Nov 16 2012
By Prof. Roger Enoka
Physical performance capabilities, which decline across the lifespan, are strongly associated with daily levels of physical activity. The decrease in physical activity across the lifespan has consequences for health and well-being. To minimize the reductions in habitual levels of physical activity and thereby lessen the decline in healthspan, it is necessary to understand how performance capabilities change from childhood to senescence. The following three examples indicate the types of adaptations that can occur across the lifespan.
Childhood
Physical activity levels decline during childhood and adolescence. This decrease is of concern because the prevalence and magnitude of obesity among children is associated with the decline in the habitual level of physical activity.
The reduction in daily levels of physical activity during puberty is greater for girls than boys, which likely contributes to the greater prevalence of high BMI values among girls. Psychosocial and environmental factors explain less than 20% of the decline in physical activity during adolescence. Rather, there is a significant role for biological processes that control energy expenditure and development.
Prior to beginning puberty, boys and girls perform similar levels of physical activity each day as indicated by the number of steps they perform. With the onset of puberty, however, the number of steps per day decreases for boys and girls, but more so for girls.
The difference between boys and girls is strongly associated with the development of the neuromuscular system, especially the strength and endurance of leg muscles. The increases in strength and endurance for boys are greater than those for girls. In addition, girls accumulate a more rapid increase in the percentage of body fat during the first few days of puberty, which further contributes to their greater decrease in the number of steps per day.
Middle-age
The National Institutes of Health in the USA has developed a Toolbox to assess neurological and behavioural function in individuals aged 3-85 years. The Toolbox comprises four domains: cognition, emotion, motor, and sensation. The motor domain assesses the ability to perform physical tasks with tests of balance, dexterity, endurance, locomotion, and strength. Dexterity is quantified with a pegboard test and strength with the handgrip test.
When young, middle-aged, and old adults perform the pegboard test (manual dexterity) and measures of hand strength, the declines in dexterity and strength differ for the three age groups. As expected, old adults are weaker and less dexterous than young adults. The hand strength of middle-aged adults, however, does not differ from that of young adults, but they take longer than young adults to complete the test of manual dexterity (pegboard test). With advancing age, therefore, it seems that dexterity begins to decline before there is a significant loss of muscle strength.
Differences in the time to complete the test of manual dexterity were most strongly associated with handgrip strength and the ability to perform steady muscle contractions. Steadiness declines with advancing age and influences our ability to perform accurate reaching movement and to move objects with our hands. In our middle-aged years, therefore, exercise programs need to include activities that stress the ability to perform accurate movements as well as exercises that maintain muscle strength.
Senescence
The decline in dexterity that begins in middle-age continues in old age and the decreases in physical function are compounded by a loss of muscle mass (sarcopenia). The decrease in the ability to perform accurate movements, such as the test of manual dexterity, can be improved with exercise programs that use light loads (~30% of maximum) but require the participant to perform steady contractions that engage as many muscle groups as possible. Such steadiness training can improve the ability of older adults to walk up and down stairs, to maintain balance, and to get up out of a chair.
After 60 years of age, the decline in physical performance capabilities increases more rapidly due to sarcopenia. Because muscle strength is directly related to the amount of muscle mass, the decrease in muscle mass (sarcopenia) results in individuals becoming weaker as they age. There is no way to prevent sarcopenia; there are no pills, creams, supplements, or medical treatments that can prevent the age-associated loss of muscle mass. Instead, the most prudent strategy is to participate in exercise programs that demand high levels of muscle strength, which likely delay the decline in muscle strength. It is never too late for a healthy older adult to begin a strength-training program.
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About Roger Enoka
Roger M. Enoka completed undergraduate training in physical education at the University of Otago in New Zealand (1968-1970) prior to obtaining an MS degree in biomechanics and a PhD in kinesiology from the University of Washington in Seattle (1974-1981).
He has held faculty positions in the Department of Exercise and Sport Science and the Department of Physiology at the University of Arizona in Tucson (1981-1993) and the Department of Biomedical Engineering at the Cleveland Clinic Foundation (1993-1996).
He is currently professor and chair in the Department of Integrative Physiology at the University of Colorado in Boulder. His research focuses on the neuromuscular mechanisms that mediate acute adjustments and chronic adaptations in response to physical activity performed by humans.
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