Chapter 19 Sports Physiology
Although 60% of ATP production is derived from aerobic processes in a 3 minute maximal effort, the value jumps to....
-90% in a 20 minute all out performance -Given this dependence on oxidative energy production, the factors limiting performance include both the cardiovascular system, which delivers oxygen rich blood to the muscles, and the mitochondrial content of the muscles involved in the activity
Although H+ is implicated in this longer relaxation time, the accumulation of inorganic phosphate (Pi) from the breakdown of ATP is...
-Also involved
Short term performances
-Events lasting 10-180 seconds
Performance goals require much more time, effort, and risk of injury than...
-Fitness goals
A high percentage of type 1 muscle fibers is associated with both a...
-Greater lactate threshold and a higher mechanical efficiency
The latter results are consistent with date from exercising humans in which force recovered before the...
-H+ concentration did
Excessive endurance training (overtraining)
-Has been associated with symptoms such as reduced performance capacity, prolonged fatigue, altered mood states, sleep disturbance, loss of appetite, and increased anxiety
Because races of this duration are not run at VO2 max, a person who can run at a high percentage of VO2 max would...
-Have an advantage -The ability to run at a high percentage of VO2 max is related to the concentration of lactate in the blood, and is one of the best predictors of race pace is the lactate threshold
In summary.....
-In events lasting 10 seconds or less, optimal performance is dependent on the recruitment of appropriate type 2 fibers to generate the great forces that are needed -Motivation and arousal are required, as well as the skill needed to direct the force -The primary energy sources are anaerobic, with the focus on phosphocreatine
In summary!!
-In moderate length performances lasting 3 to 20 minutes, aerobic metabolism provides 60%-90% of the ATP -These activities require an energy expenditure near VO2 max, with type 2 fibers being recruited, in addition to the type 1 fibers -Any factor interfering with oxygen delivery (altitude, anemia) would decrease performance, because it is so dependent on aerobic energy production. High levels of H+ accompany these types of activities
In summary
-Increases in CNS arousal facilitate motor unit recruitment to increase strength and alter the state of fatigue -Repeated stimulation of the sarcolemma can result in a reduction in the size and frequency of action potentials; however, shifts in the optimal frequency needed for muscle activation preserve force output -Under certain conditions, an action potential block can occur in the t-tubule to result in a reduction in Ca++ release from the SR
As ATP demand continues to exceed supply, a variety of reactions occur in the cell that...
-Limit work and protect the cell from damage -ATP is needed to pump ions and maintain cell structure. In this sense, fatigue serves a protective function
One sign of fatigue in isometric contractions is a....
-Longer relaxation time. -The time from peak tension development to baseline tension -This appears to be an important aspect of fatigue in fast-twitch fibers -This longer relaxation time could be due to a slowing of cross-bridge cycling, but the latter appears to be most important
The early work of Ikai and Steinhaus showed that a simple shout during exertions could increase what was formerly believed to...
-Maximal strength
Skinned fiber advantages
-Precise solutions can be applied -Possible to study myofibrillar properties, SR release and uptake, AP/Ca2+ release coupling -Metabolic and ionic changes associated with fatigue can be studied in isolation
Because radicals contribute to muscle fatigue, it is feasible that antioxidant supplementation can...
-Retard exercise induced muscle fatigue
Finally, the Ca++ must be pumped back into the...
-SR to allow the muscle to relax before the next contraction
Fatty acids can provide...
-Substantial fuel during prolonged muscular work at intensities <60% VO2 max
The discussion mechanisms starts with...
-The brain
Moderation
-Used to reduce the risk factors associated with a variety of diseases
Isolated single fiber disadvantages
1. Dissection difficult 2. Environment different to in vivo 3. K+ accumulation and other in vivo changes absent 4. Prone to damage at physiological temperatures 5. Small size makes analysis of metabolites difficult
Brain Serotonin
5-hydroxytryptamine
The brain
Influences a variety of factors that can influence the will to win and continues to the cross-bridges of the muscles themselves
What are the requirements for optimal performance?
What kind of performance? 1. Different performances require different approaches -Ex: 400 meter training is different than marathon training 2. Every performance requires a certain amount of strength and skill to apply that strength in the best way 3. Energy must be supplied in the manner needed or performance will suffer 4. Different activities require differing amounts of aerobic and anaerobic processes 5. Environment (Altitude and heat) and diet (carbohydrate and water intake) play a role in endurance performance 6. Best performances require a psychological commitment to "go for the gold"
When experiments both skinned and intact muscle fibers for done at 30 C a more physiological temperature...
the acidosis decreased force by only 10% and did not accelerate the rate of fatigue
There is evidence to support many of the sites listed in the factors affecting performance...
