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.

Saturday, August 7, 2021

A Comprehensive Overview of Muscular Fitness for Fitness Professionals

The term “muscular fitness” might conjure up images of big, bulky muscles or incredible feats of strength. While these are certainly elements of muscular fitness, they’re far from the only (and most important) aspects of muscular fitness for nearly all adults. Indeed, we use our muscles for literally everything we do and, if we do not have a well-developed muscular system, we quite literally shrivel away and die (clinically this is referred to as frailty syndrome). Since muscular fitness is critical for our survival and we know it declines with age, starting as young as 30, let’s explore the multidimensional nature of muscular fitness to better understand how it impacts clients.

What is Muscular Fitness?

This question is not as simple to answer as you might first think. Muscular fitness is not merely a singular capacity, but rather five distinct and yet interrelated capacities that all work together to allow the human body to function on a high level. These five capacities are:

  • Muscular Strength: the ability to produce high levels of force (relative to maximum force production) with no regard to how fast the force is produced. Think of doing a one rep max (1RM) deadlift or, more functionally, think of picking up a very, very heavy box off the ground. Speed doesn’t matter; it just matters that you move the object from point A to point B.

  • Muscular Power: the ability to produce moderate to high forces at high speeds (if you remember your physics class, Power = Force X Velocity). This could be an athlete doing a vertical jump, but it also could be an older person getting up from a chair. In both of these examples, speed matters. If you don’t believe me, try jumping or getting out of a chair slowly—the former is impossible, the latter is crazy hard.

  • Muscular Endurance: the ability to produce submaximal force, continuously, while resisting fatigue. Here, think of someone running a 5k, but also think about someone carrying in groceries from the car. These activities require a lower level of force production, well below max, but you must produce that force for a longer period of time.

  • Muscular Hypertrophy: the ability to increase the size of muscle fibers (hypertrophy means to make larger). While this might seem like something only bodybuilders care about, think again. Consider that we lose muscle mass with age through a process called sarcopenia. The ability to maintain and build muscle is critically important to our functional capacity, as a decline in muscle fiber size results in reductions in strength, power, and endurance.

  • Muscular Flexibility: the ability of the muscle and associated connective issues (tendons, ligaments) to go through a full, unimpeded, and functional range of motion. Not only do dancers and gymnasts need flexibility, but everyone who picks something up off the ground or puts a shirt on needs some degree of flexibility.

Now that we’re level set on what the different components of muscular fitness, we need to take a little detour to understand some neurophysiology. This understanding is absolutely essential to proper resistance training program design and subsequent improvements in muscular fitness.  

First Things, First – Motor Units

Before we get into this discussion much further, we have to acknowledge that skeletal muscle contraction is under our volitional control. Therefore, we must have a system through which we’re able to call our muscles into action. That system essentially involves the motor cortex of your brain, as well as specialized nerve fibers called motor nerves. These motor nerves carry the signals from the brain to the muscles. The motor nerve and all the muscle fibers that nerve connects to (or innervates) is referred to as a motor unit. Motor units can be very small, with one nerve connecting to only a few muscle fibers for fine motor control, like in the eye. Other motor units can be quite large, like in the quadriceps where one nerve might connect to a thousand fibers, allowing for high levels of force production (but less fine motor control).

Motor unit numbers can be as low as 100 for small muscles of the hands, to over 1000 motor units in the quadriceps. Motor units are spatially arranged in the muscle (meaning they’re located up and down the belly of the muscle). This spatial arrangement means that when one motor unit fires, fibers along the whole length of the muscle can produce force, pull on a bone, and produce movement.

What’s important to know here is even though the whole muscle may appear to be contracting, only the fibers within the active motor units are actually contributing to force production, undergoing stress, and adapting to training. This last point is so critical, I’ll say it again. Even though the whole muscle may appear to be contracting, only the fibers within the active motor units are actually contributing to force production, undergoing stress, and adapting to training. As we’ll see, it becomes important to stress the muscle with a variety of loads and velocities to ensure we maximize motor unit recruitment along the continuum from low to high threshold motor units. This is the only way to ensure muscular fitness is optimized.

Motor nerves are classified as either “fast” or “slow” based on their speed of nerve transmission, which results in muscle fibers within that motor unit being considered as fast or slow twitch. Interestingly, as we age, the fast motor nerves die off at a disproportionally greater rate than slower motor nerves. When this happens, the muscle fibers they connect to would be dormant if they don’t connect to a new nerve (remember, the nerve tells the muscle fibers what to do). Many times (but not always), this new nerve connection is from a slow motor nerve. This is why, as people age, they tend to get slower, as these former fast twitch fibers take on slow twitch characteristics due to the new (slow) nerve connection.

