Thursday, September 16, 2021

Is Strength Training the New Cardio? The Role of Muscular Fitness in Health

 


For years, cardiovascular fitness was considered the epitome of what it meant to be healthy. Someone who could walk - or run - for miles was someone with a strong heart and lungs. That strong heart and lungs would help that person live a long and healthy life.

“The heart is the most important muscle of the body” is something that exercise and medical professionals have extolled for years—but what about all the other muscles in the body? Aren’t they important as well? That answer is a resounding YES, and its causing exercise and medical professionals to rethink their paradigm around cardiovascular fitness being the most critical indicator of human health and functioning.

Dependence of Cardiovascular System on Muscular Fitness

At its most fundamental level, in order for the heart and the lungs to be stressed adequately to make functional improvement, muscles have to contract to move the body. The better those muscles are able to contract (that is, the better “fitness” they have; go here for a more specific definition of muscular fitness), the more the heart and lungs will be able to be stressed, resulting in greater improvements in cardiovascular fitness.

Think of it this way: if you don’t have adequate strength to get out of a chair, you’re likely to sit more. If you don’t have the necessary strength to walk up a flight of stairs, you probably won’t do it. If walking to the corner and back is limited by the strength and stability of your leg muscles, you’re probably not doing much walking. All of this means your heart rate is never elevated, your lungs are not forced to increase ventilation, and your cardiovascular system never improves—in fact, it declines.

With that said, you can almost view muscular fitness as the “rate limiter” for cardiovascular fitness. Weakness leads to less movement. Less movement leads to less cardiovascular stress, which leads to decline in cardiovascular function. While it’s certainly true that the cardiovascular system is essential for assisting the contracting muscle in producing force (by delivering blood and nutrients and removing waste products), it’s important to ask ourselves: what deteriorates at a faster rate with age and disuse and, furthermore, what is recoverable after deterioration?

If you examine the scientific literature, it can be difficult to parse out those two questions, as all functional capacities decline with disuse and age to some degree. That said, I would submit that cardiovascular fitness is much more quickly recoverable than muscular fitness, and also has built-in compensatory mechanisms that are fairly effective at masking decline. Consider that maximum heart rate appears to have a linear reduction of about 1bpm per year (with some variability), but cardiac output can largely be sustained at submaximal level by increasing stroke volume and oxygen/nutrient extraction at the tissue level. Stroke volume can be quickly recovered after a period of detraining due to simple mechanisms like increases in blood volume (the Starling Mechanism).

On the other hand, with aging and disuse, motor nerves can literally die off.  Motor nerves are the structures that tell muscle fibers what to do, and the motor nerves with the greatest propensity to do so are the fast twitch motor nerves. Once these nerves die off, the fibers they innervate (the so-called “fast twitch” muscle fibers) tend to be reinnervated by slower twitch nerves, which results in those formerly fast twitch fibers functionally turning into slow twitch fibers. This is not a reversible process; it is permanent. These fibers now produce less force and power and are thereby less functional. Less force and more so less power means a reduction in functional abilities, decreased activity time, increased sedentary time, and increased risk of fall/fractures with age. All of this amounts to less stress on the cardiovascular system and a functional decline in cardiovascular capacity. When muscular fitness declines, it’s a vicious cycle of less activity/movement that leads to more cardiovascular decline. One begets the other and we’re left as a shell of our former functional self.

Furthermore, the cardioprotective benefits of strength training extends to the heart (and the rest of the body) by being able to accommodate higher blood pressures due to strength training, resulting in significantly higher systolic pressure compared to traditional cardiovascular exercise. This ability to accommodate higher blood pressures, improvement in functional capacity, and finally improvements in venous return (due to a great active muscle mass) in sum all result in significant improvement in cardiovascular health. It doesn’t stop there, however, as strength training confers a whole host of other health benefits. I’ll touch on some of the most significant health benefits below.

