Showing papers in "Exercise and Sport Sciences Reviews in 1994"

Assessment and Interpretation of Aerobic Fitness in Children and Adolescents

Abstract: Our understanding of the development of children and adolescents' aerobic fitness is limited by ethical considerations and methodological constraints. Protocols, apparatus, and criteria of maximal effort used with adults are often unsuitable for use with children. In normal children and adolescents, peak VO2 increases with growth and maturation, although there are indications that girls' peak VO2 may level off around 14 years of age. Males exhibit higher values of peak VO2 than females, and the sex difference increases as they progress through adolescence. The difference between males and females has been attributed to the boys' greater muscle mass and hemoglobin concentration. It appears that boys experience an adolescent growth spurt in peak VO2, which reaches a maximum gain near the time of PHV, but data are insufficient to offer any generalization for girls. Peak VO2 has usually been expressed in relation to body mass, and with this convention it appears that boys' values are consistent throughout the developmental period, whereas girls' values decrease as they get older. This type of analysis may, however, have clouded our understanding of growth and maturational changes in peak VO2, and scaling for differences in body size may provide further clarification. If differences are shown where none were previously thought to exist, then physiological explanations must be sought. Methodological issues have also hindered the understanding of how children's blood lactate responses to exercise develop. The actual lactate level recorded during an exercise test is influenced by the site of sampling and the blood handling and assay techniques. Valid interstudy comparisons can only be made where similar procedures have been employed. In general, children demonstrate lower blood lactate levels at peak VO2 than adults, although individual variation is wide. Therefore the use of blood lactate measures to confirm the attainment of peak VO2 cannot be supported. Exercise at the same relative submaximal intensity elicits a lower blood lactate in children than in adults, but interpretation and identification of developmental and maturational patterns of response are limited by the use of different testing conditions and reference points (e.g., lactate threshold and fixed level reference points). There is growing evidence that the 2.5 mM reference level should be used in preference to the 4.0 mM level, as the adult criterion occurs close to maximal exercise in many children and adolescents. Explanations for child-adult differences in blood lactate responses to exercise are difficult to elucidate.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging skeletal muscle: response to exercise.

Abstract: The mass of many weight-bearing muscles declines in old rats, secondary to the atrophy of fibers, particularly of type IIb, with relatively little loss of muscle fibers during most of the adult life span. In humans, muscle atrophy is the result of a combination of progressive fiber loss and fiber atrophy. In both species, the proportion of histochemically determined fiber types is relatively stable across the adult portion of the life span. The loss of strength in old age is predominantly accounted for by reduced muscle mass in humans and rats. Resistance training leads to increased muscle mass and strength in old humans and rats, primarily by increasing fiber CSA. Muscle capillarity is unchanged in old rats but decreases in old dogs. Apparently, capillarity declines in truly sedentary older people. Endurance training enhances capillarity, and old rats and humans can attain levels of capillarity comparable to their active young counterparts, even when performing considerably less exercise. Blood flow during contractile activity is reduced in male rats and humans but not in old female rats or dogs. Oxidative capacity declines in many muscles of sedentary old rats and humans. With endurance training, old individuals from both species attain levels of muscle oxidative capacity quite similar to those in identically training young individuals. Muscle insulin-stimulated glucose transport is enhanced in rats after a bout of exercise, regardless of age. Endurance training elevates muscle GLUT-4 levels in young and middle-aged, but not old, rats, perhaps because the old rats trained at slower treadmill speeds. Middle-aged (47-62 yr) men and women can substantially increase muscle GLUT-4 with relatively brief (12-14 wk) endurance training; older humans (> 70-80 yr) have not been studied. Endurance training leads to reduced LDH activity without altering PFK or phosphorylase in old rats and humans. Muscle glycogen depletion, CP depletion, and lactate accumulation during contractile activity are exaggerated in old rats, apparently secondary to reduced muscle oxidative capacity and blood flow. Resting muscle glycogen concentration is diminished in older humans, probably in part because of a more sedentary lifestyle. Although several months of endurance training raises muscle glycogen concentration in older people, it does not restore it to youthful levels. Endurance training can greatly improve endurance in old age, at least in part by the same mechanism originally described in youth, i.e., an increase in muscle oxidative capacity, which contributes to reduced glycogen depletion.(ABSTRACT TRUNCATED AT 400 WORDS)

