Fatigue and Recovery in Aerobic Exercise

Paul Batman
Category: Group Exercise

The importance of fatigue lies in its ability to induce recovery that can lead to overcompensation or “the training effect.” Often you will find that your aerobic performance is stagnating with little improvement apparent. This generally means that the aerobic exercise is no longer producing enough fatigue to warrant recovery. There is no one cause of fatigue as it is very task-specific. Many times exercisers and trainers alike tend to focus on one specific cause. How often have we heard how the dreaded lactic acid interfered with performance and caused someone to stop? The reality may be that the fatigue was caused by the high lactate levels, but associated with that is the low glycogen and/or ATP levels. An untrained person whose aerobic capacity is low will experience fatigue to a greater degree than the trained person. Exercising in the heat will induce more fatigue than exercising in a cooler environment.


ATP-PC Depletion

While the ATP-PC system is almost exclusively used in sprint events, it does play a role in aerobic exercise. During aerobic exercise the ATP-PC system initiates muscle contractions at the beginning of the exercise and again when sudden bursts of energy are required. If you start the aerobic exercise too fast without slowly leading into it or start without an adequate warm-up, creatine phosphate stores can be depleted dramatically. If this occurs too quickly before the aerobic energy system and lactic acid system has time to take over, sudden fatigue will result. During steady state aerobic exercise there maybe a need to accelerate quickly. Extending beyond the capabilities of the aerobic energy system and the lactic acid system, the ATP-PC system will contribute to the energy production. If the time course for the intensity is beyond the ATP-PC capabilities fatigue will result. You will slow down forcing the aerobic energy system to assume control.

Glycogen Depletion

Marathon runners often experience the sensation of “hitting the wall” around the 30km mark. At this point acute fatigue prevails mainly due to the muscles’ inability to utilise glycogen as the main source of fuel. The body will then mobilize more free fatty acids to be oxidized through the Krebs cycle to provide the energy for the remake of ATP. As the oxygen cost of this process is increased by approximately 10%, you will fatigue and slow down below the speed previously maintained. This scenario can occur if the exercise bout is intense and long or if intense exercise bouts will follow day after day without adequate recovery or supplementation. Muscle and liver glycogen are the body’s total reserves of available energy.

Lactic Acid Accumulation

Many people continually blame lactic acid as a main cause of fatigue. How true are these accusations? In reality lactic acid is not the cause of fatigue and will only contribute if the exercise intensity is above the lactate threshold. For this to occur the you must undertake high intensity aerobic exercise. This can be seen in marathon runners at the end of the race. Many times this fatigue is due to a low energy supply rather than lactic acid as evidenced by their relatively low lactate levels.

If the body is unable to buffer the hydrogen ions from lactate accumulation, cells will eventually die. For the body to maintain homeostasis the resting pH is maintained at 7.1-7.4. In high intensity aerobic exercise the hydrogen ion increase lowers the pH to below 6.5, effectively inducing fatigue. At this low pH level fatigue is caused by the inhibition of the PFK enzyme (phosphofructokinase) normally responsible for the breaking up of glycogen and ATP production. The effect will be a decrease in glucose to the muscle and reduced energy output. In addition, the calcium in the muscle fiber used in binding to troponin and exposing the active sites on actin is also reduced. The result is the muscles’ inability to effectively contract. The sarcoplasmic reticulum can no longer store and release calcium. Oxygen transport is also reduced by the hydrogen ions inhibiting the binding ability of oxygen and haemoglobin.


The nervous system as it terminates in the muscle at the motor end plate can also be a site of fatigue. The neurotransmitter acetylcholine (Ach) is reduced across the synaptic cleft by an enzyme called cholinesterase, which can become hyperactive increasing the firing threshold of the muscle membrane, leading to fatigue.

Amino Acids

The main function of protein is to restore and rebuild cell components. In prolonged aerobic exercise protein can also be used as a fuel source. Although relatively insignificant this process is used to maintain blood glucose so that tissues such as the brain can function. Protein consists of amino acids linked together. During exercise the branch chain amino acids of valine, leucine and isoleucine are the preferred forms of protein fuel. Another amino acid used by the brain for maintenance is tryptophan. This amino acid is necessary to build a brain neurotransmitter called 5HT. The main function of 5HT is to move messages around the brain. In a fatigued state the amount of branch chain amino acids decreases in the blood on its way to being used as a fuel. When this occurs the amount of tryptophan entering the brain is increased causing a central fatigue represented by tiredness and lethargy.


One of the benefits of warm-up is the increased blood flow to the working muscle. This prolongs the aerobic activity by assisting the transport and delivery of oxygen to the muscle and speeding up aerobic enzyme responses. During prolonged aerobic exercise, particularly in the heat, blood can be diverted away from the exercising muscle to the skin for the purpose of cooling the body. During aerobic exercise as much as 60-70% of the total energy of the human body is degraded as heat. As the body reaches approximately 39ºC there is a shift in blood flow to the skin to help with heat dissipation. The decreased blood flow to the muscle will reduce its ability to consume oxygen and increase the role of the anaerobic glycolytic system in producing the additional energy for the remake of ATP. The consequence will be an increased rate of lactate production and fatigue.

