Human Movement Systems: Muscular System

Introduction In this lesson you will learn about: • The Muscular System • Types of muscle tissue • Skeletal muscle structure • Muscle contraction

• Skeletal Muscle fiber types • Principles of force generation • Muscle soreness

The Muscular System

 Muscles generate internal tension that, under the control of the nervous system, manipulates the bones of our body to produce movement.

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Muscular System  Three types of muscle tissue: – Skeletal • Attaches to the skeleton via tendons, contracts to move bones

• Voluntary • Striated appearance

– Smooth • Found on the walls of hollow organs and tubes (e.g., stomach and blood vessels)

• Involuntary • Smooth appearance

– Cardiac • Forms the walls of the heart • Involuntary (controlled by autonomic Nervous system) • Smooth appearance and intercalated discs

Structure of Skeletal Muscle  Muscle is the compilation of many individual muscle fibers neatly wrapped together with connective tissue to form bundles:  Each layer of connective tissue extends the length of the muscle, helping to form the tendon. – Fascia: Outer-most layer of connective tissue surrounding the entire muscle. – Epimysium: Layer of connective tissue under fascia immediately surrounding the muscle. – Perimysium: Surrounds bundle of muscle fibers called fascicles. – Endomysium: Surrounds each muscle fiber (individual muscle cells)

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Muscle Fibers  Contain typical cell components: •

Cellular plasma called sarcoplasm (contains glycogen, fats, minerals, and oxygen-binding myoglobin)

•

Nuclei

•

Mitochondria (transform energy from food into energy for the cell)

 Unlike typical cells, they also have structures called myofibrils.

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Contractile Elements  Sarcomere: The contractile unit of the muscle.  Myofibrils: actual contractile components of muscle tissue: – Actin: thin filaments – Myosin: thick filaments

 Lines and bands define parts of sarcomere: – Sarcomere is defined as Z-line to Z-line.

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Muscle contraction  Muscle contraction – A contraction occurs when an electronic impulse is transmitted from the brain to the muscle. – Muscle contraction is the result of the interaction of the actin and myosin filaments, which causes a shortening of the individual sarcomeres, and therefore, a shortening of their associated muscle fibers.

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Neuromuscular anatomy  Motor unit – A motor nerve and all its associated muscle fibers – All fibers comprising a motor unit are homogeneous (they are either all fast-twitch or all slow-twitch). – Motor units made up of 5–10 fibers are responsible for fine, delicate movements such as blinking the eye.

– Motor units made up of thousands of fibers are responsible for forceful movements such as jumping.

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Motor Unit Recruitment Patterns  All-or-none principle: Motor units give an all-or-none response  Size principle: The order of motor unit recruitment is directly related to the motor neuron size  Principle of orderly recruitment: Motor units are activated on the basis of a fixed order – type I → type IIa → type IIx

 Asynchronic vs Synchronic: Synchronizing more motor units and thus more muscle fibers produces more force

Sliding Filament Model The process of how the contraction of the filaments within the sarcomere takes place:  A sarcomere shortens as a result of the Z lines moving closer together.  The Z lines converge as the result of myosin heads attaching to the actin filament and asynchronously pulling (power strokes) the actin filament across the myosin.

Skeletal Muscle Fiber Types  Type I: Slow Twitch – – – –

Higher in capillaries, mitochondria, and myoglobin Increased oxygen delivery and slow to fatigue Smaller motor units and fiber size tend to produce less force Tend to increase capillaries, mitochondria and myoglobin rather than hypertrophy of fibers. – Long-term contractions (utilized primarily for posture & stabilization)

 Type IIx: Fast Twitch – Lower in capillaries, mitochondria, and myoglobin – Decreased oxygen delivery and quick to fatigue – Larger motor units & fiber size tend to produce more force: Contain more myosin cross-bridges per cross-sectional area of fiber and produce 10–20% more force than slow-twitch muscle fibers – Tend to hypertrophy more readily than Type I – Short-term contractions (force and power)

Muscle-fiber Types: Intermediate

 Type IIa-Intermediate muscle fibers – Have a high capacity for both fast anaerobic and slow aerobic movements

– Are more efficient at using oxygen to generate ATP to fuel continuous muscle contractions due to their higher concentrations of myoglobin, larger number of capillaries, and higher mitochondrial enzyme activity. – Adaptable based upon the training stimulus.

A Photomicrograph Showing Type I, Type IIa, and Type IIx Muscle Fibers

Type I (black), type IIa (white), and type IIx (gray) muscle fibers

Muscle-fiber Distribution

Muscle-fiber distribution is largely determined by genetics. • Most people have about equal percentages of FT and ST fibers. • Persons better at low-intensity endurance activities may have a larger percentage of ST fibers. • Persons better at high-intensity, sudden bursts of activity probably have a larger percentage of FT fibers. • Intermediate fibers can be trained to do either.