Growth hormone effects on metabolism, body composition, muscle mass, and strength

Abstract: It is clear that the anthropometric ramifications, especially with respect to muscle mass, of the metabolic actions of GH and IGF-I treatment in intact and GH-deficient adults require further study. At present, it appears that daily GH or IGF-I treatment modestly increases nitrogen retention in most normal adults, probably by separate but permissive mechanisms, but only for a short period of time (approximately 1 month). During prolonged GH administration, resistance to the anabolic actions of GH seems to occur, and optimizing the anabolic effects of GH or IGF-I treatment will require a better understanding of the interactions among GH, GHBP, IGF-I production, IGFBPs, the GH dose regimen, and other unidentified regulatory factors. On the basis of the similar increases in muscle protein synthesis, muscle cross-sectional area, and muscle strength observed in placebo and GH-treated exercising young adults, it is doubtful that the nitrogen retention associated with daily GH treatment results in an increase in contractile protein, improved muscle function, strength and athletic performance. Even in catabolic or GH-deficient populations, GH treatment provides only modest increments in nitrogen retention, muscle size, strength, and exercise capacity. Further, the side effects of GH treatment (water retention, carpal tunnel compression, insulin resistance) would be a detriment, rather than an aid, to athletic performance. In addition, whether prolonged (> 6 months) GH treatment alone or in combination with other agents used by athletes (e.g., anabolic steroids, beta-agonists) is associated with other adverse side effects (e.g., cancer, diabetes) has not been evaluated. Therefore, health professionals should continue to discourage the use of GH by exercise enthusiasts.

The potential role of physical activity in the prevention of non-insulin-dependent diabetes mellitus: the epidemiological evidence.

Abstract: Examining the epidemiological literature, a relationship between physical activity and the development of NIDDM seems not only plausible but likely. What is needed is a randomized clinical trial to confirm the metabolic and epidemiological research findings to date. To maximize success of a diabetes prevention trial, both diet and physical activity should be part of the intervention. The relative importance of diet or physical activity as components of the intervention will vary, depending upon the participant. However, it is critical that compliance with the physical activity and diet intervention be maintained, for this question to be answered. If the intervention is successful, the participants will have made lifestyle changes that have the potential of lasting beyond the closure of the clinical trial.

Analysis and simulation of mechanical loads on the human musculoskeletal system: a methodological overview.

Abstract: Load in the human body can be quantified as force, stress, or strain, depending on the anatomical structure and the measuring technique. Direct measurements of these variables are invasive and only possible in animals or in small-scale in vivo studies in humans. Miniaturization of transducers and electronics may open new possibilities for direct measurements of load in the human body. Studies with a large number of human subjects, and routine analysis of patients, are done using noninvasive techniques: EMG analysis for muscle forces, kinematic analysis for ligament forces, and inverse dynamics for resultant joint loads. Inverse dynamics is the most general method and is applicable to all joints in the human body. Important limitations of inverse dynamics are due to the "distribution problem": the separation of resultant loads into the individual forces in muscles and other structures. Dynamic optimization is the most promising solution method for this problem. Inverse dynamics also relies heavily on the assumption that body segments are rigid. The errors caused by this simplification are most severe in impact and vibration studies. Computer simulation is a well-established method for load analysis in mechanical engineering but is relatively rare in biomechanics. Replacing the human test subject by a mathematical model has many advantages, mainly for reproducibility and understanding of the results. Models for mechanical properties and control of muscles are an important and difficult part of computer simulation. For this reason, computer simulation has only been applied for load analysis in impact simulations, where the muscles can be regarded as passive, or for certain special problems where similarly simple muscle models can be used. In the future, we may see more applications of computer simulation for analysis of more complex activities, such as gait and sports.

Preemployment physical evaluation.

