Hey Readers! Thanks for being here! In today's blog post, I want to clarify various terms used to describe skeletal muscles in the human body. Skeletal muscles generate internal tension that manipulates the bones of our body to produce movement, all under the guidance of the nervous system (click here to read more about the cellular mechanisms that govern skeletal muscle contraction). Skeletal muscle tissue is quite fascinating, and there are several attributes, or characteristics, that govern how this type of tissue functions, including: flexibility, stability, endurance, strength, power, and size. Certified personal trainers (CPTs), coaches, fitness instructors, and yoga teachers often use these muscular terms pretty regularly, but clients and students might not know exactly what they mean.
Having a relatively solid understanding of these various attributes helps you to move, and respond to, your body with greater awareness and efficiency. This information is also super helpful when designing or planning your fitness programming, as certain activities are designed to enhance some of these attributes over others. In today's blog post, I will give a brief overview of each skeletal muscle attribute. I hope that this information helps you to prevent, or reduce, pain and injury in your body, while also helping to inform your physical wellness routine to best match your fitness goals.
Flexibility is defined as the extensibility of muscle tissue, and the surrounding connective tissue, that allows a joint to move through its full range of motion (ROM). Adequate flexibility allows for the freedom of movement needed to perform every tasks, such as bending over to pick something up or reaching overhead to put dishes away. Several studies have found a link between a lack of flexibility and increased risk of injury. When you move through your environment, some muscles contract to create that movement, while opposing muscles relax and stretch to allow the muscle contraction to occur. Each muscle in the body needs an appropriate amount of flexibility (not too much and not too little), so that fluid and smooth movement can occur.
If a particular muscle has reduced flexibility, meaning that it cannot lengthen optimally, it can reduce the contractile ability of its opposing muscle AND it can potentially pull nearby joints out of alignment. For example, if you need your biceps muscle to bend the elbow to lift something, the triceps muscle (i.e. the opposing, or antagonist, muscle) needs to be able to lengthen optimally to allow the biceps to contract with adequate force. If the triceps has reduced flexibility, it can affect how well the biceps can contract, AND it can affect the resting position of the shoulder and/or elbow joint, affecting movement at those joints. Flexibility is usually improved through different types of stretching (e.g. static, dynamic, etc. - stay tuned for a future blog post on the different types of stretching).
Stability refers to the body's ability to maintain optimal control of joint movement and position. In other words, muscle stability is about controlling static (i.e. stillness) and dynamic (i.e. movement) posture in order to keep the body steady, firm, and solid. Muscle stability comes about through the coordinated actions of the surrounding muscles, tissues, AND the nervous system. When a muscle has adequate stability, it can fire (or activate) at the right time, with the right amount of force, and in the proper plane of motion. And, stable muscles are more likely to keep a joint in an optimal position, reducing the risk for dysfunctional movement, pain, and injury. Muscle stability is usually improved through balance training (e.g. tree pose in yoga; BOSU ball squat; etc.) and isometric contractions (e.g. holding plank pose).
Muscular endurance refers to the ability to produce AND maintain force production for prolonged periods of time. Improving muscular endurance can make it much easier to complete activities of daily living (ADLs). When muscle endurance is low, fatigue and exhaustion set in rather quickly during any type of movement. If your endurance is poor, even walking to and from the car can feel really challenging. Muscle endurance is related to stability. If your postural and stabilization muscles (e.g. abdominal muscles, spinal muscles, rotator cuff, etc.) have reduced endurance, it will likely be very difficult to maintain proper joint position during static and dynamic postures. Developing adequate muscle endurance can increase core and joint stabilization, improving overall stability in the body and keeping joints and tissues safe from possible injury. Muscle endurance is usually improved through completing many repetitions of some type of activity (e.g. running, swimming, resistance training, etc.).
Strength is the ability of of a muscle to produce enough internal tension to overcome an external load. For example, if you have to lift your tantruming child off the floor, you need adequate strength to lift his/her body. Strength is absolutely required for performing other ADLs, such as standing up from a chair or carrying your grocery bags. Strength is not just a function of muscle tissue, but rather, it is also a function of the nervous system, since the nervous system controls what muscles contract, how forcefully, and when. Strength requires an adequate foundation of stabilization and flexibility. Muscles must have enough stability in order to efficiently generate enough tension for contraction, and antagonistic muscles must have enough flexibility to allow the opposing muscle to contract adequately (see flexibility section).