-"Weak links" in the development of the muscle tension needed for optimal performance
Maximal performances in the 10-60 second range are still predominantly...
->70% anaerobic, using the high force, fast twitch fiber
Factors affecting performance in ultra endurance events
-A class of their own -166 km mountain runs, triple iron triathlon, paris to beijing run -In a recent study to evaluate the factors related to ultra endurance performance, 14 subjects ran as many kilometers as possible in 24 hours on a treadmill. They covered and average of 149.1 km in 18 hours and 39 minutes of actual treadmill running in the 24 hours. -2 of the most important variables related to performance included VO2 max and the percentage of VO2 that they could maintain over the 24 hours, 2 of the 3 variables linked to the regular endurance performances we just discussed -Studies that have tracked metabolic responses before and after ultra endurance events find that fat oxidation is markedly elevated after an ultra endurance race, with plasm free fatty acids being 3.5 times higher -Given the low % of VO2 max maintained in many of these events (<60%) the metabolic response is not surprising. However, it does not mean that carbohydrate is not shown a 50% reduction in muscle glycogen stores. -Although ultra endurance runs increase the potential for hyponatremia (a dangerous condition in which the body's sodium store is diluted because of the ingestion of too much water) only 4 % of the runners experienced a mild form of the condition
Merton's classic experiments showed no difference in tension development when...
-A voluntary maximal contraction was compared to an electrically induced maximal contraction.
Glucose supplementation during long term performances provides the fuel needed for....
-ATP generation for the cross-bridges and for the protection of muscle membrane excitability
Fatigue can be viewed as the result of a simple imbalance between the...
-ATP requirements of a muscle and its ATP-generating capacity
When a maximal performance is extended to 3 minutes...
-About 60% of the energy comes from the slower aerobic, ATP-generating processes.
If you are looking for potential sites of fatigue, you might track the movement of the...
-Action potential from nerve to the muscle, and within the muscle, its progress along the sarcolemma and t-tubule to the sarcoplasmic reticulum (SR) where Ca++ is stored
As the duration of an all out performance increases, more demand is placed on the...
-Aerobic sources of energy -Environmental factors such as heat and humidity, and dietary factors such as water and carbohydrate ingestion play a role in fatigue
Given that the ATP-PC system can supply ATP for only several seconds, the vast majority of the ATP will be derived from....
-Anaerobic glycolysis -This will cause an accumulation of H+ in muscle as well as blood -The elevated H+ concentration may actually interfere with the continued production of ATP via Glycolysis, or the contractile machinery itself, by interfering with troponin's ability to bind with Ca++ -Following exhausting exercise, muscle tension recovers before H+ concentration does, indicating the complex nature of the fatigue process. -In effort to slow H+ accumulation, some athletes have attempted to ingest buffers prior to a race
Free radicals
-Are highly reactive molecules that contain an unpaired electron in their outer orbital -This unpaired electron results in molecular instability; therefore, radicals are highly reactive and capable of damaging proteins, lipids, and DNA in the cell -Radical-mediated damage to cellular constituents is called oxidative stress, and high levels of oxidative stress can lead to cellular damage and dysfunction
The arterial oxygen content (CaO2) is influenced by the...
-Arterial hemoglobin content [Hb] , the fraction of inspired oxygen (FIO2), and PO2 of the inspired air
Differences in running economy are due to...
-Bio mechanical and/or bioenergetic factors -In this case, measurements of both VO2 max and running economy would be needed to predict performance
For many endurance activities that are performed at higher exercise intensities (marathon running) must have....
-Carbohydrate to oxidize or performance will decline
The maximal stroke volume is a crucial key to a high...
-Cardiac output and is influenced by both genetics and training
Noakes and others have developed the "central governor" model of central fatigue that focuses primarily on the...
-Conscious and subconscious brain and does not involve the spinal cord or motor unit
As a result of the transition from anaerobic to aerobic energy production, maximal running speed...
-Decreases as the duration of the rate increases from 10-180 seconds.
Experiments using the powerful antioxidant N-acetyl-cysteine reveal that this unique antioxidant can...