Abstract: There is a growing trend toward using preemployment tests to select employees for physically demanding jobs. Women are, in increasing numbers, entering physically demanding occupations that were traditionally dominated by men. Under current Federal employment law, it is illegal to disqualify an employee for a job because of race, color, religion, sex, national origin, and with the recent passage of the American Disabilities Act (ADA), handicap. Because of gender differences in strength, body composition, and VO2max, preemployment tests for physically demanding jobs tend to screen out more females than males. Employers are using preemployment tests not only to enhance worker productivity, but also to minimize the threat of litigation for discriminatory hiring practices and to reduce the risk of musculoskeletal injuries. The primary ergonomic methods used in industry to reduce the risk of back injuries are preemployment testing and job redesign. When a test results in adverse impact, the validity of the test must be established. Validity in this context means that the test represents or predicts the applicant's capacity to perform the job. Criterion-related, content, and construct validation studies are the means used to establish validity. The validity of preemployment hiring practices for physically demanding jobs has been decided in the courts. The most common reason for ruling an employment practice invalid is the failure to show that the test measured important job behaviors. Much of this litigation has involved height and weight requirements for public safety jobs. The courts have generally ruled that using height and weight standards as a criteria for employment is illegal because they were not job related. If fitness tests comprise part or all of the preemployment test, it is essential to demonstrate that the fitness component is related to job performance. Although there are many factors to consider when establishing a cut score, there is a growing trend toward establishing the cut score on the basis of the job's physical demands, defined by VO2max and strength. This literature is limited because most validation studies are not published. They more typically take the form of a technical report to the governmental agency or company that funded the project. There are published preemployment validation studies for outdoor telephone craft jobs involving pole-climbing tasks; firefighters; highway patrol officers; steel workers; underground coal miners; chemical plant workers; electrical transmission lineworkers; and various military jobs.

Endurance exercise in aging humans: effects on energy metabolism.

Abstract: In summary, data suggest that the decline in RMR with advancing age is primarily related to the decline in fat-free mass However, in addition to the erosion of fat-free mass, other factors such as Na-K pump activity, fat mass, maximal aerobic power, and menopausal status are important determinants influencing the decline in RMR in older individuals Second, we provide revised prediction equations for RMR that are both gender- and sex-specific and use easily measured variables to facilitate their use in clinical and field settings Third, preliminary studies suggest that older individuals may have a reduced energy expenditure following meal ingestion, although this is not a universal finding among investigators Furthermore, several studies suggest that physically active older men exhibit higher thermic responses to a meal than sedentary older men Data on total energy expenditure in free-living elderly persons are sparse However, the available data suggest that there is large variation in total energy expenditure in the elderly population, caused primarily by differences in physical activity The heterogeneity in physical activity makes estimation of individual energy requirements difficult However, preliminary studies have suggested that measurement of VO2max and other activity indices may be useful markers for estimating energy requirements on an individual basis Furthermore, attempting to "normalize" total energy expenditure in the elderly by prescribing physical activity is not as straightforward as it seems, due to exercise-induced compensatory reductions in physical activity during the remainder of the day Levels of insulin-like growth factor-1 decline with advancing age Preliminary evidence from cross-sectional and exercise intervention studies suggest that the lower serum levels of IGF-1 in older individuals may be partially due to diminished physical activity Aging is associated with an increase in fasting levels of norepinephrine, primarily influenced by an elevated rate of norepinephrine into circulation; however, the clinical significance of the elevated sympathetic tone is unclear Endurance training in older individuals has been found to increase basal levels of norepinephrine appearance into circulation, and this has been associated with an increased RMR and enhanced fat oxidation Optimal exercise interventions need to be identified for the elderly, which maximally increase daily energy expenditure and offset metabolic deterioration with advancing age

Transgenic animal models.

Abstract: Exercise adaptations induce the differential expression of genes that encode components of the contractile apparatus, metabolic pathways, organelle systems, and membrane components. These changes in gene expression are presumably brought about by the activation of intracellular signaling pathways. The use of transgenic technology is particularly well suited for exercise studies because it uniquely allows investigators to evaluate gene regulatory mechanisms in the intact animal at all stages of the life cycle. Transgenic mice can be generated that harbor any of the varied genes involved in exercise adaptation, for the purpose of determining what sequences within this gene regulate the response to any exercise regimen. Once the corresponding transcriptional factor(s) have been identified and the gene(s) cloned, a series of similar studies (gene networking) could be undertaken to determine the signal transduction pathway. Alternatively, transgenics can be produced that overexpress a particular protein or express an isoform of a protein in a tissue where it is not normally expressed. The goal of these studies would be to determine if function is improved and if exercise adaptation is enhanced or facilitated. Ultimately, gene targeting can be used to either mutate or knock out a gene to determine whether its gene product is indispensable for function, exercise performance, and/or adaptation.