When muscles have reduced strength, it can cause the body to compensate by using other muscles to complete the task. The problem with compensation is that certain muscles are designed for certain tasks. If your body recruits the "wrong" muscle to carry out a motor plan, that "wrong" muscle is prone to injury since it is not designed to do what your body is asking it to do. For example, if your gluteal muscles are weak, your low back muscles may have to pick up the slack when you are standing or walking, causing pain or injury in the low back since your low back muscles are not designed to do the job of the glutes (click here to read more about the glutes). Muscle strength can usually be improved through resistance training, where you challenge the muscles to contract against heavy loads (e.g. free weights, cable machines, resistance bands, medicine balls, etc.). The amount of the load needed to improve muscle strength will vary person-to-person and is based on your fitness goals. Please seek the advice of a CPT or strength and fitness coach to help you determine the right load for your body and goals.
Power refers to the ability of a muscle to produce the greatest possible force in the shortest possible time. Essentially, power is "strength in action." Power requires adequate stabilization, strength, flexibility, and endurance, and it is actually a property of both muscle AND neural tissue. The muscle tissue is responsible for contracting appropriately. And the nervous system is responsible for determining how many muscle cells are activated, the synchronization between them, and the speed at which they are activated.
While power is essential for most athletes, it is also super important for the average person. Safe and efficient movement requires muscles to contract with enough strength at realistic speeds, so you can complete ADLs without injury. When neuromuscular tissue has reduced power, muscles may be slow to respond to the environment, potentially resulting in injury. For example, if you accidentally step on one of your dog's toy bones on the floor, your leg and core muscles need to have adequate power in order to quickly contract to keep you from falling and hurting yourself. Muscle power is usually improved through a combination of improving strength AND using activities with low resistance and a high velocity (e.g. medicine ball activities).
Increased muscle size, known as hypertrophy, is the enlargement of skeletal muscle fibers. Muscle hypertrophy occurs when muscle tissue is repeatedly asked to contract against increased levels of tension. The increased tension literally forces the muscle tissue to adapt by getting bigger. In contrast to the other muscle attributes discussed above, having huge muscles is not necessarily required for safe and efficient movement. Obviously, we do not want our muscles wasting away (which they will do if you do not use them), but being built like a bodybuilder is more of a personal preference than a biomechanic requirement. Muscle hypertrophy is usually achieved by lifting heavy loads with moderate-to-low repetitions. If you are interested in the cellular mechanisms that govern hypertrophy (and atrophy - muscle shrinkage), please refer to my earlier blog post on this topic (click here). If your fitness goals involve muscle hypertrophy, please seek out the advice of a CPT, as she/he can tailor your workout program to help you achieve those results.
Skeletal muscle contractions move our skeleton so that we can interact with our daily environment. Multiple attributes govern how well skeletal muscles create movement in the body. Flexibility allows muscles to extend and lengthen. Endurance allows muscles to contract for prolonged periods of time. Stabilization allows muscles to keep joints in proper alignment during static and dynamic postures. Strength allows muscles to contract against an external load. Power allows muscles to contract at realistic, functional speeds. And hypertrophy allows muscles to grow in order to continue meeting the demands placed on them. Each of these attributes are trained in different ways. Understanding the differences between these concepts helps you to better streamline your fitness program, so you can cause the appropriate adaptations for your body and fitness goals. I highly recommend that you seek the advice and services of a certified personal trainer (CPT), as CPTs are specifically trained to help you tailor your workout to best meet your wellness goals.
As always, the information presented in this blog post is derived from my own study of neuroscience, human movement, anatomy, and yoga. If you have specific questions about your muscles or fitness programming, please consult with your physician, physical therapist, personal trainer, or private yoga teacher. If you are interested in private yoga and/or personal training sessions with me, Jackie, email me at email@example.com for more information about my services. Also, please subscribe to my website so you can receive my weekly newsletters (scroll to the bottom of the page where you can submit your email address). This will help keep you "in-the-know" about my latest blog releases and other helpful yoga and wellness information. Thanks for reading!
~Namaste, Jackie Allen, M.S., M.Ed., CCC-SLP, RYT-200, RCYT, NASM-CPT
Clark, M.A. et al. (2018). NASM Essentials of Personal Fitness Training. 6th Edition. Jones & Bartlett Learning. Burlington, MA.