Fatigue and Recovery

KEY KNOWLEDGE The multi-factorial mechanisms (including fuel depletion, ACCUMULATION of metabolic by-products and thermoregulation) associated with muscular fatigue as a result of varied exercise intensities and durations Passive and active recovery methods to assist in returning the body to pre-exercise levels

What is fatigue? Simply defined, fatigue is the inability to continue exercise at a given intensity and can occur during exercise of maximal effort lasting a few seconds or sub-maximal intensity exercise that lasts hours. It is a decline in muscular force and power, which must be reversible in recovery, as distinct from muscle damage. A more recent description of fatigue encompassing the disciplines of physiology, biomechanics and psychology is sensations of tiredness and associated decrements in muscular performance and function . The advantage of this definition is that it does not limit fatigue to only physiological mechanisms.

What is fatigue? Fatigue is a complex phenomenon involving both physiological and psychological factors. It can manifest itself in many forms including physical, mental, local muscular, general and chronic fatigue.

Factors that effect fatigue: The onset of fatigue will vary according to a number of factors, including exercise intensity and duration, environmental conditions (for example, heat, humidity and altitude) and training status of the person (their fitness level).

Types of Fatigue Central fatigue: mechanisms operating outside the muscle that inhibit muscle activation and reduce force production (that is, within the central nervous system) Peripheral fatigue: mechanisms operating within the muscle that affect the ability of the muscle itself to produce force (for example, fuel depletion)

Types of Fatigue Local (muscular) fatigue Local fatigue is related to a particular muscle or muscle group that has been involved in a specific activity or training session. It can be defined as ‘the inability to maintain the required power output to continue muscular work at a given intensity’. An alternate definition is ‘any exercise-induced reduction in the maximal capacity to generate force or power output’ To distinguish local muscular fatigue from muscle weakness or damage, one can think of local muscular fatigue as being reversible by rest. Experienced in quadriceps after mogul skiing Experienced in triceps after performing maximum dips fitness test

Types of Fatigue General fatigue General fatigue is where the athlete experiences feelings of fatigue in all muscle groups as a result of an extended training program or performance. Mental fatigue can also be experienced in this state. Experienced after a full weights session Experienced after completing a marathon run (42.2 kilometres) Experienced after 36 holes of golf in one day Chronic (long-term) fatigue Chronic fatigue is where the athlete experiences an unhealthy level of fatigue caused by the continual breakdown of the body's defences. Insufficient recovery may lead to physiological and psychological problems, such as severe lethargy and depression. Experienced as chronic fatigue syndrome Experienced as overtraining syndrome Experienced as recurrent minor illnesses Experienced as recurrent viral infections Experienced as insomnia

Mechanisms (causes) of fatigue – there are numerous factors that cause fatigue. (multi-factorial) From the study design- key knowledge: the multi-factorial mechanisms (including fuel depletion, metabolic by-products and thermoregulation) associated with muscular fatigue as a result of varied exercise intensities and durations

Mechanisms (causes) of muscular fatigue – there are numrous factors that cause fatigue. (multifactorial)

Muscular Fatigue

Accumulation of metabolic by-products The accumulation of metabolic by-products, has long been considered a factor that contributes to muscular fatigue. Metabolic by-products or metabolites are substances produced as a result of chemical reactions within the body associated with the production of energy for ATP resynthesis. They are the ‘leftovers’, and include lactic acid (or more accurately, lactate and hydrogen ions), as well as inorganic phosphate (Pi) and adenosine diphosphate (ADP). Key term: Fatigue is multi-factorial in nature. One cause is the accumulation of metabolic by-products such as ADP, Pi and H+

Accumulation of metabolic by-products Lactic Acid lactic acid within the muscle quickly dissociates (splits) into lactate and hydrogen ions (H+). Recent research into fatigue during high-intensity activity has focused more on the effects of the hydrogen ions and other metabolic by-products.

Removal of Lactic Acid following exercise • Recent evidence – 70% of lactic acid is oxidized  Used as a substrate by heart and skeletal muscle – 20% converted to glucose – 10% converted to amino acids • Lactic acid is removed more rapidly with light exercise in recovery – Optimal intensity is ~30–40% VO2 max

Blood Lactate Removal Following Strenuous Exercise

LIP(Lactate Inflection Point) or LT2 (Lactate transition 2)

LIP(Lactate Inflection Point) or LT2 (Lactate transition 2) LIP – the point at which lactic acid production exceeds lactic acid removal. An accumulation of lactate and hydrogen ions results in a number of changes within the muscle, including: an increase in muscle acidity (lowers pH) leading to inhibition of the glycolytic enzyme (phosphofructokinase, PFK) decreased calcium (Ca2+ ) release from the sarcoplasmic reticulum. These changes result in a reduced rate of ATP resynthesis and muscle contraction due to a slowing of the glycolysis process and a reduction in the myosin coupling action (that is, cross-bridging).

