This subproject is one of many research subprojects utilizing the resources provided by a Center grant funded by NIH/NCRR. The subproject and investigator (PI) may have received primary funding from another NIH source, and thus could be represented in other CRISP entries. The institution listed is for the Center, which is not necessarily the institution for the investigator. A decrease in muscle mass and muscle power with aging has been well documented. These decreases are related to 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 investigator hypothesized that those individuals who are extremely physically active will exhibit a shorter activation time for contraction and a greater rate of torque development and peak torque compared to their sedentary counterparts. 40 women between 65 and 80 years were recruited into highly active or sedentary groups and performed a series of maximal voluntary muscle contractions using the quadriceps muscle group. A reaction task was used to measure premotor time, electromechanical delay, rate of torque development, peak torque, and onset EMG magnitude during knee extension following a visual stimulus. A preliminary examination of the available data indicates that inactive women have a 20% longer delay between excitation of the muscle and the production of force. This delay is related to excitation contraction coupling and could include impaired sodium and potassium diffusion across the sarcolemma, slowed calcium release from the sarcoplasmic reticulum, or a decrease in the activity of myosin ATPase. When examining how quickly muscle force is being produced, the inactive group exhibits a 19% reduction in the rate of muscle torque development when controlled for body mass. This can be due to a number of factors including less muscle mass, a decreased ability to recruit available motor units, decreased motor unit firing rate, and higher fat mass relative to muscle mass in the inactive group. This discrepancy is also seen in peak muscle torque for perhaps the same reasons. Interestingly, there appears to be a reduction of 33% for the rate of EMG rise and 36% for the ratio of onset EMG to peak EMG in the inactive women. These measures reflect the ability of the nervous system to maximally activate muscle during the high speed application of force. These results indicate that women over age 65 with a history of lifetime activity can generate a greater motor output when rapid increases in muscle force are needed when compared to their less active pee
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