-Delay fatigue during prolonged sub maximal exercise
Chronic severe training over many years has been lined to improvements in...
-Delta and gross efficiencies (+1.75% and + 1.87%) that contributed to the consecutive victories of the tour de France champion lance armstrong
Depending on the nature of the exercise, the subject, and the experimental approach used to study fatigue...
-Different results may be obtained
To date, studies using antioxidant vitamins...
-Do not support the concept that dietary antioxidants can improve human performance
Ultra short term performances
-Events 10 seconds or less -High jump, long jump, 50, 100 meter springs -These events require that tremendous amounts of energy be produced in a short period of time (high-power events) and type 2 muscle fibers must be recruited
For performances greater than 1 hour, the ability of the muscle and liver carbohydrate stores to supply glucose may be...
-Exceeded
In Summary.......
-Factors limiting performance are specific to the type of performance -Short term explosive performances are dependent on type 2x fibers that can generate great power through anaerobic processes -Longer duration aerobic events require a cardiovascular system that can deliver oxygen at a high rate to muscle fibers with many mitochondria -It is clear that the testing and training of athletes must focus on the factors limiting performance for the specific event EX: Dietary carbohydrate and fluid ingestion are more crucial for the long distance runner than for the high jumper.
In summary...
-Fatigue is directly associated with a mismatch between the rate at which the muscle uses ATP and the rate at which ATP can be supplied -Cellular fatigue mechanisms slow down the rate of ATP utilization faster than the rate of ATP generation to preserve the ATP concentration and cellular homeostasis
Between 40%-75% VO2 max type 2a fast twitch...
-Fatigue resistant fibers are recruited, in addition to the type 1 fibers -These fast twitch fibers are rich in mitochondria, as are the type 1 fibers, making them dependent on oxygen delivery to this fiber type is decreased, or the ability of the fiber to use oxygen is decreased (due to low mitochondrial number) tension development will fall, requiring type 2x fiber recruitment to maintain tension
Data from skinned muscle fibers studied at 20 C room temperature showed than an increase in H+ was linked to a decrease in...
-Force and the maximal velocity of shortening
In summary!!!!
-In long-term performances of 1-4 hours duration, environmental factors play a more important role as the muscle and liver glycogen stores try to keep up with the rate at which carbohydrate is used -Diet, fluid ingestion and the ability of the athlete to deal with heat and humidity all influence the final outcome
In summary!
-In short term performances lasting 10-180 seconds, there is a shift from 70% of the energy supplied anaerobically at 10 seconds to 60% being supplied aerobically at 180 seconds -Anaerobic glycolysis provides a substantial portion of the energy, resulting in elevated lactate levels
Later work showed that electrical stimulation of a muscle fatigued by voluntary contractions resulted in an...
-Increase in tension development
In summary!!!
-Intermediate length activities lasting 21-60 minutes are usually conducted at less than 90% VO2 max and are predominantly aerobic -Given the length of the activity, environmental factors such as heat, humidity, and the state of hydration of the subject play a role in the outcome
Because speed is a prerequisite in races lasting fewer than 20 minutes, type 2 a fibers, which are rich in mitochondria are...
-Involved in supplying the ATP aerobically in addition to the already recruited type 1 fibers
Sarcolemma and Transverse Tubules
-It has been hypothesized that the sarcolemma might be the site of fatigue due to its inability to maintain Na+ and K+ concentrations during during repeated during repeated stimulation. -When the Na+/K+ pump cannot keep up, K+ accumulates outside the membrane and decreases inside the cell. This results in a depolarization of the cell and a reduction in action potential amplitude. The depolarization of the sarcolemma could result in altered t-tubule function, including a block of the t-tubule action potential. If the latter occurs, Ca++ release from the SR will be affected, as will muscle contraction -However, the evidence indicates that the typical reduction in the size of the action potential amplitude has little effect on force output by the muscle. In addition, the lower frequency of action potential firing with repeated stimulation of muscle seems to protect the muscle from further fatigue (rather than cause fatigue) by shifting the activation to a lower, more optimal, rate of firing. -This does not mean that the t-tubule is not involved in the fatigue process. Under certain stimulation conditions an action potential block can occur in the t-tubule, leading to a reduction in Ca++ release from the SR. -As a result, myosin cross-bridge activation would be adversely affected. One of the beneficial effects of training is an increase in the capacity of the Na+/K+ pump, which may contribute to the maintenance of the Na+/K+ gradient and reduce the potential for fatigue via this mechanism
Do animal studies mimic the changes experienced by humans?
