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Review principles for Muscle Mechanics
1. Same principles apply to contraction of a single fiber and a whole muscle
2. Contraction produces tension, the force exerted on teh load or object to be moved (muscle tension)
- 3. Contraction does not always shorten a muscle
- *Isometric Contraction: no shortening; muscle tension increases but does not exceed load
- *Isotonic Contraction: muscle shortens because muscle tension exceeds the load
4. Force and duration of contraction vary in response to stimuli of different frequencies and intensities
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Motor Unit: The Nerve-Muscle functional unit
Define
- motor unit: a motor neuron and all (four to several hundred) muscle fibers it supplies

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Motor Unit:
- -Small motor units in muscles that control fine movements (fingers, eyes)
- -Large motor units in large weight bearing muscles (thighs, hips)
-muscle fibers from a motor unit are spread throughout muscle so that a single motor unit causes a weak contraction of the entire muscle
-motor units in a muscle ususally contract asynchronously; helps prevent fatigue
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Muscle Twitch
-Response of a muscle to a single, brief threshhold stimulus
-Simplest contraction observable in the lab (recorded by a myogram)
- Three phases of a Twitch
- 1. Latent period: events of E-C coupling
- 2. Period of Contraction: cross bridge formation; tension increases
- 3. Period of Relaxation: Ca2+ reentry into the SR; tension declines to zero

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Muscle Twitch Comparisons
Different strength and duration of twitches are due to variations in metabolic properties and enzymes between muscles
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Graded Muscular Responses
- -variations in the degree of muscle contraction
- -required for proper control of skeletal movement
- Responses are graded by:
- 1. changing the frequency of the stimulation
- 2. changing the strength of the stimulation
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Treppe
- When frequency and strength are constant

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Response to change in stimulus STRENGTH
Threshold stimulus: stimulus strength at which that first observable muscle contraction occurs
-Muscle contracts more vigorously as stiimulus strength is increased above threshold
-Contraction force is precisely controlled by recruitment (multiple motor unit summation), which brings more and more muscle fibers into action
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Size Princple
Size principle: motor units with larger and larger fibers are recruited as stimulus intensity increases
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Muscle Tone
-Constant, slightly contracted state of all muscles
-Due to spinal reflexes that activate groups of motor units alternately in response to input from stretch receptors in muscles
-Keeps muscles firm, healthy, and ready to respond (helps posture and stabilize joints)
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Isotonic Contractions
-muscle changes in length and moves the load
- -Isotonic contraction are either concentric or eccentric
- 1. Concentric: the muscle shortend and does the work
- 2. Eccentric: the muscle contracts as it lengthens (like your calf muscle when you walk uphill)
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Isometric Contractions
-The load is greater than the tension the muscle is able to develop
-Tension increases to the muscle's capacity, but the muscle neither shortens nor lengthens
*object is not lifted or something is being held in position
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Muscle Metabolism: Energy for contraction
-ATP is the only source used directly for contractile activities
-Available stores of ATP are used within 4-6 seconds
- -ATP is regenerated by
- 1. Directy phosphorylation of ADP by creatine phosphate (CP)---> it gives enough ATP for 15-16 seconds and is replenished when resting
- 2. Anaerobic Pathways
- 3. Aerobic respiration
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Anaerobic Pathway
-At 70% of maximum contractile activity:
- *bulging muscles compress blood bessels
- *oxygen delivery is impaired
- * pyruvic acid is converted into lactic acid
- Lactic acid:
- *diffuses into the bloodstream
- *used as fuel by the liver, kidneys, and heart
- *converted back into pyruvic acid by the liver

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Aerobic Pathway
-Produces 95% of ATP during rest and light to moderate exercise
-Fuels: stored glycogen, then bloodstream glucose, pyruvic acid, and free fatty acuds
-occurs int he mitochondria
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Muscle Fatigue
-Physiological inability to contract
- Occurs when:
- *Ionic imbalances (K+, Ca2+, etc) interfere with E-C coupling
- *Prolonged exercise damages the SR and interferes with Ca2+ regulation and release
-Total lack of ATP is rare, during states of continuous contraction, and causes contractures (continuous contractions) like ragamortis
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Oxygen Deficit
Extra O2 needed after exercise for:
- -The replenishment of:
- * oxygen reserves
- * glycogen stores
- *ATP and CP reserves
-Conversion of lactic acid to pyruvic acid, glucose, and glycogen
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heat production during Muscle Activity
About 40% of the energy released in muscle activity is useful as work
The other 60% is given off as heat
-dangerous heat levels are prevented by radiation of heat from the skin and sweating
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Force of Muscle Contraction
The force of the conraction is affected by:
* number of muscle fibers stimulated (Recruitment)
* Relative size of the fibers --hypertrophy of cells increases strength (increase size by regular resistence exercise)
*F requency of Stimulation- higher frequency allows time for more effective transfer of tension to noncontractile components (summation of contractions)
- *Length-Tension Relationship: muscles contract most strongly when muscle fibers are 80-120% of their normal resting length (when cross bridges can happen)
 
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VELOCITY and Duration of the contraction
-How fast they can contract and how long it takes for it to fatigue
- Influenced by:
- 1. Muscle fiber type
- 2. Load
- 3. Recruitment
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Muscle Fiber Types
Classified according to 2 different characteristics:
- 1. Speed of contraction: slow or fast, according to
- *speed at which myosin ATPases split ATP
- *Patter of electrical activity of the motor neurons
- 2. Metabolic oathways for ATP synthesis:
- *oxidative fibers--use aerobic pathways
- *Glycolytic fibers--use anaerbic glycolysis
- Split into 3 types:
- 1. Slow oxidative fibers (SO)
- 2. Fast oxidative fibers (FO)
- 3. Fast Glycolytic fibers (FG)
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Influence of Load
larger the load = the larger the latent period, smaller the contraction, and shorter duration of contration
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Influence of recruitment
Recruitment--> faster contraction and longer duration of contraction
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Effects of AEROBIC exercise
Endurance exercise
- leads to increased:
- 1. Muscle capillaries
- 2. number of mitochondria
- 3. Myoglobin synthesis
results in greater endurance, strength, and resistance to fatigue
May convert fast glycolytic fibers into fast oxidative fibers
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Effects of Resistance Exercise
Typically Anaerobic
- Results in:
- 1. Hypertrophy (due to increase in fiber size)
- 2. Increased mitochondria, myofilaments, glycogen stores, and connective tissue
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The Overload principle
Forcing a muscle to work hard promotes increased muscle strength and endurance
Muscles adapt to increasing demands
Muscles can be overloaded to produce further gains
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