Exercise and fatigue

Chapter 9

pages 263 - 277

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

Single Muscle Fiber Isometric and Isotonic Twitch Contraction

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

Isotonic Twitch Contractions with Different Loads

Velocity of skeletal muscle shortening and lengthening

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

Variation in muscle tension at different lengths of muscle fiber

At what length do skeletal muscles generate the most force?

1 2 3 4 5

20% 20% 20%20%20%1. Fully contracted

2. Partially contracted

3. Relaxed

4. Partially stretched

5. Fully stretched

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

Isometric contractions produced by multiple stimuli

How does the nervous system control the tension produced by a single muscle fiber?

1 2 3 4

25% 25%25%25%1. Alter level of muscle [Ca2+]i during single AP

2. Alter level of muscle [Ca2+]i with multiple APs

3. All of the above

4. None of the above

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

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

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

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

Muscle fatigue during a maintained isometric tetanus

Which of the following is NOT responsible for muscle fatigue?

1 2 3 4

25% 25%25%25%1. ADP increase inhibits cross bridge cycling

2. Conduction failure due to elevated [K+]o

3. Sudden decrease in [Ca2+]i levels

4. Decreased pH due to lactic acid buildup

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

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

Rate of fatigue development in the 3 types of muscle fibers

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

Recruitment of Muscle Fibers

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

Which muscle fibers are recruited first by the nervous system during muscle contraction?

1 2 3 4

25% 25%25%25%1. Fast glycolytic

2. Fast oxidative

3. Slow glycolytic

4. Slow oxidative

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

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