MUSCLE FUNCTION and AGING

BACKGROUND

A decrease in muscle mass and power with aging has been well documented. These decreases are related to increased fall risk, inability to perform activities of daily living, health, and a loss of independence. The ability to develop force quickly is a function of the initiation of the motor signal, excitation-contraction coupling of muscle, and the rate and magnitude of muscle force production.

The goals of this line of research are to:

1:  assess the effect of physical activity on the temporal aspects of muscle activation and rate of muscle torque development in elderly individuals.

2:  determine the effect of physical activity on muscle activation by assessing motor unit activation strategies.

3.  develop muscle power training interventions to restore muscle function in previously sedentary older adults.

PROCEDURES

We are currently studying the differences in muscle activation and force production between highly active and more sedentary women age 65+.  One task involves electrically activating the muscle to assess underlying muscle function in the absence of nervous innervation.  Another uses a timing light, surface electromyography, and measures of muscle force to determine premotor time, electromechanical delay, rate of torque development, peak torque, and onset EMG.  Participants are asked to produce as much muscle force as possible, as quickly as possible, following a visual stimulus.  A preliminary examination of our data indicates that more active women generate more force, do it more quickly, and have enhanced neurological activation of the muscle compared to their sedentary counterparts.  The figure below illustrates how these variables are calculated.

PREMOTOR TIME is the time that it take from the onset of the visual stimulus until muscle cell depolarization occurs and is representative of reaction time and the generation of a motor output by the central nervous system. 

ELECTROMECHANICAL DELAY is the time from muscle cell depolarization until muscle force output can be measured.  EMD is indicative of the time it takes for excitation-contraction coupling to occur as well the time to take up the slack of the muscle-tendon unit.

RATE OF FORCE DEVELOPMENT is a measure of how quickly force can be generated and is measured as the slope of the force vs. time curve.

ONSET EMG (electromyography) is a comparison of the amount of motor unit activity seen during the initiation of a forceful contraction to that seen at the maximum force.  This is the first "hump" seen in the EMG signal that occurs simultaneously with the rapid onset of force production.

RATE OF EMG RISE is calculated as the slope of the EMG vs. time curve and represents how quickly the nervous system can activate muscle.

Supported by the Vermont Genetics Network

through NIH Grant Number P20 RR16462

 from the INBRE Program of the National Center for Research Resources