-Many of the space flight induced changes in skeletal muscle are observed in rodents, nonhuman primates, and humans. Fiber atrophy caused by the selective loss of myofilaments has been observed in all species. There are species differences in the time course of the adaptive process. For example, rats flown in space show a faster rate of fiber atrophy than do humans. -Flights as short as 2 or 3 weeks have been shown to increase soleus muscle velocity in both rats and humans, but in rats the increase was in part due to a conversion of approximately 20% of the slow type 1 fibers to fast twitch fibers containing fast myosin isozymes -In humans, short duration space flight does not cause fiber type conversion. Recent data suggest that such conversions from slow to fast twitch do occur in humans following long duration (6 months) space flight
As exercise intensity increases, muscle fiber recruitment progresses from type 1, type 2a, type 2x. This means that the ATP supply needed for tension development becomes...
-More and more dependent upon anaerobic metabolism. -In this way, fatigue is specific to the type of task undertaken -If a task requires only type 1 fiber recruitment, then the factors limiting performance will be very different from those associated with tasks requiring type 2x fibers.
In summary....
-Muscle fibers are recruited in the following order with increasing intensities of exercise: 1. Type 1 2. Type 2a 3. Type 2x -The progression moves from the most to the least oxidative muscle fiber type. Intense exercise (>75% VO2 max) demands that type 2x fibers (in addition to the type 1 and type 2a fibers) be recruited, resulting in an increase in H+ production
Cross-bridges
-Myosin head, which connects thick filaments and thin filaments during a contraction
These studies suggest that the upper limit of voluntary strength is....
-Psychologically set, given that certain motivational or arousal factors are needed to achieve a physiological limit
Exercise promotes the production of...
-Radicals in skeletal muscles and prolonged and/or intense exercise can lead to oxidative stress in the exercising muscle -This exercise induced oxidative damage is a key contributor to muscle fatigue during longed exercise size (> 30 minutes)
This longer relaxation time would result in a reduction in....
-Reduction in stride rate, a feature of fatigue in 400 meter races
When the Ca++ is released it interacts with...
-Regulatory proteins (troponin) that causes movement of the cross-bridges to generate tension
Skinned fiber Disadvantages
-Relevance to fatigue can be questionable -May lose important intracellular constituents -Relevant metabolites to study must be identified in other systems
When the muscle was fatigued by voluntary contractions, electrical stimulations could not...
-Restore tension -This suggested that the CNS was not limiting performance and that the periphery was the site of fatigue
Exercise, especially eccentric exercise, can cause a physical disruption of the...
-Sarcomere and reduce the capacity of the muscle to produce tension. -However, a high H+ concentration, which generally occurs with heavy exercise, may contribute to fatigue in a variety of ways: 1.Reduce the force per cross-bridge 2.Reduce the force generated at a give Ca++ concentration (related to H+ ion interference with Ca++ binding troponin) 3.Inhibit SR Ca+ release
The importance of both VO2 max and running economy in predicting distance running performance was recently confirmed and evidence exists that both are important in...
-Shorter 1500 m races
Optimal performance is affected by...
-Skill and technique
Are elite athletes being treated like racing cars in which engineers and mechanics (scientists and coaches) try to spot weaknesses that compromise performance, and then recommend solutions?
-Some say yes and indicate the worthiness of such an enterprise. -Others may suggest that this has the potential to be dehumanizing if the athlete is reduced to no more than a collection of working parts that are evaluated by a variety of specialists. -Much would appear to depend on the goal of the research. -If we are trying to understand how we function, and we develop healthy and safe methods that allow us to overcome personal limitations, we would appear to be on the right track. -Institutional Review boards (IRBs) approve research proposals so that the rights of the subject are protected. This process also forces the investigator to provide a strong rationale showing that the risk to the subject is worth the benefits that might occur.
Dr.David Costill: Advanced the study of endurance performance
-Studied the factors affecting performance Addressed a variety of variables influencing endurance performance -Role of VO2 max -Contribution of the percent of VO2 max -Importance of running economy -How blood/plasm lactate concentration is linked to performance -Strategies for loading muscle glycogen -Fluid replacement with emphasis on characteristics of sports drinks -Role of muscle fiber type -Whether ergogenic aids like caffeine, L-carnitine work
Up to about 40% of VO2 max, the type 1 slow-twitch oxidative muscle fiber is recruited to provide...