Increase Bone Mineral Density

Bones lose their density because we load them less and less as we age. With bone mineral density (BMD), certainly hormone status plays a role, such as the loss of estrogen during menopause, as does dietary intake of things like calcium and Vitamin D (for the most part, higher levels are associated with better BMD). But the most significant effect of bone demineralization is lack of loading of the bones, which can lead to osteoporosis or its subclinical precursor osteopenia. We load our bones when we do impact-based activities, like walking and running, but also when we put stress, tension, and strain on the bones when lifting objects. This loading stimulates osteoblast activity in our bones. These are cells that literally lay down new bone. Less activity and less loading results in osteoclast activity, that breaks down bone, exceeding the osteoblast activity, with the net result being bone loss and reduced BMD.

Strength training is the ideal modality for loading bones, as it can place stress, tension, and strain on all the major bones of the body from different angles and positions, thereby maximizing BMD. If you think about how many ways you load the femur when walking, you’ll find it’s really one way. If you consider how many leg-based strength training exercises you can do to load the femur from different angles and positions, you’ll find you almost have too many to count. Bottom-line: to improve BMD and reduce the risk of osteoporotic fractures in the spine and hip, strength training is the most efficacious vehicle.

Type II Diabetes

Diabetes is a significant problem with over 34 million American having Type II Diabetes (T2D). Another 84+ million Americans are pre-diabetic and well on their way to developing full blown T2D. Because disease management strategies for T2D are so effective, people can live for a long time with this condition. While this is good for longevity, it is very costly for our medical system. Cost for managing a newly diagnosed diabetic can range in the thousands of dollars per year. Treating end-stage diabetics (with retinal, renal, or neurological issues) can cost in the tens to hundreds of thousands. Cost aside, T2D can limit functional capacity, increase body mass and systemic inflammation, and predispose an individual to acute cardiovascular or cerebrovascular events (i.e., heart attack and stroke).

While overall physical activity and exercise have proven very effective in reducing diabetes symptoms (see the DPP for more info on this), strength training plays a unique role. One of the big hallmarks of strength training is some level of increase in muscle mass. Muscle is essentially the reservoir of carbohydrate in our body. Carbs either go to our muscles, our liver, or our fat cells when they need to be stored for energy use in the future. Liver carb stores are limited, and fat is clearly not an ideal place to store carbs. That leaves us with skeletal muscle as the best storage site for carbs. As we lose muscle (with age or disuse), we lose storage capacity. This can result in carbohydrates staying in the blood steam longer, causing vascular injury, insulin resistance, and hyperglycemia.  The net effect of this is the diabetic cascade of events that increase cardiovascular disease as well as a slow death of our Pancreatic Beta Cells that produce insulin. Maintaining (or ideally increasing) muscle mass increases the gas tank for carbs in your body. This allows you to accommodate much more carbohydrate and reduce the likelihood of diabetes.

Falls

Falls are one of the most debilitating events as people age. Even the fear of falling causes activity to decline, as the psychological threat of falling and injuring one’s self is too great. Fear of falling causes reduced activity, which reduces muscle mass and cardiovascular capacity, which further causes function to decline (see above), and results in more fear of falling. This terrible cycle plays out until someone is nearly paralyzed by their fear.

If falls do take place, they can result in facture (often in the hip, arm, or spine) and head injury (concussion). Falls leading to hip fractures are particularly deleterious, as this outcome can be predictive of mortality in some individuals.

To not fall, once must keep their center of mass (located in the middle-ish of the torso) within their base of support (their two feet). When something causes us to get our center of mass outside of our base of support and we can’t correct fast enough to get it back in there, we fall. Also, like when we stub our toe, we can often fall if we can’t recover in time—more specifically we can’t dorsiflex the ankle fast enough.

The ability to maintain stability on your own two feet is dependent on strength (co-contraction of muscles on both sides of a joint). The ability to recover from the center of mass getting outside of the base of support is dependent on power (or speed-strength). In essence, you have to have the power to contract muscles at a relatively high rate of speed with some decent force in order to recover quickly enough to prevent a fall. The only way to develop this capacity is to strength train and to do so with heavy enough loads to get strong, and fast enough loads in order to be powerful. You can’t train these capacities of strength and power any other way.

