www.healthiestwell.com

Muscular Strength and Muscular Endurance

INTRODUCTION

The development of muscular strength and muscular endurance through physical training is significant to overall muscle fitness and function in performance. Muscular strength can be defined as the maximal exertion of force against resistance, whereas muscular endurance can be described as the submaximal repetitive or sustained exertion of force against resistance. Both components are developed during conditioning processes in an interrelated manner relative to the number of repetitions and the load lifted.

MUSCLE STRUCTURE AND MUSCLE CONTRACTION

Skeletal muscle is basically composed of muscle fibers and myofibrils. The muscle fibers or cells are individually enclosed in layers of endomysium connective tissue, bound together in bundles of fasciculi, and covered by perimysium tissue. Muscle fasciculi are then combined to form specific muscle tissue and are held together by epimysium and fascia band coverings, respectively (Figure 4.1).

Muscle fibers contain cytoplasm or sarcoplasm within which are found mitochondria and myofibrils covered by a plasma membrane or sarcolemma. A membranous system of channels made up of sarcoplasmic reticuli or storage areas and transverse tubules or pathways, cross through the cellular sarcoplasm. The myofibrils are composed of actin and myosin, light and dark protein filaments. These filaments are arranged in units of sarcomeres bound by Z lines surrounding I and A, light and dark band filament sections. During muscle contraction, the Z-lined sections move toward the center and shorten, the I bands narrow, and the A bands remain the same, relative to length (Figure 4.2).⁴

Muscle contraction processes begin with the release of acetylcholine from the motor end plate of a motor neuron due to electrical and chemical stimulus depolarization (Figure 4.3).³⁹ In turn, muscle cell membrane depolarization occurs, and the acetylcholine is conducted through the transverse tubules, generating action potential impulses throughout the fibers innervated. Calcium ions are then secreted from the sarcoplasmic reticulum and attach themselves to the tropomyosin-troponin complex (protein process governing substances).²⁵ This process results in a conformational change in the position of actin, allowing the myosin globular heads to bind to these sites (Figure 4.4). These globular heads contain myosin adenosine triphos

Epimysium Layers Cover Muscle Tissue

Perimysium Layers Bind

Bundles of Muscle Fibers Endomysium Layers Enclose Muscle Fibers

Muscle Fibers Myofibrils Actin/Myosin Filaments

FIGURE 4.1 Muscle structure.

phatase (ATPase) enzymes which are activated upon binding. At this point, adenosine triphosphate (ATP) splits and energy is released. Muscle contraction occurs when the myosin globular arms or heads pull the actin filaments toward the center in a ratchet action.

MUSCLE FIBER TYPES

There are basically two muscle fiber types, slow twitch and fast twitch. Twitch refers to contraction response to stimuli. Type I (slow oxidative) fibers are high in aerobic capacity, slow in contraction speed, high in fatigue resistance, and low in force production. Type II A and Type II B fibers, two fast-twitch subtypes (fast oxidative/glycolytic and fast glycolytic), are high in anaerobic capacity, fast in contraction speed, low in fatigue resistance, and high in force production. In reference to resistance training, slow-twitch fibers, because of their low force production, would be used for low-to-moderate-resistance, high-repetitive muscle endurance activities, whereas fast-twitch fibers, because of their high force production, would be used for high-resistance, low-repetitive muscle strength activities (Figure 4.5).²²

MUSCLE STRENGTH AND MUSCLE ENDURANCE DETERMINANTS

Muscular strength and muscular endurance development can basically be determined by muscle size and cross section, muscle length and angle of pull, and muscle contraction speed and force production. There is a significant relationship between muscle size and cross section, and the generation of muscle strength.^3,19 The larger the size and cross section, the greater the force produced. This relationship is

Z Line

attributed to the hypertrophy of the myofibrils in size and number, which activates a greater amount of actin-myosin cross bridges.^{14,17,26,32,34,42}

Force production can be shown as an interaction between muscle length and angle of pull. While the greatest force production occurs with muscle tissue at extended length, the greatest force production relative to angle of pull is set at a range of 80° to 120°.^17,29,35 The equal interaction of, or predominance of either length or angle of pull in training must be determined by the type of activity or movement pattern to be executed. The magnitude of actin-myosin cross bridge activation would be the predominant factor, because the production of force would be related to the increasing or decreasing site numbers activated during shortening and lengthening patterns of movement (Figure 4.6).^21-22,29,39

Muscle contraction speed and force production are based on resistance load.^5,13 As resistance increases, muscle contraction speed slows. Similarly, as resistance decreases, muscle contraction speed increases. The variance in load is the principal

Relaxation Contraction

muscle (1) membranemembrane excitation

transverse sarcoplasmictubule reticulum

Ca2+ Ca2+

(5)

Ca2+ uptake ADP + Pi ATP

(2) Ca2+ release Ca2+

removal of Ca2+ (3) Ca2+ binds to troponinfrom troponin restoresremoving blocking actionblocking action of tropomyosin or tropomyosin

troponin tropomyosin

actin thin filament

(4) cross-bridge myosinATP movement thick filament

FIGURE 4.3 Diagram of neural excitation and muscle contraction. (From Vander, A.J., J.H. Sherman, and D.S. Luciano, Human Physiology: The Mechanisms of Body Function, 3rd ed., McGraw-Hill, New York, 1980. Reproduced with the permission of the McGraw-Hill Companies.)

MUSCLE STRENGTH TRAINING Resistance -High Repetitions -Low

MUSCLE ENDURANCE TRAINING Resistance - Low to Medium Repetitions -High

Visit & Buy from: http://www.drugswell.com/wow/index.php

Pages: 1 · 2 · 3 · 4