-Tension development -This fiber type is dependent on a continuous supply of blood to provide the oxygen needed for the generation of ATP from carbohydrates and fats -Any factor limiting the oxygen supply to this fiber type (altitude, dehydration, blood loss, anemia) would cause a reduction in tension development in these fibers and necessitate the recruitment of type 2a fibers to generate tension
Neuromuscular junction
-The action potential appears to reach the neuromuscular junction even when fatigue occurs -In addition, evidence based on simultaneous measurements of electrical activity at the neuromuscular junction and in the individual muscle fibers suggests that neuromuscular junction is not the site of fatigue
In summary.
-The cross-bridge's ability to cycle is important in continued tension development. Fatigue may be related, in part, to the effect of a high H+ concentration and the inability of the sarcoplasmic reticulum to rapidly take up Ca++. The end result may be a longer relaxation time, which affects the rate of muscle contraction
Type 2x
-The fast twitch muscle fiber with a low mitochondrial content -This fiber can generate great tension via anaerobic sources of energy, but it fatigues quickly -It is recruited at about 75% VO2 max, adding to the tension from type 1 and type 2a fibers and making heavy exercise dependent upon its ability to develop tension
The longer the performance...
-The greater the chance that environmental factors will play a role in the outcome
What changes occur in skeletal muscle due to space travel?
-The primary change in skeletal muscle with space travel is fiber atrophy due to a selective loss in the myofilaments. The anti gravity muscle of the legs are more affected than fast twitch muscles such as the gastrocnemius -Due to the loss of myofilaments, muscle fibers generate less force and power. Following short duration (< 3 weeks) space flight, slow type 1 fibers show an elevated maximal shortening velocity, which is not caused by an expression of fast type myosin. -It has been hypothesized that the increased velocity results from a selective loss of the thin filament actin, which increases the space between the filaments, causing the myosin cross-bridge to detach sooner at the end of the power stroke -Recent results from international space station experiments have shown the elevated velocity to be a transient change, with slow fiber velocity showing a significant decline following long duration flights. -This change, plus additional fiber atrophy, contributes to the considerably greater loss in slow fiber power after long duration, compared to short duration, flights. -Space flight appears to increase the muscle's reliance on carbohydrates and reduce its ability to oxidize fats. This metabolic change is not caused by a reduced activity of any of the enzymes of the B-oxidative pathway or the Krebs cycle. -The loss in fiber power and increased reliance on carbohydrates cause a reduced work capacity. Post flight crew members experience muscle soreness due to an increased susceptibility to eccentric contraction induced fiber damage
Are there any intervention (training) strategies being employed to reduce the impact of space flight on skeletal muscle?
-The primary countermeasure used to protect skeletal muscle from microgravity induced loss has employed endurance exercise on either a bicycle or treadmill. This type of modality has not been completely successful, as crew members still lose up to 20% of their leg muscle mass following 6 months in space -High intensity exercise has been incorporated into the counter measure program on the ISS, but the loads have been insufficient to adequately protect limb muscle size or power. Recently a new high resistance device has been installed on the ISS, but its effectiveness has not yet been established
Mechanical factors
-The primary mechanical factor that may be related to fatigue is cross-bridge "cycling". The action of the cross-bridge depends on: 1.The functional arrangement of actin and myosin 2. Ca++ being available to bind with troponin to allow the cross-bridge to bind with the active site on actin 3. ATP, which is needed for both the activation of the cross-bridge to cause movement, and the dissociation of the cross-bridge from actin
Recent studies indicate that it is not the serotonin alone, but...
-The ratio of serotonin to dopamine that contributes to tiredness on the one hand and arousal on the other, and that brain levels of norepinephrine also contribute to the picture
Is maximal oxygen uptake important in distance running performance?
-VO2 max is directly related to the rate of ATP generation that can be maintained during a distance race, even though it is not run at 100% VO2 max -The rate of ATP generation is dependent on the actual VO2 that can be maintained during the run ( ml x kg[-1] x min [-1] ) which is a function of the runners VO2 max and the percent VO2 max at which the runner can perform. -To run a 2:15 marathon, the runner would have to maintain a VO2 of about 60 ml x kg [-1] x min [-1] throughout the race. -A runner working at 80% VO2 max would need a VO2 max of 75 ml x kg [-1] x min [-1] -In this way, the VO2 max sets the upper limit for energy production in endurance events but does not determine the final performance. -It is clear that both the percent of VO2 max that can be maintained over the course of the run (estimated by the lactate threshold) and running economy have a dramatic impact on the speed that can be maintained over distance -The progressive reduction in VO2 max with age appears to be the primary physiological mechanism associated with a reduction in the endurance performance of master athletes, along with the reduction in the velocity at the lactate threshold
The accumulation of Pi reduces the total ATP cost per unit of force, suggesting an improvement in efficiency.
