1. Exercise and fatigue
Chapter 9
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2. Contraction types
Isometric contraction – maintain same length in presence of load
Example: arm wrestling, yoga plank pose
Isotonic contraction – maintain same force while contracting
Example: arm curl with fixed weight
Lengthening contraction – muscle provides resistance while load extends muscle
Example: quadriceps during knee bend
Isometric tension time course
Brief latent period for AP to initiate the isometric contraction
Tension rises and falls with Ca2+ levels
Tension due to contracted myosin cross-bridges and stretched actin filaments

3. Single Muscle Fiber Isometric and Isotonic Twitch Contraction

4. Muscle length and tension
Isotonic contraction time course
Tension rises and falls with Ca2+ levels
Muscle cannot shorten until tension exceeds load
Muscle length time course for isotonic contraction
Fastest shortening occurs when tension >> load
Isometric when tension = load
Lengthening when tension < load
Shortening does not begin until enough cross bridges
have been formed and the muscle tension just exceeds
the load on the fiber, also eventually a load is reached
that the fiber is unable to lift

5. Isotonic Twitch Contractions with Different Loads

6. Velocity of skeletal muscle shortening and lengthening

7. Muscle length and tension
Muscle tension changes with length
Tension depends on overlap of actin and myosin filaments
Relaxed muscle fibers typically have optimal length for force generation
Tension drops off when muscles contract or when they are stretched by external load

8. Variation in muscle tension at different lengths of muscle fiber

9. At what length do skeletal muscles generate the most force?
Fully contracted
Partially contracted
Relaxed
Partially stretched
Fully stretched

10. Frequency and tension
Contraction in response to single AP is called a twitch
Nervous system controls contraction intensity and duration via repetitive APs
Repetitive APs will produce individual twitches at low frequency (< 5 Hz)
Repetitive APs > 10 Hz prevent complete recovery of normal Ca2+ levels, leads to constant contraction known as tetanus
Higher frequency APs increase level of basal contraction
AP frequency controls contraction strength
Fused tetanus – APs at such high frequency that Ca2+ levels never fall below level that saturates troponin

11. Isometric contractions produced by multiple stimuli

12. How does the nervous system control the tension produced by a single muscle fiber?
Alter level of muscle [Ca2+]i during single AP
Alter level of muscle [Ca2+]i with multiple APs
All of the above
None of the above

13. Metabolism and fatigue
Most tissues that have a relatively constant rate of ATP consumption
Rate of ATP consumption in muscle drastically increases during contraction
Muscle tissue has specialized mechanisms for ATP production to account for variable consumption rate
Also requires constant ATP production during sustained activity
Muscle fatigue occurs when ATP production and reserves cannot sustain consumption

14. The three sources of ATP production during muscle contraction:
1) Creatine phosphate 2) oxidatgive phosphorylation 3) glycolysis

15. Sources of ATP in muscle
Creatine Phosphate (CP) is initial source of ATP during contraction
ATP subsequently produced by glycolysis and oxidative phosphorylation of glucose
Glycolysis becomes more important during intense activity, rapid but inefficient!
Glucose comes from glycogen stored in muscle or from blood
Fatty acids are metabolized during sustained exercise
CP and glycogen levels must be restored following activity
Requires ATP energy from oxidative phosphorylation
Oxygen debt – sustained hyperventilation after cessation of activity to provide O2 for restoring CP and glycogen

16. Fatigue
Involuntary decline and cessation of muscle contraction during tetanic stimulation is known as fatigue
Muscle must recover during quiescent period before subsequent stimuli can elicit contraction
High-frequency fatigue due to brief intense activity (fast recovery)
Conduction failure from buildup of extracellular K+ in transverse tubules, effect is to depolarize membrane potential
Lactic acid buildup following glycolysis lowers pH
Elevated ADP prevents dissociation of ADP from myosin
Low-frequency fatigue due to sustained moderate activity and decrease in energy sources (slow recovery)
Hypoglycemia or low blood glucose
Reduced glycogen in muscles
Brain may sense hypoglycemia and prevent presynaptic APs

17. Muscle fatigue during a maintained isometric tetanus

18. Which of the following is NOT responsible for muscle fatigue?
ADP increase inhibits cross bridge cycling
Conduction failure due to elevated [K+]o
Sudden decrease in [Ca2+]i levels
Decreased pH due to lactic acid buildup

19. Types of skeletal muscle fibers
Classified by major source of ATP
Oxidative fibers consume high levels of O2
Tend to be smaller to improve O2 diffusion
Surrounded by many blood vessels
Contain many mitochondria
Contain O2 carrier known as myoglobin that gives red color
Fatigue resistant due to low glycogen consumption and low lactic acid production
Glycolytic muscle fibers
Don’t need high O2, can be larger to generate more force
Also require more glycogen storage, white muscle fibers
Fatigue rapidly due to high glycogen consumption and high lactic acid production

20. Types of skeletal muscle fibers
Also classified by contraction velocity
Fast vs. slow contraction determined by rate at which myosin ATPase hydrolyzes ATP to during contraction cycling
Fast fibers generate force faster for bursts of activity
Slow fibers needed for sustained activity
Four possible combinations, slow-oxidative, fast-oxidative-glycolytic, fast-glycol
Slow -glycolytic fibers not found

21. Rate of fatigue development in the 3 types of muscle fibers

22. Fiber recruitment Whole muscles consist of a mix of fiber types
Control tension and velocity through order in which fiber types begin contracting
Process known as fiber recruitment
Thinner oxidative fibers are recruited first
Larger glycolytic fibers are recruited later
Fiber size determines response to presynaptic APs and postsynaptic AChR activation
Therefore a larger fiber may require higher frequency of presynaptic APs to be recruited

23. Recruitment of Muscle Fibers

24. Fiber recruitment
Recruitment provides a large range of forces and shortening velocities that muscles can produce
Tension is controlled by the number of contracting myofibrils acting in parallel
Remember forces in parallel add
Velocity is controlled by fast/slow fiber type
Velocity also controlled with total force generated by muscle relative to applied load
This is how muscles can produce motion or force that ranges from delicate to powerful

25. Which muscle fibers are recruited first by the nervous system during muscle contraction?
Fast glycolytic
Fast oxidative
Slow glycolytic
Slow oxidative

26. Adaptation to exercise
Muscle fibers can change size after disuse or excessive use
Atrophy – reduction in muscle fiber size
Hypertrophy – increase in muscle fiber size
Number of muscle fibers stays relative constant through adult life
Muscles can also change their capacity for ATP production
Repetitive exercise of high intensity short-duration exercise hypertrophies fast glycolytic muscle and increases glycolytic enzymes
Increase in glycolytic enzymes also occurs in oxidative fibers and can convert these to glycolytic
This leads to big, bulky muscles in body builders

27. Adaptation to exercise
Low intensity sustained exercise (aerobic exercise) increases mitochondria and blood vessels in oxidative fibers
These fibers may even atrophy slightly to reduce ATP consumption and increase resistance to fatigue
Can reduce glycolytic enzymes to convert some glycolytic fibers into oxidative fibers
This leads to trim, toned muscles in aerobic athletes
Not clear what signals initiate these changes in muscle
Anabolic steroids accelerate myofibril growth in response to exercise by increasing rate of protein synthesis
Training regimens have gotten very specialized to increase strength and endurance where needed
Muscle soreness is inflammatory response due to myofibril damage
Excessive stress or repetitive lengthening contractions