Neuromuscular Adaptations with Aging by Hayden Giuliani

[Hayden Giuliani recently finished her Master’s degree at the University of North Carolina Chapel Hill, where she now works as a research coordinator. She is currently in the Coaching Mentorship Program at Athletic Lab.]

As we age, there are obvious changes in our performance and the way our body responds to loads or exercise. Many adaptations occur within the body, specifically the muscles, that allow for these more global decrements. But, for some, it seems there is less of a decrease in performance and quality of life. Exercise is the key method in attenuating many of the muscular adaptations with aging. The primary adaptations that occur with aging are decreased muscle size (sarcopenia), decreased muscle strength (dynapenia), and decreased muscle quality, which other muscle architectures and structural adaptations contribute to. With a growing number of older adults, this is a very relevant topic to discuss.

Muscle Size

Sarcopenia differs from atrophy in that it occurs despite lifestyle and weight changes. Beyond the overall decrease in lean mass and inherent increase in body fat percentage, research has shown that muscle size decreases 25-36% in the thigh musculature with aging, compared to younger adults. (Lexell, Overend) An invasive study by Lexell and colleagues found that size changes within the vastus lasteralis can begin as early as 25 years and decrease as much as 10% by age 50. This loss of muscle is different between the upper and lower body, with the lower body reducing at a rate twice that of the upper body. This decrease in size is caused by a reduction in muscle fiber size and quantity. Neuromuscular fiber changes occur through a continual process of denervation and reinnervation, which leads to not only a loss of fibers and motor units but also a remodeling of the remaining fibers. There is a greater loss of type II (fast-twitch) than type I fibers, seen by less specific enzyme activity of type II fibers. (Lexell) Fiber type grouping is a term used to describe this change, with slow-twitch motor units reinnervating the denervated fast-twitch motor units and increased percent area of type I fibers.

Muscle Strength

It has been shown that dynapenia, the age-related loss of muscle strength, is more associated with physical disability. Studies have shown strength losses of 24-30% in the leg extensors and flexors after adjusting for sarcopenia, indicating that sarcopenia’s effects are secondary to dynapenia. (Clark) A longitudinal study by Goodpaster and colleagues has shown a 3% loss of muscle strength per year over a period of three years in older adults, and those who exhibit less strength show an increased risk of falling. (Perry – Figure 1) In addition to the loss of muscle size contribution to a decrease in muscle strength, there are also neuromuscular function deficits. With the process of denervation and reinnervation, as mentioned above, there is an increase in the motor unit grouping. Basically, there is a loss of type II motor units, and a greater number of type I, relatively. Type II muscle fibers mostly contribute to power and strength development, and with a loss of those fibers, there is less ability to generate high forces. In addition to the remodeling of the muscle, there is a reduction in voluntary activation of the muscles, meaning older adults cannot fully activate the muscle. Motor unit firing frequency may be contributed to the change in muscular properties, but also impaired neural drive, especially in those who are less active or have a neuromuscular disease. (Stevens, Harridge) A component of strength is the rate of force development (RFD), or how quickly a muscle can produce near-maximal force. It has been shown by Izquierdo and colleagues (1999) that RFD was significantly lower in older adults than young and middle-aged adults, even up to 64%. RFD is not only important in athletes but is highly associated with the prevention of falls, which is important as we get older.

Muscle Quality

Another way to evaluate strength is relative to muscle size. This is termed specific strength and has also been shown to decrease with aging, up to 5-8%. (Goodpaster) Specific strength was the first method of investigating the term “muscle quality.” Because it has been shown that muscle strength decreases at a greater rate (2-5 times) than the decrease in muscle size, it is suggested that something else must be contributing to this loss of performance. (Delmonico) This has led to other methods, such as CT scan and ultrasonography, to investigate muscle quality as defined by intramuscular composition. Numerous studies have now found that there is an increase in fat and connective tissue infiltration within the muscle, up to 50% specifically within the quadriceps muscles. (Delmonico, Csapo) This occurs alongside the remodeling process when muscle fibers are not reinnervated. Obesity also exacerbates poor muscle quality, by increasing the infiltration of intramuscular fat. This measure of muscle quality has shown to be significantly correlated with walking speed and functional tests in older adults, with poor quality leading to slower times. (Rech, Wilhelm)

Muscle-Tendon Architecture

There is no one mechanism that contributes to the loss of strength and function, so here are a couple other adaptations that occur within and around the muscle unit. Pennation angle and fascicle length compose the architecture of the muscle (alignment of the fibers). Cross-sectional studies have found a 10-19% reduction in fascicle length, suggesting fewer sarcomeres in series, while other studies have found a 13-20% reduction in pennation angle, indicative of fewer sarcomeres in parallel. This also contributes to the decrease in strength. Another contributing factor to the reduction in RFD in older adults is muscle-tendon compliance. A decrease in tendon stiffness creates additional “lag” time between muscle activation and muscle force production, which occurs with aging and may be a primary cause of the loss of power and RFD. This may lead to the increased risk of falls.

The Answer: Exercise

There is good news in all of this… We can mitigate the degree of these changes and maintain proper muscle size, strength, and function. For example, it has been shown that those who maintained their physical activity through adulthood attenuated the increase in intramuscular fat infiltration (18.4 v 2.3%). (Goodpaster – Figure 2) The same study found that the physical activity protocol completely prevented the loss of strength as well, and strength actually increased by 2.5%. One study compared three training programs and concluded that any of the training implemented improved strength and endurance more so than those not training, but strength training had the greatest effect on all exercises. Resistance training maintains lean mass, muscle strength, and bone mineral density. These are all qualities that are associated with an older adult’s ability to perform activities of daily living. A person with less muscle strength is at greater risk for falls and prolonged recovery time from falls, but it becomes negated with the implementation of exercise. But even if you haven’t maintained your activity level, by implementing a new program, you will still see great results. A large number of studies have investigated this using strength training. Most studies showed that the higher the intensity, typically 80% of 1-repetition maximum (1RM), produce better results. For example, strength has (1RM) increased up to 90%, lean mass and muscle size has increased up to 7-10%, isometric force production up to 150-200%, increased muscle fiber size, increased muscle activation, and increased rate of torque development. (Lovell, Kalapotharakos, Fiatarone, Frontera, Ferri, Barry) All of these improvements also enrich the quality of life, both mentally and physically, which is the most important benefit of all.

References

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By | 2017-11-21T11:32:17+00:00 November 21st, 2017|Training Info|0 Comments

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