRTR may be similar between the sexes.Key words: gender strength differences, muscle contraction, rate of torque, tetanus, twitch

ISSN:1066-7814    

DOI:10.1139/h94-021

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Performance in endurance activities depends on maximal aerobic capacityand the ability to sustain a high percentage ofover time. This study examined whether noninvasive laboratory measures would be valid predictors of endurance performance in an individual-start bicycle race (TT). Eight experienced male cyclists (age = 25.1 ± 3.3 years, weight = 75.0 ± 5.7 kg,) performed a progressive incremental exercise test to exhaustion on a cycle ergometer., maximum power output, and ventilatory threshold were determined. Later the subjects completed a 40-km TT. Power output at the ventilatory threshold (VT watts) was correlated with race performance time and calculated power output during the competition (r = −0.81; r = 0.82). VT watts andaccounted for 75% of the variance between subjects (r = 0.91) in performance time. These data indicate that simple laboratory measures can predict TT performance in trained cyclists. Individual differences may be accounted for by motivation, aerodynamic position, and efficiency.Key words:exercise, laboratory methods,, athletes

ISSN:1066-7814    

DOI:10.1139/h94-022

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

Skeletal muscle lactate transport was investigated in vitro in isolated fast-twitch (EDL) and slow-twitch soleus (Sol) skeletal muscles from control and exercised mice. Exercise (23 m/min, 8% grade) reduced muscle glycogen by 37% in EDL (p < 0.05) and by 35% in Sol muscles (p < 0.05). Lactate transport measurements (45 sec) were performed after 60 min of exercise in intact EDL and Sol muscles in vitro, at differing pH (6.5 and 7.4) and differing lactate concentrations (4 and 30 mM). Lactate transport was observed to be greater in Sol than in EDL (p < 0.05). In the exercised muscles there was a small but significant increase in lactate transport (p < 0.05). Lactate transport was greater when exogenous lactate concentrations were greater (p < 0.05) and more rapid at the lower pH (p < 0.05). These studies demonstrated that lactate transport was increased with exercise.Key words: soleus, EDL, treadmill exercise

ISSN:1066-7814    

DOI:10.1139/h94-023

出版商:NRC Research Press    

年代:1994

数据来源: NRC

ISSN:1066-7814    

DOI:10.1139/h94-024

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

This article presents the author's views about the neural drives to breathing during exercise. Two hypotheses are developed, the first being that the rapid changes in ventilation at the start and end of exercise are due to a fast neural drive whose magnitude is related to the frequency of limb movement. Experimental data are presented that this drive persists throughout exercise but declines as exercise continues. Second, the excessive increase in ventilation that occurs above the first ventilatory threshold during an incremental exercise test is due to a heavy exercise neural drive whose magnitude is related to the motor commands to the exercising muscles. Using the electromyographical activity of the working muscles as an index of the strength of the motor commands, experimental evidence is presented showing the coincidence of the first ventilatory threshold and that for the electromyographic activity of the working muscles during incremental exercise tests.Key words: neural drive hypotheses, treadmill speed vs grade, start and end of exercise, incremental exercise tests, hyperoxia

ISSN:1066-7814    

DOI:10.1139/h94-025

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

The stability of arterial blood gas tensions and pH during steady-state moderate exercise has suggested an important humoral element of ventilatory control in humans. However, the involvement of central and peripheral chemoreflexes in this humoral control remains controversial. This reflects, in large part, technical and interpretational limitations inherent in currently used estimators of chemoreflexes "sensitivity." Evidence suggests that the central chemoreceptors (a) contribute little during moderate exercise, given the relative stability of cerebrospinal pH, (b) constrain the hyperpnea of high-intensity exercise, consequent to the respiratory compensation for the metabolic acidemia, and (c) may play a role in the respiratory compensation during chronic metabolic acidemia. In contrast, the peripheral chemoreceptors appear to (a) exert considerable influence on ventilatory kinetics in moderate exercise, but are less important in the steady state, and (b) induce much of the respiratory compensation of high-intensity exercise.Key words: medullary chemoreceptors, carotid chemoreceptors, hyperoxia, ventilatory dynamics, metabolic acidemia

ISSN:1066-7814    

DOI:10.1139/h94-026

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

For optimal performance of exercising muscle, the charge state of proteins must be maintained; the pH environment of protein histidine imidazole groups must be coordinated with their pK. During exercise, increasing temperature and osmolality as well as changes in strong ions affect the pK of imidazole groups. Production of strong organic anions also decreases the concentration difference between strong cations and anions (strong ion difference, or [SID]), causing a metabolic acidosis in peripheral tissues. Central chemoreceptors regulatein relation to the [SID] of brain fluids to maintain a "constant" brain [H+]. In addition, increased osmolality, angiotensin II, and vasopressin during exercise may stimulate circumventricular organs of the brain and interact with chemical control of ventilation. Changes in [SID] of brain fluids during exercise are negligible compared to systemic decreases in [SID]; thus, regulation ofto maintain brain [H+] homeostasis cannot simultaneously compensate for greater changes in [SID] in peripheral tissues.Key words: circumventricular organs, central chemoreception, angiotensin II, vasopressin, alphastat theory

ISSN:1066-7814    

DOI:10.1139/h94-027

出版商:NRC Research Press    

年代:1994

数据来源: NRC

摘要:

In this paper two types of evidence are presented which question the commonly presumed role of carotid chemoreceptor stimulation as the primary mediator of the hyperventilatory response to heavy exercise. First, carotid-body denervation in ponies increases their hyperventilatory response to heavy exercise. Second, the awake dog and the goat at rest show an immediate and substantial depression of tidal volume and of ventilation when their isolated carotid chemoreceptors are made hypocapnic. Accordingly, it is proposed that during heavy exercise the carotid chemoreceptors are inhibitory to respiratory motor output and that the cause of the hyperventilatory response originates from extrachemoreceptor, locomotor-linked, feed-forward stimuli.Key words: hypocapnic inhibition, medullary chemoreceptors, locomotor-linked stimuli, feed-forward

ISSN:1066-7814    

DOI:10.1139/h94-028

出版商:NRC Research Press    

年代:1994

数据来源: NRC

ISSN:1066-7814    

DOI:10.1139/h94-029

出版商:NRC Research Press    

年代:1994

数据来源: NRC