Skeletal Muscle Mechanics Dr. Abdelrahman Mustafa LECTUERER, Physiology Department Basic Medical Sciences Division of Physiology Faculty of Medicine Almaarefa Colleges

Learning Objecetives By the end of this lecture you should be able to Define the muscle tension List the factors that affect the muscle tension Define the muscle twitch List Phases of Twitch Define Twitch Summation and Tetanus Identify Motor Unit Recruitment Identify Lever Systems List the types of Contraction Identify Muscle fatigue

Muscle Mechanics Muscle mechanics include. Coordination of skeletal muscle. Muscle Tension and Muscle twitch Motor Unit Recruitment Performance of skeletal muscle. Lever Systems Optimal Length and Types of Contraction Muscle Fatigue Metabolism of skeletal muscle Source of energy Oxygen Debt

Skeletal Muscle Mechanics Muscle consists of groups of muscle fibers bundled together and attached to bones Connective tissue covering muscle divides muscle internally into bundles Connective tissue extends beyond ends of muscle to form tendons – Tendons attach muscle to bone

Muscle Tension Definition: The pulling force on the tendons that muscle cells generate when contracting Production :within the sarcomere (contractile component),as a result of cross-bridge cycling resulting.in sarcomere shortening. Transmission: to move the bones, the tension generated by these contractile component must be transmitted to the bones via the connective tissue and tendons. Connective tissue + tendons = series elastic components

Muscle Tension Factors influencing extent to which tension can be developed – Frequency of stimulation – Length of fiber at onset of contraction – Extent of fatigue – Thickness of fiber

Muscle Twitch Definition :A brief contraction-relaxation response to a single action potential Phases of Twitch 1.Latent period before contraction: – the action potential moves through sarcolemma – causing Ca 2+ release 2.Contraction phase: – calcium ions bind – tension builds to peak 3.Relaxation phase: – Ca 2+ levels fall – active sites are covered – tension falls to resting levels

Muscle Twitch

Twitch Summation and Tetanus Single action potential in muscle fiber produces twitch. Which is very weak & brief. Contraction of longer duration and greater tension can be produces by repeated stimulation of fiber. Two twitches from two action potential add together, or sum, to produce greater tension in the fiber. Twitch summation is possible because the duration of action potential is much shorter than duration of twitch(contraction)

Summation and Tetanus

Twitch Summation and Tetanus Tetanus – Occurs if muscle fiber is stimulated so rapidly that it does not have a chance to relax between stimuli, – Definition: smooth sustained contraction of maximum strength occurs known as tetanus.

Motor Unit Recruitment Definition :the successive activation of the same and additional motor units with increasing strength of voluntary muscle contraction. Properties 1.Greater the number of fibers contracting greater the total muscle tension 2.The number of muscle fibers contracting within a muscle depends on the extent of motor unit recruitment. 3.The pattern of motor unit recruitment varies, depending on the inherent properties of specific motor neurons.

Motor Unit Recruitment – Muscles that produce precise, delicate movements contain fewer fibers per motor unit e.g. external muscles of eye – Muscles performing powerful, coarsely controlled movement have larger number of fibers per motor unit e.g. muscles of thigh One motor neuron innervates number of muscle fibers but each muscle fiber is innervated by only one motor neuron

Motor Unit Recruitment

Lever Systems A lever is a rigid structure that can move around a fixed point called a Fulcrum. A lever is acted on at two different points by two different forces: – The Effort is the force due to muscular contraction ( which causes movement) – The load or (resistance) is the weight that is moved or some resistance an object to being moved ( which opposes movement).

Types of levers There are 3 types of levers that differ on the positions of the fulcrum, effort, and load. – First-class levers are not common: the fulcrum is between the effort and the load. – Second-class levers are uncommon: the load is between the fulcrum and the effort. – Third-class levers are common: the effort is between the fulcrum and the load.

Types of levers

Types of Contraction Two primary types – Isotonic Muscle tension remains constant as muscle changes length – Isometric Muscle is prevented from shortening Tension develops at constant muscle length

CONTRACTION Isometric contraction: occurs when muscle contracts without shortening in length. (iso= same, metric= measure or length) Isotonic contraction: occurs when muscle contracts with shortening of length but against a constant load, thus, the tension on the muscle remains constant (iso= same, tonic= tension)

Physiologic basis of Isometric & Isotonic contractions: The same internal events occur in both isotonic and isometric contractions: Muscle excitation starts the sliding filament cycling; the cross bridges start cycling; and filament sliding shortens the sarcomeres, which exert force on the bone at the site of the muscle’s insertion. During a given time, a muscle may shift between isotonic & isometric contractions. E.g. when you lift a book up it is isotonic contraction and when you keep holding the book up while reading it is isometric contraction. NOTE: Since Work=Distance X Load, Isotonic contractions do work where as Isometric do not.

MUSCLE FATIGUE Definition: Fatigue occurs when prolonged & strong stimulation of an exercising muscle reaches a stage when the muscle is no longer able to respond to the stimulation with the same degree of contractile activity. Is of 2 main types: 1.Muscle fatigue: occurs in the muscle & is a defense mechanism that protects the muscle by preventing it from reaching a point where no ATP will be available. 2.Central fatigue: more psychological. Occurs when CNS no longer activates the motor neurons supplying the muscles. Person stops exercising even though the muscles can still perform.

MUSCLE FATIGUE CAUSES: 1.Depletion of Glycogen energy stores. 2.Accumulation of Hydrogen ions from lactic acid- interfere with cross- bridge functions. 3.Intracellular acidosis from lactic acid inhibits glycolysis enzymes & slows ATP production. 4.NT depletion at the NMJ. 5.Central fatigue- lack of will & sleep. 6.Accumulation of extracellular K +

Q1 Which is of the following factors DOSENOT influencing tension: A)Frequency of stimulation B)Length of fiber at onset of contraction C)Extent of fatigue D)Type of muscle contraction

Q2 Which of the following muscles has larger number of fibers per motor unit A)Muscles of the eye B)Muscles of thigh C)Muscles of the mouth D)Muscles of the arm

Q3 A lever system is: A)A rigid structure that can move around a fixed point B) The pulling force on the tendons that muscle cells generate when contracting C) A brief contraction-relaxation response to a single action potential D) the successive activation of the same and additional motor units with increasing strength of voluntary muscle contraction.

Q4 Physiological inability of the muscle to contract is the definition of : A)Muscle tension B)Muscle fatigue C)Muscle twitch D)Muscle tetanus

References Human physiology by Lauralee Sherwood, 7 th edition Text book physiology by Guyton &Hall,12 th edition Text book of physiology by Linda.s contanzo,third edition 27