Impaired voluntary neuromuscular activation limits muscle power in mobility-limited older adults

Authors: CLARK, DAVID - University Of Florida; PATTEN, CAROLYNN - Us Army Medical Department; REID, KIERAN - Jean Mayer Human Nutrition Research Center On Aging At Tufts University; CARABELLO, ROBERT - Spaulding Rehabilitation Hospital; FIELDING, ROGER - Jean Mayer Human Nutrition Research Center On Aging At Tufts University

Submitted to: Journals of Gerontology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/15/2010
Publication Date: 5/4/2010
Citation: Clark, D.J., Patten, C., Reid, K.F., Carabello, R.J., Fielding, R.A. 2010. Impaired voluntary neuromuscular activation limits muscle power in mobility-limited older adults. Journals of Gerontology. 65(5):495-502.

Interpretive Summary: Problem: Maximal voluntary muscle power declines with aging, and the magnitude of this deterioration has been shown to be predictive of mobility function in older adults. Since power is determined force/strength and speed of movement, it is more sensitive than traditional measures of muscle strength (e.g., static force or the one repetition maximum) to physiological changes in the motor system that occur with aging. Muscle power may be highly dependent on the ability of the nervous system to activate or initiate movement (neuromuscular activation). Mobility tasks often require rapid alterations of muscle activation, and slowing of activation with aging may pose a particular challenge when task demands are higher such as during fast walking, stair negotiation and balance recovery after tripping. We compared neuromuscular activation in older adults to middle-aged controls with the expectation that this narrower age range would reveal differences that are more focused and relevant to age-related declines in mobility function. We found that, relative to middle-aged adults, neuromuscular activation was be slightly reduced in healthy older adults and more substantially reduced in mobility-limited older adults. We also found that neuromuscular activation rate would be positively associated with dynamic muscle performance and mobility function.

Technical Abstract: Background. Age-related alterations of neuromuscular activation may contribute to deficits in muscle power and mobility function. This study assesses whether impaired activation of the agonist quadriceps and antagonist hamstrings, including amplitude- and velocity-dependent characteristics of activation, may explain differences in leg extension torque and power between healthy middle-aged, healthy older, and mobility-limited older adults. Methods. Torque, power, and electromyography were recorded during maximal voluntary leg extension trials across a range of velocities on an isokinetic dynamometer. Results. Neuromuscular activation was similar between middle-aged and older healthy groups, with differences in torque and power explained predominantly by muscle size. However, the older mobility–limited group demonstrated marked impairment of torque, power, and agonist muscle activation, with the greatest deficits occurring at the fastest movement velocities. Agonist muscle activation was found to be strongly associated with torque output. Conclusions. Similar neuromuscular activation between the middle-aged and older healthy groups indicates that impaired voluntary activation is not an obligatory consequence of aging. However, the finding that the mobility-limited group exhibited impaired activation of the agonist quadriceps and concomitant deficits in torque and power output suggests that neuromuscular activation deficits may contribute to compromised mobility function in older adults.