Muscle Primer: What It Is, Why It Matters
Muscle mass is likely the most important tissue in your body when it comes to managing insulin action and blood glucose levels. It is also one of the most important things to maintain to live well and independently as you age. In other words, you can’t afford to lose your muscle mass if you want to live long and well.
What exactly is muscle? It’s comprised of about 20 percent protein, with the rest mainly made up of water plus a few minerals and some carbohydrate and fat stores. We generally talk about having two types of muscle fibers, slow twitch and fast twitch, which refers to the speed at which the contractile proteins can do their work and be ready to contract again. Their speed determines whether they’re good for doing long, slow endurance exercise (slow twitch) or explosive power activities like sprinting (fast twitch).
Muscle fibers are still currently categorized into three types in humans: slow oxidative (SO, or Type I), fast, oxidative glycolytic (FOG, or Type IIa), and fast glycolytic (FG, or Type IIx). The Type IIb fibers we used to think that humans had only exist in lower mammals like rats apparently; we have Type IIx instead. Oxidative fibers can use a variety of fuels during activities, including carbohydrates, protein and fat, which is why they’re recruited for doing activities lasting longer than two minutes. Glycolytic fibers primarily use glycogen (stored glucose) readily available in muscle and other compounds in muscle like ATP and creatine phosphate that can only fuel activities for 10 seconds or less; accordingly, these fibers are only adequately fueled during shorter activities (30 seconds to two minutes) or bursts of speed or power.
In reality, muscle fibers really exist on a continuum from slow to fast and oxidative to glycolytic, and specific exercise training can make them change some of their metabolic and speed capabilities (1). They are recruited in a step-wise fashion, meaning that the harder an activity becomes or the more power or force you need to produce, the more fibers get recruited. You start with the slower ones since they’re hard to fatigue—which is good since they’re in postural muscles—and then add in the faster fibers as needed to produce more power or speed.
To be honest, the muscle fiber types you have (or don’t have, based on genetics) are not nearly as important as recruiting as many of them as possible when you train. As you age, you lose the muscle fibers that you don’t use regularly. Since it’s unusual for older individuals to sprint or lift heavy weights, the natural losses include more of the faster twitch fibers that are normally used to produce power and speed. Of course, that means that you are also losing some of your strength over time, which can make it harder to do ordinary things or even basic self-care.
Recently, I have had online "altercations" with a number of people zealous about their latest fitness training craze, CrossFit (CF), when I have suggested that CF may not be appropriate for many older individuals with diabetes and that many CF trainers have inadequate training to really teach proper form to prevent injuries. Many CF enthusiasts also seem unwilling to consider that some of the more tried-and-true methods of strengthening and building muscle, such as progressive resistance training, actually work as well as (or better than) CF, which is known for its explosive exercises done to extreme fatigue. Recent research has shown that muscle soreness is not required for muscle remodeling and strength gains (2).
Although any type of training can cause repetitive (overuse) injuries if done excessively or with poor form, most injuries are preventable. Moreover, it is my humble opinion that any type of training that increases the risk for spine injuries in particular (i.e., CF) should be undertaken with much care and adequate precautions to prevent injuries (3). Back problems are certain to limit people’s ability to train, but having to stop training altogether when doing any type of exercise due to injuries is a sure-fire way to lose muscle mass and should be avoided.
When it comes to managing your diabetes, having more muscle mass lets you take up more blood glucose and store it as muscle glycogen. Muscles have a limit to how much they can store, and having more muscle mass results in a greater potential storage capacity, whereas losing muscle mass through aging and being inactive limits it. Enhanced muscle mass is likely the explanation for why lifestyle changes, such as those made in the Diabetes Prevention Program, are more effective in older individuals than younger ones. While aerobic activities like walking may not enhance muscle mass as much due to their lower intensity, you don't have to improve your fitness levels to have better control over your diabetes and metabolism, as long as you exercise regularly (4).
So, do whatever you can to keep as much muscle as you can as you age, but avoid getting injured--if at all possible, which means training smart and not necessarily pushing yourself too hard all the time. Stay regularly active, do resistance training, and eat adequate amounts of protein and calories. Your good health well into your later years will be your reward!
Ingalls, CP. Nature vs. nurture: can exercise really alter fiber type composition in human skeletal muscle? J Appl Physiol, 97(5): 1591-1592, 2004 (http://jap.physiology.org/content/97/5/1591)
Flann KL, LaStayo PC, McClain DA, at al. Muscle damage and muscle remodeling: no pain, no gain? J Exp Biol. 214(Pt 4):674-9, 2011 (http://www.ncbi.nlm.nih.gov/pubmed/21270317)
Hak PT, Hodzovic E, Hickey B. The nature and prevalence of injury during CrossFit training. J Strength Cond Res. Nov 22, 2013. [Epub ahead of print] (http://www.ncbi.nlm.nih.gov/pubmed/24276294)
Pandey A, Swift DL, McGuire DK, et al. Metabolic effects of exercise training among fitness-nonresponsive patients with type 2 diabetes: The HART-D study. Diabetes Care. 38(8):1494-501, 2015 (http://www.ncbi.nlm.nih.gov/pubmed/26084342?dopt=Abstract)