Lower Back Pain

Lower Back Pain (LBP) can be one of the more debilitating orthopedic conditions an individual can encounter. Given the lower back’s role in stability of the hips, spine, and scapula, pain and dysfunction in the lower back can limit nearly any physical activity. The prevalence of LBP is quite high, with upwards of 80% of Americans experiencing an acute incident of LBP in their lifetime. Further, at any one point in time, LBP may affect approximately 30% of our population. It is estimated that the annual cost of LBP exceeds $100 billion, with more than two-thirds of that cost coming from lost wages and productivity.

Although the causes of LBP are multifactorial in nature and a true etiology is nearly impossible to isolate, one thing is clear: common postures of daily life don’t lend themselves to back health. We sit more now than we ever have. The result is a spinal alignment that is largely kyphotic (rounded forward). This postural alignment results in the elongation of the musculature on the posterior side of the body, which leads to progressive weakening of several muscle groups in the upper and lower back. Over time this results in the inability to effectively stabilize the torso in extension, thereby predisposing it to a whole host of potential spinal pathologies (that we can generalize as LBP).

Combatting postural issues is challenging because it’s not likely our high volume of sitting is going to change any time soon. One of the most fruitful interventions for addressing postural issues as well as LBP is strength training; more specifically, strength training of upper body posterior chain musculature such as the lats, mid-traps, lower traps, spinal erectors, and other deep spinal stabilizers. In fact, research suggest that spinal erector strength improvements, facilitated by resisted spinal extension exercises, results in reduced LBP symptoms and prevalence of acute LBP episodes.

Frailty  

According to the National Institutes of Health, frailty is a “clinically recognizable state of increased vulnerability resulting from aging-associated decline in reserve and function across multiple physiologic systems such that the ability to cope with every day or acute stressors is comprised.” In essence, frailty causes life to get very small very quick. Individuals with extreme frailty do not even perform some of the most instrumental activities of daily life (cooking, dressing themselves, personal hygiene habits). Even when not at that extreme, frailty still results in a decline in functional capacity that leaves life as a shadow of its former self.

While the aging process and frailty is complex, it is clear that the decline in muscular function (strength, power, and muscle mass) contributes significantly to frailty and functional decline. Age-related declines in muscle function are inevitable to some degree, but can be greatly slowed with strength training. In fact, research shows the preservation of strength, power, and muscle mass with age in the presence of a progressive resistance training program. Many studies have found that resistance-trained individuals in their 60s and 70s have the muscular fitness of untrained controls that are in their 40s and 50s. If you read that sentence again (and then one more time for good measure), you’ll be astounded by its implication. Most specifically, strength training turns back the hands of time and makes you (functionally) younger. Now while I won’t be hyperbolic and call it the fountain of youth, these are very significant findings. Furthermore, research also shows individuals can see positive improvements in muscular fitness well into their 90s, with the great improvements in muscular fitness coming in individuals who have the lowest muscular fitness to start.


Putting it All Together

To optimize health, one must be physically active, get adequate sleep, and eat a well-balanced diet. Traditionally the physical activity category has largely focused on cardiovascular fitness and associated training modalities. While this emphasis is not incorrect, it fails to recognize the importance of strength training and muscular fitness on overall health. As we have seen in this article, there are many areas of health strength training will preferentially enhance over the improvement seen from traditional cardiovascular exercise. Clearly the ideal exercise program for health would include both aerobic exercise and resistance training. However, if you had to choose just one, the ever-increasing evidence continues to point towards resistance training.

Finally, for those interested in a more comprehensive review in the peer-reviewed literature, check out this article: The Benefits of Strength Training on Musculoskeletal System Health: Practical Applications for Interdisciplinary Care.

Is Strength Training the New Cardio? The Role of Muscular Fitness in Health

  For years, cardiovascular fitness was considered the epitome of what it meant to be healthy. Someone who could walk - or run - for miles...