Use it or lose it? (Implications for aging muscle and why training is vital to health)


I get it. You eat healthy. You don't smoke. You walk your dog around the block most days. As you will read in my past posts, and posts to come, I want to make a case for exercise. Not just light dog-walking most days, but higher volumes and intensities of exercise. Exercise plays a HUGE role in the gracefullness of how we age... and it can literally be VITAL to our health and wellness. Here, I want to make a case for how and why exercise can prevent a decline in muscle and muscle function that is thought to be inherent with the aging process.

Sarcopenia is the term for programmed cell death that occurs in the human body. Sarcopenia is thus associated with impaired whole body muscle performance that occurs as we age. Although it seems to have an onset in humans around the 7th decade of life, many factors influence sarcopenia in the body such as nutrition, disease, inflammation, level and intensity of physical activity, and endocrine factors, potentially delaying or changing the morphology or severity of onset of sarcopenia. Interestingly, it appears that the quality of the muscle seems to be affected with natural aging more so than the quantity. Although the atrophying of muscle (shrinking of muscle cross sectional area) does play a role, it is more about the loss of quality contraction, and more specifically, loss of motor units (the functional unit of the muscle) that have the biggest detriment as we age.

Noticeable changes in physical characteristics of the muscle are the loss of power (both force and velocity), decrease in muscle mass, loss of activation of muscle fibres, impaired excitation contraction coupling (think stimulation of the nerve paired with the contraction of the stimulated muscle fiber), and a change in the muscle architecture in terms of how it is organized into fascicles (think pods of muscle fibers within a whole muscle). Recent research points towards loss of motor units as one of the main mechanisms for which these changes occur. A motor unit (MU) consists of an alpha motor-neuron and all of the skeletal muscle fibres it innervates. A motor unit can consist of many muscle fibres or very few, depending on the location in the body and type of movement produced (large whole-body movement or small refined movements). Recruiting more or less motor units for the movement can modulate force in a given muscle.

It is commonly known that as we age, there is a decrease in strength, but the mechanism or the "HOW do we lose strength" is less understood. Recent studies have shown that the loss in strength is first preceded by a loss of MUs, beginning at the neuromuscular junction. Individuals will not demonstrate a loss of strength with the initial loss of MUs, and instead might have other compensatory mechanisms for maintaining strength that mask the real loss of MUs. Despite these losses, restructuring can occur whereby neighbouring MU’s axons will re-innervate fibres that had been de-innervated. This re-innervation process is not perfect, and thus sometimes other muscle fibres will forever lose their innervation and thus ability to contract. When this process, termed collateral re-innervation occurs, the MUs are typically larger and demonstrate properties of Type 1 fibres in that they are slower twitch and produce less force. This is interesting in that although it appears that MU loss and full innervation to each muscle fibre will surely decrease with age, there are mechanisms that somewhat slow the process. The question then becomes, what if we maintain a very high level of activity throughout our lives? Will it have a protective effect on the loss of these MU’s?

A recent study by Power et al. did just this, and attempted to examine the difference in MUs between young, old age, and old aged masters athletes who had maintained high levels of running their whole lives. They measured the number of functional motor units (MUNEs) of the tibialis anterior muscle (crucial in running), and hypothesized that because of chronic activation of this muscle and motor unit pools throughout their lifetimes, the master’s athletes would have less of a loss of total MUNEs than the old population. Sure enough, the result of the study showed no significant difference between the young and master’s athletes number of MUNEs, while the old group had a significant decrease as expected.

These results are very valuable when combined with another study by Power et al. that showed that preservation of MU’s are site specific. The masters athletes, although showing no real loss in MUNEs in their tibialis anterior’s, showed decreases in their biceps brachii muscles, proving that if you don’t use it, you lose it! Reasons for this include that potentially different mechanisms of re-innervation may occur in different muscles, such as between a fast or slow twitch muscle. Fast twitch muscles (such as the biceps brachii) perhaps do not benefit from collateral re-innervation because with aging may change to be innervated by larger MUs and thus take on Type 1 (slow and low force) properties.

There is undoubtedly a protective effect of high levels of physical activity in preserving muscle motor units and activation of muscle. There is no doubt, either, that high levels of training, in this case endurance training in high volumes, are site specific, and thus in terms of loss of motor units with aging, perhaps full-body athletes such as triathletes have the right idea. Although resistance training has not been addressed in this post, maintaining strength training as we age has also proven to be very valuable in preventing bone loss, and preserving muscle function with age. Because we do not see immediate decreases in strength with loss of motor units, older individuals should be aware of the benefits of maintaining high levels of activity if possible throughout their lives. This is important for maintaining functional muscle mass into old age, and maintaining quality of life and independence, along with the numerous other health benefits of training.

For future studies, it would be interesting to examine the effect of endurance type training versus resistance training, as well as a combination of both for maintaining healthy junctional connections and full innervation of the muscle fibre, as well as mainlining the size and muscle type of fibres in the body.

Remember, strength and fitness are VITAL for your well being and health. #VitalStrength

References

Power, G. a., Dalton, B. H., & Rice, C. L. (2013). Human neuromuscular structure and function in old age: A brief review. Journal of Sport and Health Science, 2(4), 215–226. doi:10.1016/j.jshs.2013.07.001

#aging #muscle #motorunits #qualityoflife #resistancetraining #innervation #nerves #musclefibre #agingmuscle #strength #conditioning #disease #inflammation #sarcopenia #contraction #forceproduction #athlete #athletes #masterasthlete #Type1 #Type2 #preservation

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