Exercise and fatigue

1. Exercise and fatigue Chapter 9 pages 263 - 277

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