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1 Clinical Research Branch, National Institue on Aging, Baltimore, MD, USA
2 School of Nursing, Uniformed Services University of the Health Sciences, Bethesda, MD, USA
3 National Institute of Neurological Diseases and Stroke, Bethesda, MD, USA
* To whom correspondence should be addressed. E-mail: metterj{at}grc.nia.nih.gov.
Poor muscle strength is associated with mortality presumably due to low muscle mass. Notably muscle power declines more rapidly than muscle strength with increasing age, which may be related to more complex central nervous system movement control. We examined arm-cranking power against 4 workloads and isometric strength measured in the upper extremities of 993 men longitudinally tested over a 25-year period. Muscle mass was estimated using 24-hour creatinine excretion; physical activity was assessed by self-reported questionnaire. Muscle power and strength were modeled by time using mixed effects models, which developed regression equations for each individual. The first derivative of these equations estimated rate of change in strength or power at each evaluation. Survival analyses, using the counting method, examined the impact of strength, power and their rates of change on all-cause mortality while adjusting for age. Arm cranking power (rr=.984 per 100 kgm/min, p<.001) was a stronger predictor of mortality than was arm strength (rr=.986 per 10 kg, p=NS), while rate of power change (rr=.989 per 100 kg/min/year) and rate of arm strength change (rr=.888 per 10kg/year) were risks independent of the power or strength levels . The impacts of power and strength were partially independent of muscle mass and physical activity. The risk of mortality was similar across the 4 power work loads (rr=.93-.96 per 100kgm/min), while the lowest load generated less than half the power as the higher loads. Arm cranking power is a risk factor for mortality independent of muscle strength, physical activity and muscle mass. The impact is found with loads that do not generate maximal power suggesting an important role for motor coordination and speed of movement.
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