Motor units operate off of a very logical size principle. In terms of strength or power output, the greater the demand on the muscle, the more motor units we recruit, preferentially recruiting larger and larger motor units progressively. This is known as the Size Principle, and it makes sense. You wouldn’t want to recruit 90% of your motor units in your biceps to lift a spoon, but if you were lifting a heavy box, you certainly would. The implications for strength training mean that the loads you lift and the speeds you lift at dictate what motor units (and thereby muscle fibers) are active. If the load is too low and/or the speed is too slow, you won’t recruit the motor units that are only stimulated by high load and/or high velocity demands. Put another way: if you only train slow, with high reps (> 15), and low weight (< 75% of maximum), you’re likely failing to recruit at least 1/3rd (or more) of your muscle fibers—remember what I said twice above about the whole muscle appearing to contract. This leaves of a lot of inactive muscle fibers “on the table,” and has very real implications for training adaptation and functional capacity for all clients we work with. Quick side note, higher rep sets taken completely to failure do likely recruit a large percentage of motor units/muscle fibers, but for your average client, this is less of a consideration, as they’re typically not training intensely to all out failure. 

If you’re wondering why we took this trip down neurology lane, I think you’ll see why very quickly below, as it is the properties of these nerves that drive some of the most important adaptations and improvements in muscular fitness. With this understanding in place, let’s start to dive into each component of muscle fitness to understand how the body makes improvement in each capacity and what those improvements mean from a functional outcome perspective for our clients.

Muscular Strength

Improving muscular strength happens through several different mechanisms; some of these are neurological, others are anatomical. The chart below provides a summary of these mechanisms.

Neurological

Anatomical

Increased recruitment of motor units

Hypertrophy of muscle fibers

Improved synchronization of motor units

 

Increased Muscle Spindle Activity

 

Decreased GTO Activity

 


The neurological mechanisms largely focus on increasing recruitment of motor units. The more motor units we recruit, the more force we produce. To keep our soft tissue safe, we can’t recruit all of motor units when we’re not well-trained (smart on our body’s part, so we don’t rip tendons from bones). Over time, with the right training, we recruit more and more motor units and produce more and more force. We can also increase the synchronization of motor units over time. This means that instead of the thousands of motor units you have in your quadriceps firing milliseconds apart, they can all fire together, producing a larger summative force than if they fired in an asynchronous fashion.

There are also some neurological changes that happen in our local spinal reflexes. Muscle spindles, found embedded within our muscles, sensing change in length of the muscle, get more sensitized. Since muscle spindles exert an excitatory influence on muscle contraction, this sensitization leads to greater force production. Conversely, the GTO, found at the junction between our tendon and our muscle, sensing change in force transmission through the tendon, get less sensitized. Since the GTO inhibits muscular contraction (for fear the tendon is going to be ripped off the bone), less sensitization means greater force production. The net effect of more spindle activity and less GTO activity equates to greater reflexive force production.

Finally, the mechanical tension higher loads place on muscle fibers results in damage. This damage leads to a cascade of events that first cause skeletal muscle repair (back to baseline) and subsequently growth of muscle fibers due to increased protein synthesis. By increasing the size of the muscle fibers, the muscle is able to produce more force.

Early on in training, neurological adaptations are predominant. Indeed, people can get stronger very quickly (within one or two workouts) far faster than muscle can be built. As time goes on and muscle tissue is gained, anatomical changes will start to progressively contribute more and more to the ability to produce higher levels of force.

For developing optimal strength use the following training parameters:

Muscular Strength Parameters

Frequency

2-3 Whole Body Workouts/Week

Sets/Muscle Group

2-4

Reps/Set

4-8 (1-6 for more advanced trainees)

Load

80-90% of 1RM (or 4-8 RM loads)

Rest between Sets

2-3 minutes

Muscular Power

The adaptations that lead to power development are not altogether different than strength development. The chart below provides a summary of these mechanisms.

Neurological

Anatomical

Increased recruitment of motor units

Elasticity of connective tissue

Improved synchronization of motor units

Hypertrophy of muscle fibers

Increased Muscle Spindle Activity

 

Decreased GTO Activity

 

Improved Stretch Shortening Cycle reflex

 


Here we see significant improvement in neurological mechanisms as well, just in a slightly different way than with the strength mechanisms. Recruitment and synchronization of motor units is certainly important, but with power training, we work to recruit the motor units faster to increase speed of movement. There are certainly more nuanced differences in the neurological mechanisms for strength versus power training, but those are beyond the scope of this article. Just know that strength-based neurological adaptations lend themselves to more force production responses whereas power-based neurological adaptation favor more speed-based responses.