OBLA Onset of Blood Lactate Accumulation At rest, everyone has lactic acid in their muscles When exercise begins the muscular levels of lactic acid begin to rise The level of hydrogen ions (H+) also rise This is known as OBLA

Lactate Threshold Now better known as Lactate Inflection Point (LIP) Lactic acid production is greater than lactic acid removal At exercise intensities beyond the LIP blood lactic acid concentration increases Beyond the lactate threshold/LIP the athlete has to stop or reduce muscle effort Trained athletes can and aim to increase their tolerance to lactic acid accumulation

Determining the LT/LIP Scientific testing in labs (eg: AIS) is the best method Some rough estimates are in table 2.3, p.76 Untrained athlete ~60% max HR Trained athlete ~90% max HR

Determining the LIP VO2 max Untrained athlete: LIP = ~50% VO2 max

LIP & Fatigue Exercise intensities beyond the LIP are associated with fatigue The greater the exercise intensity above the inflection point, the more rapid the fatigue This fatigue is generally considered to be a consequence of a greater reliance on the anaerobic systems to supply the adenosine triphosphate (ATP) and the resultant accumulation of the by-products of anaerobic metabolism Lactic acid and hydrogen ions

Lactic Acid Removal/Recovery When lactic acid builds up an active recovery is best Keep moving and keep HR slightly elevated This keeps blood flow higher to help break down lactic acid Lactic acid  pyruvic acid  ATP (Kreb’s cycle) Lactic acid is now thought to have some positive effect on performance When oxygen is available it can help to produce ATP

LIP(Lactate Inflection Point) or LT2 (Lactate transition 2)

Mechanism to explain the LIP

Cori Cycle

Oxidation of lactate in the mitochondria

Oxidation of lactate in the mitochondria The lactate shuttle. On the left we see a fast twitch muscle fiber producing lactate.  This lactate then travels to neighboring muscle tissues (slow twitch) as well as distant tissues like the heart and other muscle cells, where it is used as a fuel.  In the middle you see the liver, representing the conversion of lactate to glucose (the Cori Cycle)

Fuel Depletion Refers to both the depletion of adenosine triphosphate (ATP), but also the depletion of phospho-creatine (PC) and glycogen which are used to resynthesise ATP. Fuel depletion is determined by intensity and duration of exercise.

Fuel Depletion When do we use these fuels? ATP PC Glycogen

Fuel Depletion ATP depleted in first few seconds PC – high intensity, short durations 6-10 sec. Used to resynthesise ATP from ADP and Pi Glycogen - Glycogen is the preferred fuel used by the aerobic system during exercise of sustained sub-maximal intensity (endurance exercise). Also used in Anaerobic Glycolysis resulting in lactic acid as a metabolic by-product.

Fuel Depletion Glycogen - Muscle glycogen stores can be depleted within as little as 40 minutes during intense prolonged exercise … although more typically glycogen stores can fuel continuous sub-maximal exercise for periods of 90–120 minutes.

Fuel Depletion Glycogen - Furthermore, an increased reliance on fat oxidation further reduces an individual's exercise capacity as less ATP is generated from fats compared to glycogen per litre of oxygen consumed (see table 4.2). TABLE 4.2 Energy released per litre of oxygen consumed Substrate or fuel Energy per litre of oxygen, kcal (kJ) Carbohydrate 5.05 (21.2) Fat 4.70 (19.7) Protein 4.82 (20.2) Source: Abernathy 2005, The biophysical foundations of human movement, p. 138.

Fuel Depletion Summary In summary, fuel depletion contributes to metabolic fatigue within the muscle due to a lack of intracellular energy to power muscle contractions. In essence, the muscle is unable to continue contracting at the same rate or force, basically because it lacks the energy to do so.

Thermoregulation

Thermoregulation muscular contraction, results in heat production, which in turn brings about an increase in both muscle and core body temperature. Although an increased temperature of the skeletal muscles enhances the ability of muscles to contract, performance becomes markedly impaired when the body's core temperature rises sufficiently. A rise in core body temperature to a temperature greater than 37.5–38.3°C (depending on the source or reference) is referred to as hyperthermia

Thermoregulation Increase core temp (hyperthermia)causes increase skin blood flow Increase skin blood flow means decrease working muscle blood flow Decrease muscle blood flow mean reduced O2 reaching muscle cause increase reliance on anaerobic energy resulting in metabolic by-products. Dehydration is also a factor FATIGUE

Thermoregulation dehydration The decrease in blood plasma volume leads to a reduction in the amount of blood available to working muscles as well as for blood flow to the skin surface for dissipating heat. A number of factors influence the level of increased body temperature and dehydration that may occur when athletes exercise under conditions where heat stress may result in fatigue. These factors include: the duration of the training session or competition the particular environmental conditions such as the air temperature, humidity levels and even the playing surface the athlete's acclimatisation to conditions.

Thermoregulation At the same time it appears that central fatigue mechanisms (mainly changes of the brain's ability to sustain sufficient activation of the skeletal muscles) are also implicated as core and brain temperatures rise. A Central fatigue mechanism

Thermoregulation hypothermia The ability to perform coordinated movements is severely compromised, and many people who develop moderate hypothermia appear to be extremely lethargic, or even in a stupor. This has much to do with the greatly decreased blood flow to the extremities as blood is redirected to the major organs to sustain their function. In severe hypothermia the person may be comatose, with death usually occurring if core temperature falls below 28°C. A Central fatigue mechanism

CNS fatigue - psychological ‘Clearly there is some form of concious central nervous system involvement in most forms of fatigue … the stress of exercise often leads to a concious decision on the part of the athlete to terminate the activity rather than tolerate further discomfort.’ A Central fatigue mechanism

Multi-factorial

Summary Summary of fatigue mechanisms From this and the preceding sections on the mechanisms of fatigue, it is clear that the cause of muscular fatigue during both high-intensity, short-duration exercise and prolonged sub-maximal exercise is complex and may actually involve several mechanisms occurring simultaneously and synergistically. Fatigue appears to be determined by a delicate interplay between physiological and psychological factors of both peripheral and central origin. A major challenge for the field of exercise science is to establish the relative importance of these mechanisms in the fatigue process.