-What is interesting is that the cell does not run out of ATP, even in cases of extreme fatigue. -Typically the ATP concentration falls to only 70% of its pre-exercise level. -The factors that cause fatigue reduce the rate of ATP utilization faster than ATP generation so that ATP concentration is maintained. This is believed to be a protective function aimed at minimizing changes in cellular homeostasis with continued stimulation
What are the signals to the muscle cell that energy utilization must slow down?
-When ATP-generating mechanisms cannot keep up with ATP use, inorganic phosphate (Pi) begins to accumulate in the cell (Pi and ADP are not being converted to ATP) -An increase in Pi in the muscle has been shown to inhibit maximal force, and the higher the Pi concentration, the lower the force measured during recovery from fatigue -The Pi seems to act directly on the cross-bridges to reduce its binding to actin and also inhibits calcium release from the sarcoplasmic reticulum
Peripheral fatiuge
-Where neural, mechanical, or energetic events can hamper tension development -The vast majority of evidence points to the periphery for being a site of fatigue
Although optimal levels of antioxidants can postpone fatigue, high doses of antioxidants can...
-impair muscle performance -Therefore in-discriminant antioxidant supplementation could be detrimental to athletic performance
Long term performances
1-4 hours
Over the past decade, a considerable amount of attention has been directed at brain serotonin as a....
1. A factor in fatigue due to its links to depression, sleepiness, and mood 2. There is evidence that increases and decreases in brain serotonin activity during prolonged exercise hasten and delay fatigue, respectively
Central Nervous System (CNS) would be implicated in fatigue if there were a...
1. A reduction in the number of functioning motor units involved in the activity 2. A reduction in motor unit firing frequency
Muscle in vivo Advantages
1. All physiological mechanisms present 2. Fatigues can be central or peripheral 3. All types of fatigue can be studied 4. Stimulation patterns appropriate for fiber types and stage of fatigue
When exercise begins and the need for ATP accelerates a series of ATP generating reactions occur to replenish the ATP
1. As the cross-bridges use ATP and generate ADP, phosphocreatine provides for the immediate re-synthesis of the ATP (PC + ADP----> + C) 2. As the phosphocreatine becomes depleted, ADP begins to accumulate and the myokinase reaction occurs reaction to generate ATP (ADP + ADP ---> ATP + AMP) 3. The accumulation of all these products stimulates glycolysis to generate additional ATP, which may result in an H+ accumulation
Isolated muscle Advantages
1. Central fatigue eliminated 2. Dissection simple
Factors affecting performance
1. Diet- Carb and water intake 2. CNS function- Arousal and motivation 3. Strength/skill- Practice, natural endowment, body type, muscle fiber type 4. Environment- Altitude, heat, humidity 5. Energy production- Anaerobic sources- PC, Glycolysis Aerobic sources- VO2 max, cardiac output, O2 delivery- (Hb, PO2) ,O2 extraction, Mitochondria
The CNS is intimately involved in...
1. Exercise, including the psyching up prior to exercise, the recruitment of motor units, and the continual feedback from a host of receptors sensing tension, temperature, blood gases, blood pressure. 2. The brain integrates these various signals and generates commands that automatically reduce power output to protect the organism 3. Exercise begins and ends in the brain
Central fatigue
1. Fatigue that originates in the CNS 2. There is evidence both for and against this
Fatigue
1. Inability to maintain a power output or force during repeated muscle contractions 2. The causes of fatigue vary and are usually specific to the type of physical activity
Isolated muscle Disadvantages
1. Mixture of fiber types 2. Inevitable extracellular gradients of O2, CO2, K+, Lactic acid 3. Mechanisms of fatigue biased by presence of extracellular gradients 4. Drugs cannot be applied rapidly because of diffusion gradients
Muscle in vivo Disadvantages
1. Mixture of fiber types 2. Complex activation patterns 3. Produces correlative data; hard to identify mechanisms 4. Experimental interventions very limited
Isolated single fiber Advantages
1. Only one fiber type present 2. Force and other changes (ionic, metabolic) can be unequivocally correlated 3. Fluorescent measurements of ions, metabolites, membrane potential, etc. possible 4. Easy and rapid application of extracellular drugs, ions, metabolites, etc.