As we’ll discuss below when laying out the training parameters for power, loads are lighter in order to move them faster. Because of that, less mechanical tension results, which limits the growth response of the muscle fiber. This is not to say you don’t get any growth from power training, it’s just less than strength-based training. Finally, there are some anatomical adaptations made to the elastic elements of connective tissue as well as some reflex adaptations (i.e., the Stretch Shortening Cycle) that also aid in enhancing speed of contraction.

Before we talk about the training parameters, I want to highlight one last important point – everyone needs muscular power! As we talked about before, getting out of a chair requires power, so does walking upstairs, recovering from a slip to prevent a fall, etc. Power is much more than just an athletic attribute. More importantly, we disproportionally lose power with age (more so than muscle size or strength). Loss of power with age can be one of the more functionally debilitating aspects of the aging process and can exponentially increase the risk of falls. As such, appropriate power should be incorporated into all client’s workouts regardless of age.

To increase power, use the following training parameters:

Muscular Power Parameters

Frequency

2-3 Whole Body Workouts/Week

Sets/Muscle Group

2-4

Reps/Set

4-8 (1-6 for more advanced trainees)

Emphasize speed of movement

Load

30-60% of 1RM (or 10-20 RM loads)

Rest between Sets

2-3 minutes

Muscular Hypertrophy

Hypertrophy-based training is all about getting the muscle to grow, therefore most of the adaptations here are anatomical in nature. When the muscle grows, it grows several ways (like everything in the human body, it’s more complex than what you think).

The first type of growth is myofibrillar hypertrophy. This happens by increased synthesis of contractile proteins (like actin and myosin). This growth can happen in parallel, like cramming more sardines into a can, or in series, like adding links onto the end of a chain. The differences between parallel and series-based hypertrophy are beyond the scope of the article, but it’s at least good for you to know there’s a difference.

The second type of hypertrophy that occurs is referred to as sarcoplasmic hypertrophy, which basically means the fluid content increases inside of the muscle fiber to give it a bigger appearance. This increase in fluid content happens for a number of reasons, including: increased intramuscular carbohydrate storage, fluid retention, and the inflammatory response associated with muscle damaged caused by strength training.

Both myofibrillar and sarcoplasmic hypertrophy are driven by three primary mechanisms. Mechanical tension (load placed on muscle fibers), muscle damage (tears/damage to muscle fibers), and metabolic stress (the buildup of metabolites like lactic acid). It appears that, based on current research, the most critical mechanism for hypertrophy is mechanical tension, but the other factors do contribute to the holistic hypertrophic response.  For a more complete review of these mechanisms go to this article by Schoenfeld: The Mechanisms of Muscle Hypertrophy and Their Application to Resistance Training

It should be noted that gaining muscle is not just for the bodybuilders of the world. All of us need sufficient muscle tissue and we lose that tissue with age (even if we’re weight training, albeit at a slower rate). The loss of muscle tissue with age is referred to as sarcopenia. Less muscle tissue means less strength and power production and therefore less functional potential. Worse yet, as people age, they not only lose muscle, but gain fat. This is where sarcopenic obesity occurs and it’s a real double whammy. The loss of muscle tissue makes it harder to move around and the gain for fat mass means there’s more tissue to move around – a bad combo to say the least. Finally, loss of muscle tissue reduces metabolism (since muscle is so metabolically active), which increase the risk of gaining body fat.

Last thing I’ll say: to all of those who worry that strength training will make you big, muscle bound, and bulky -- if only it were that easy!!! Even the most highly trained and dedicated bodybuilder can only gain small amounts of muscle each year (a half pound per month at best, provide they’re not using “vitamin S,” that is). The average trainee lifting weights would do well to put on 0.25lbs of muscle per month (for women the rate of growth is even less). At that rate of gain, no one will certainly ever look bulky. In fact, the extra muscle tissue will likely increase metabolism to a great enough degree that they’ll likely lose more fat than they’ll gain muscle tissue; thereby weighing less and looking more toned.

Hypertrophy programs can be a little more complex to structure in terms of frequency and workout splits (i.e., what is trained on what days). Below is a general overview:

Muscular Hypertrophy Parameters

Frequency

Each muscle trained 2-3 times/week

Can be “split” into multiple workouts

Sets/Muscle Group

2-8

Reps/Set

8-12

Load

70-80% of 1RM (or 8-12 RM loads)

Emphasis on training at or near failure

Rest between Sets

1-2 minutes

Muscular Endurance

The adaptations that result in improved muscular endurance are largely metabolic in nature. This means they help the muscle produce energy for a longer period of time. Remember, muscular endurance is submaximal force production while resisting fatigue. As such, rarely is force production the limiting factor (although if you get stronger, you’ll be able to do the endurance activity at a lower percentage of max force production, which makes it less taxing).

