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 1/3^rd (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.