Massage Therapy Journal Spring 2024

Spring 2024 • 39

Muscles in the Human Body: Skeletal Muscles

this allows these types of muscles to sustain contractions for long periods of time without fatiguing quickly. They are well-suited for endurance activities, such as long-distance running or swimming, which require lower effort but sustained performance. Fast-twitch or type-2 muscle fibers are larger in size and have less myoglobin content with fewer mitochondria in comparison to slow-twitch fibers. These muscle fibers contract rapidly, generating a higher force in a shorter amount of time. Fast-twitch muscle fibers generally rely on anaerobic metabolism (without the use of oxygen) to produce ATP, or energy. This allows these muscle fibers to generate rapid bursts of power but leads to quicker fatigue since oxygen is not being used. Most muscles in the body contain a mix of both slow-twitch and fast-twitch fibers. The proportion can vary based on genetics, training and the function of specific muscle groups. Athletic performance and training adaptations can be influenced by the types of muscle fibers predominant in a person’s body. Postural muscles act mainly to sustain the posture of the body. Due to the endurance required for this activity, these muscles contain mainly slow-twitch muscle fibers that have a greater capacity for constant work. As such, they may be prone to hypertonicity. Phasic muscles are designed to move the body through space and contain mostly fast twitch muscle fibers. Phasic muscles are prone to inhibition and can fatigue more easily than postural muscles. Postural muscles tend to shorten in response to overuse, underuse or trauma, whereas phasic muscles tend to lengthen and weaken in response to these sorts of stimuli. These effects can lead to musculoskeletal imbalance and joint instability when postural and phasic muscles are located on opposing sides of the agonist/antagonist relationship. These relationships can be key to understanding common patterns of postural imbalance, such as upper and lower crossed syndromes.

Skeletal muscle is one of three types of muscle tissue, along with cardiac muscle and smooth muscle, and skeletal muscle tissue is what we rely on to move joints, maintain posture and provide stability. It is composed of individual muscle fibers that are under our voluntary control and can run as long as the length of an entire muscle. Skeletal muscle is able to perform three distinct types of contractions. • The first is an isometric contraction , which is where the muscle contracts without shortening or lengthening. • The other two are both forms of isotonic contractions, meaning the muscle contracting is either shortening or becoming longer. During an isotonic concentric contraction, the muscle contracts and shortens. Conversely, during an isotonic eccentric contraction, the muscle contracts but also lengthens. This type of contraction typically fatigues the muscle quicker and is more common during a muscle strain. Cardiac muscle , as its name implies, is found only in the heart. Smooth muscle is found in places such as the intestines, pupils, skin and blood vessels. The fiber composition of our skeletal muscles is divided into two types that play different roles in physical activities. Commonly known as fast twitch and slow-twitch muscle fibers, they differ in their construction, purpose and their way of utilizing energy, all of which in turn dictates their performance capabilities. Slow-twitch muscle fibers , also known as type-1 muscle fibers, are categorized by their smaller size. They have a higher content of myoglobin, which is a red pigmented protein that binds to oxygen, similar to hemoglobin in the red blood cells. Slow-twitch muscle fibers also have numerous mitochondria, which are the structures inside cells that are responsible for energy production. Slow-twitch fibers contract slower compared to fast-twitch fibers. Slow-twitch muscle fibers utilize aerobic metabolism (the use of oxygen) to produce adenosine triphosphate (ATP), or energy, and

Postural Muscles Levator scapula Upper trapezius Suboccipitals Scalenes Sternocleidomastoid Biceps brachii Pectoralis major Pectoralis minor Latissimus dorsi Erector spinae Quadratus lumborum Hamstrings Adductor group Tensor fasciae latae Piriformis Iliopsoas Rectus femoris Soleus Gastrocnemius Phasic Muscles Longus capitis and colli Middle trapezius Lower trapezius Serratus anterior Triceps Deltoids Rectus abdominis Transverse abdominis

Internal oblique External oblique Gluteus maximus Gluteus medius Gluteus minimus

Vastus lateralis Vastus medialis Fibularis longus and brevis Tibialis anterior

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