Possible fatigue mechanisms
1. Psyche/brain -Motivation (arousal) (Motor recruitment) 2. Spinal cord -Reflex drive 3.Peripheral nerve -Neuromuscular transmission 4.Muscle sarcolemma -Muscle action potential 5.Transverse tubular system (K+,Na+) -Excitation 6.Ca++ release -Activation -Energy supply 7.Actin-myosin interaction 8.Cross-bridge tension + heat 9.Force/power output
The far from perfect agreement among scientists about the exact causes of fatigue:
1. The fiber type and training of the subject 2. Whether the muscle was stimulated voluntarily or electrically 3. The use of both amphibian and mammalian muscle preparations with some isolated from the body 4. The intensity and duration of the exercise and whether it was continuous or intermittent activity
In agreement with these results (that the CNS can limit performance) are two studies by Asmussen and Mazin
1. Their subjects lifted weights 30 times a minute, causing fatigue in 2-3 minutes. 2. Following a 2 minute pause, the lifting continued. 3. These investigators showed that when either a physical diversion, consisting of the contraction of non-fatigued muscles, or a mental diversion, consisting of doing mental arithmetic, was used between fatiguing bouts of exercise, work output was greater than when nothing was done during the pause. 4. They also found that if a person did a series of muscle contractions to the point of fatigue with the eyes closed, simply opening the eyes restored tension. 5. These studies suggest that alterations in central nervous system "arousal" can facilitate motor unit recruitment to increase strength and alter the stage of fatigue
Factors affecting fatigue in aerobic performances lasting 1-4 hours
1.% Type 1 fibers 2.Lactate threshold 3.Supplement during exercise -diet 4.Liver and muscle glycogen stores 5. % VO2 max 6.Dehydration -Heat load 7.VO2 max 8.Steady state VO2 1.% Type 1 fibers 2.Biomechanics -Bioenergetics 3.Running economy
Factors affecting fatigue in aerobic performances lasting 21-60 minutes
1.% Type 1 fibers 2.Lactate threshold 3.Dehydration/heat load 4.%VO2 max 5.Steady state VO2 1.%Type 1 fibers 2.Biomechanics 3.Bioenergetics 4.Running economy
Factors affecting fatigue in short term events
1.Buffers 2.Muscle and blood H+ 3.Fiber type distribution and recruitment 4.Glycolysis + PC (40-90% ATP) 5.VO2 (10-60% ATP)
Factors that affect fatigue in ultra short term events
1.Practice 2.Skill and technique 3.Motivation and arousal 4.Fiber type distribution and recruitment 5.PC and glycolysis 6.Muscular power
Current evidence indicates that radical production can contribute to Muscle fatigue in 2 ways
1.Radicals can damage key contractile proteins including myosin and troponin. Damage to these muscle proteins reduces the calcium sensitivity of myofilaments and limits the number of myosin cross-bridges in the strong binding state. It follows that when fewer myosin cross-bridges are bound to actin, muscle force production is reduced 2. A second mechanism to explain how radicals may contribute to muscle fatigue is that high radical production can depress sodium/potassium pump activity in skeletal muscle. Impaired sodium/potassium pump function results in disturbances in muscle potassium levels, and this disruption in potassium homeostasis has been linked to muscle fatigue
Factors affecting fatigue in aerobic performances lasting 3-20 minutes
1.Training 2.Mitochondrial and capillary density 3.Genetics 4.Fiber type 5.FIO2, [Hb] , PO2 6.(CaO2 - CvO2) 7.VO2 max 1.Genetics and training 2.HR x SV max 3.Maximal cardiac output 4.VO2 max
Races lasting fewer than 20 minute are run at....
90%-100% of maximal aerobic power, so the athlete with the highest VO2 max has a distinct advantage -However, because type 2 x fibers are also recruited and H+ accumulation would affect tension development as previously described
In all out performances lasting 21-60 minutes, the athlete will generally work at...
<90% VO2 max -A high VO2 max is certainly a prerequisite for success, but now other factors come into play -For example: An individual who is an economical runner can move at a higher speed for the same amount of oxygen compared to a runner who is not economical
Intermediate length performances
Events lasting 21-60 minutes