Muscle-fiber Response to Training  All three muscle-fiber types are highly trainable. – Adapt to the specific demand placed on them – Muscle-fiber types are recruited sequentially in response to force generation: ST then FT

– Increasing muscular strength assists endurance performance. – Increasing muscular endurance may interfere with strength/power performance. – Training endurance and strength separately appears to lesson interference (e.g. work endurance for legs the same day as upper body strength training and visa versa)

Muscle Force  The amount of force generated during a muscle group’s contraction depends on the following: – The size of the individual muscle fiber (the larger the fiber, the greater the force during contraction)

– The number of muscle fibers recruited (more fibers equal more force) – The length of the muscle fiber prior to contraction (a muscle generates maximum force when it begins its contraction at 1.2 times its resting length) – The speed of contraction (the slower the movement, the more force that is produced) 17

Chronic Adaptations: Resistance training  Neural adaptations – Improved motor unit recruitment patterns – Improved motor learning – Neural adaptations are predominant for first 8-10 weeks) with little or no change in muscle cross-sectional area  Hypertrophy of fast-twitch fibers predominates after 10 weeks of training  Increased size and number of actin and myosin filaments  Increased lean body mass  Increased connective tissue strength  Decreased risk of joint injury  Increased bone mineral density 18

Factors promoting muscle hypertrophy  Mechanical tension – increased intensity represents overload.

 Muscle Damage – inflammatory response stimulates satellite cell growth process.

 Metabolic stress – Anaerobic by-products stimulate hormonal factors leading to hypertrophy (e.g. H+, lactate, Pi).

– Could explain how body builder’s high volume workouts initiate growth. Source: Bubbico, A. & Kravitz, L. (2011). Muscle hypertrophy: new insights and training recommendations. IDEA Fitness Journal. 8 (10): 23-26

Muscle hypertrophy: before & after resistance training

Other research related to muscle hypertrophy  Upper vs. lower body growth rates – Upper body appears to hypertrophy faster than lower body.

 Ideal intensity for hypertrophy – 80-95% 1RM

 Difference in strength/power training vs body building training – Strength/Power have more hypertrophy in Type II fibers – Body builders have hypertrophy in both Type I & II fibers

 Single or multi-joint exercises better for growth – Multi-joint increase anabolic hormone Source: Bubbico, A. & Kravitz, L. (2011). Muscle hypertrophy: new insights and training recommendations. IDEA Fitness Journal. 8 (10): 23-26

Muscle Soreness There are generally two types of muscle soreness  Acute Muscle Soreness: – Due to metabolite build-up in muscles. – Causes “stiffness” soon after exercise bout but goes away quickly.

– Reduce this type of soreness with cool-down and post exercise stretching.

 Delayed onset muscle soreness – Peaks 24-48 hours after starting an exercise program, progressing too quickly, or doing something new. – Exacerbated by eccentric contractions and overstretching. – It does go away in a few days (about a week). – Attempt to reduce DOMS by starting at a low intensity and progressing slowly through the first few weeks while minimizing eccentric actions. 22

Summary of Muscle System    

Skeletal muscles produce force for movement and stabilization. The contractile unit of the muscle is the sarcomere. The sliding filament model explains how muscles produce force. The motor unit is an important concept which explains how muscle fibers work together to produce smooth and controlled movements.  Changes in force production and muscular endurance can be explained by muscle fiber types (I, IIx, IIa), motor unit recruitment patterns (asynchronic vs synchronic) and changes to muscle structure (hypertrophy and hyperplasia).  A variety of factors contribute to muscle hypertrophy.  There are two types of muscle soreness-acute and DOMS

Endocrine System System of glands that secrete hormones that control bodily function: • Consists of host organs, chemical messengers, target cells. • Target cells bind specifically to hormones.

• Regulates body functions (growth, metabolism, and response to stress).

Endocrine Glands Primary glands of the endocrine system include:  Pituitary “master” gland  Hypothalamus  Thyroid gland

 Adrenal gland

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Endocrine Glands Pituitary gland: Master control gland. Secretes the following hormones related to exercise: – Growth Hormone (GH)- major anabolic agent – Adrenocorticotropic hormone (ACH)-stimulates adrenal glands – Thyroid-stimulating hormone (TSH) -stimulates thyroid

– Follicle-stimulating hormone (FSH) and Luteinizing hormone (LH)-stimulates sex organs.

Endocrine Glands  Thyroid gland: Regulates metabolism.  Adrenal glands: – Epinephrine (a.k.a. adrenaline) and Norepinephrine. “Fightor-Flight” hormone. Epinephrine is released during exercise, which increases heart rate, elevates blood glucose, and opens airways.

– Cortisol produced in adrenal is main catabolic and antiinflammatory agent. – Testosterone is produced in adrenal glands and testes; men produce 10 times more than women. Major anabolic agent. – Estrogen is produced in adrenal glands and ovaries. Women produce significantly more than men.

Blood Glucose Control  Control of blood glucose levels regulated by the pancreas to prevent wide swings in blood glucose levels.  Insulin: Brings glucose into cells from blood stream, resulting in net drop in blood sugar levels.  Glucagon: Signals the liver and muscles to break down and release glycogen stores; results in net rise of blood sugar levels.

 Exercise improves body’s utilization of glucose.

Summary of NASM Chapter 2  The three components of the kinetic chain all work together to produce movement.  The nervous system and endocrine systems represent controlling factors.  The nervous is composed of billions of neurons that transfer information throughout the body, through two interdependent systems: the CNS and the PNS.  The skeletal system is the body’s framework and is made up of bones and joints in two divisions: axial and appendicular.  The muscular system is made up of many individual fibers attached to bones by way of the tendons. Muscles generate force through neural activation, the sliding filament model, and excitation–contraction coupling.