The metabolic adaptations that occur are related to an increase in enzyme content, intramuscular fuel stores, and increased number of mitochondria, the powerhouse of the cell that produces energy. Additionally, there are a number of cardiovascular adaptations that can occur centrally (in the heart) and also near the muscle (like increased in capillary bed size to reduce diffusion distance with the blood stream). The net effect of all of these adaptations is that the muscle can generate more fuel for a longer time resulting in great fatigue resistance or endurance. Lastly, there is increased buffering capacity of lactic acid (hydrogen ions more specifically), which limits the discomfort associated with a high level of anaerobic metabolism.

Muscular endurance is certainly important for athletic endeavors such running races and triathlons, but also important for many activities of daily life. A number of activities that we perform require sustained force production at a lower level (walking, carrying objectives, performing repetitive occupational tasks, and so on). Although strength and power are likely more directly related to the limits of functional capacity, muscular endurance challenges may predominate during the majority of our muscular activities during the day. 

To improve muscular endurance, use the following training parameters:

Muscular Endurance Parameters

Frequency

Each muscle trained 2-3 times/week

Sets/Muscle Group

2-4

Reps/Set

15-20+

Load

50-60% of 1RM (or 15-20+ RM loads)

Rest between Sets

30-60 seconds

Muscular Flexibility

This has everything to do with the muscle and connective tissue to go through a full, unimpeded, and functional range-of-motion (i.e., the ROM you need to do the things you need to do with your muscles).

There are a number of complex adaptations to flexibility training. Among these are some dealing with the muscle proprioceptors we discussed earlier (muscle spindles and GTOs). Other adaptations happen to the connective tissue itself, such as improved compliance of the some of the structural elements of connective tissue (like elastin and collagen). Still other improvements in flexibility come through improved nourishment of articular cartilage (on the end of bones). The reality is: flexibility is the complex interplay of adaptations on the neurological and anatomical level, with some adaptations occurring within the muscle itself and other happening to the connective tissue (tendons, ligaments, and cartilage).

The need for flexibility seems clear to most people; we need to be flexible to move, this is unquestionable. What’s interesting is, most stretching research has shown mixed results on improvements in range-of-motion (ROM), performance, and injury prevention. I suspect this is because it’s hard to quantify the intensity of a stretch (it’s subjective, unlike load lifted or heart rate achieved in resistance and aerobic training research, respectively). My sense is that flexibility training does confer benefits in terms of performance, injury prevention, and ROM, so don’t ditch the stretching yet.

Since the emphasis of this article is more strength training, I’ll focus on more on the role of strength training in improving flexibility. You heard me right – STRENGTH TRAINING improving flexibility. It turns out there’s a significant amount of research that supports this claim, but there’s also practical observation as well. No one actually needs flexibility devoid of strength (or force output by the muscle). You’re not just sitting there trying to achieve a ROM for the fun of it; you’re trying to do something, and that something always couples muscular force production (strength) with ROM (flexibility). You want to pick something up off the ground, or squat down to play with your dog, or reach up a grab something above you or behind you. There is always a force production element involved in dynamic flexibility. Because of that, doing full range of motion strength training (within appropriate ROM limits) is the best way to improve functional flexibility.

A good full ROM squat (even with an unloaded bar on the back) will do wonders for your hip, knee and ankle flexibility. A full ROM RDL will result in amazing improvements in flexibility to the low back and hamstring. A chest flye? You guessed it: great for shoulder mobility. Now to be clear, I’m not the meathead who’s saying not to do stretching. I’m merely saying that by emphasizing full ROM on strength training, you can greatly improve functional flexibility. 

Possibly a better way to think of flexibility in a more practical context is mobility, as this capacity is the intersection of strength, flexibility, and technical skill (i.e., the ability to perform a motor skill efficiently). It is through being strong, having functional flexibility, and having adequate kinesthetic awareness that we can be truly mobile, and that is perhaps the most critical functional capacity of them all.

Putting It All Together

Phew, there was a lot in this article. Nice work getting through it. When looking at things from a bird’s eye view, it’s clear there are several different types of workouts to improve overall muscular fitness optimally. How to introduce this type of training to someone with minimal-to-no experience is beyond the scope of this article. Needless to say, you can’t just take someone who has never lifted a weight in their life before and start them on a heavy strength cycle. This article also doesn’t cover how to structure training in a systematic fashion to optimize adaptation (this is called periodization). The goal of this article was merely to get you thinking about what comprehensive muscular fitness looks like and how to improve each of its components. Just knowing there are different ways to attack the various aspect of muscular fitness is the first step to optimizing the functional capacity of all clients at all ability levels.

 

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...