Delayed Onset Muscle Soreness (DOMS)

While it has been established that a build-up of lactate will induce fatigue during the exercise, it is equally well known that it does not remain elevated for more than 60 minutes after exercise. In many cases you can experience delayed onset muscle soreness but not exercise intensely enough to approach lactate threshold during. It seems unlikely that lactic acid is a major cause of extended fatigue. Acute and chronic muscle soreness can be the result of eccentric contractions during aerobic exercise.

Another explanation for DOMS is the muscle spasm theory. It was thought that severe muscle contractions could cause reduced blood flow to the active muscles, which releases pain substances that stimulate nerve endings causing a muscle spasm. Again this is an unlikely cause. In aerobic exercise there is always the possibility of some microscopic muscle or connective tissue damage. The site of potential damage is in the myofibril at the Z lines where the sarcomeres are joined together. The swelling of the muscle tissue that stimulates free nerve endings causes the pain.


To improve maximum aerobic capacity progressive overload is required. Overload increases the stress the body must overcome to overcompensate and produce the training effect. To facilitate the greatest training gains, structured recovery sessions must be incorporated into the training program. The greatest training gains are made when you can sustain high volumes of high intensity exercise. If this training regime cannot be maintained, a state of overtraining will develop. To ensure that the best results will occur recovery activities must be structured within the overall program. Recovery refers to the physical and/or psychological restoration as a consequence of fatigue from the exercise session.


During aerobic exercise there is occasionally the need to exceed the anaerobic or lactate threshold for a short period of time. It may be that on an aerobic run you might need to run very fast for a few seconds or minutes. Once the lactate threshold is exceeded there is a necessity to use the ATP-PC system and the lactic acid system in combination with the aerobic energy system. Once these systems have been used and you move back into the aerobic energy zone, phosphagen must be remade and lactic acid removed. The aerobic energy system provides the energy for these recovery processes to occur.

Replenishment of Myoglobin

Myoglobin is a protein found in the muscle whose role it is to store oxygen and assist in its transport into the mitochondria of the muscle cell. When the muscle demands extra oxygen the myoglobin can release a maximum of 11mls of oxygen per kilogram of muscle tissue. Myoglobin is primarily found in Type1 fibers (ST) and gives the fibre its red coloring. While this is not a large amount, in situations when oxygen is required rapidly it can be significant, particularly in delaying the accumulation of lactic acid. In exercise that requires continual bursts of fast energy while in the aerobic zone, myoglobin can account for nearly 20% of the total ATP required. During recovery myoglobin is restored at the same time phosphagen is remade. The oxygen brought into the lungs and transported in the blood to the muscle is diffused across the muscle cell membrane binding to the myoglobin to be used again.

Restoring Muscle Glycogen

During high intensity aerobic exercise there is always a significant amount of glycogen converted to glucose and used a fuel for energy. If you are exercising every day or completing double workouts per day there is every likelihood that eventually you will deplete your glycogen stores. In the event this was to happen your performance would deteriorate significantly with your fuel usage now coming from fats or protein. Anyone who is participating in high intensity aerobic exercise can deplete liver glycogen by as much as 55%, while longer duration training of approximately two hours can completely empty glycogen stores. By consuming approximately 60 grams of carbohydrate during the high intensity, aerobic exercise, glycogen levels can improve performance by approximately 15-30%. Supplementing carbohydrates during aerobic exercise can spare muscle glycogen and maintain a stable level of blood glucose during aerobic exercise. The optimal benefits from carbohydrate feeding occurs at approximately 75% VO2max. To further increase muscle and liver glycogen storage levels, carbohydrate loading may also be considered. To initiate glycogen overcompensation, a diet rich in carbohydrates is eaten for approximately 3-4 days prior to the aerobic event. As high intensity aerobic exercise has the potential to significantly deplete glycogen, restoring it during recovery becomes very important. In the event that glycogen stores are replenished promptly at the rate of 5-7% per hour it will still take over 20 hours to restore levels back to pre-exercise conditions. Muscle glycogen repletion is most rapid during the first 10 hours of recovery.

Lactate Removal

When exercising aerobically at or close to the lactate threshold, a build-up of lactate will contribute to fatigue. During recovery it becomes increasingly important to rid the body of the lactate as quickly as possible.

It has been demonstrated that active recovery is more effective in reducing lactate levels post exercise than passive recovery. This is due to the increased blood flow to the muscle during active recovery, enhancing the transport of lactate from the muscle to the removal sites. The clearance of lactate during recovery is due to its oxidation to carbon dioxide and water, its transport to the liver where it is converted back to glucose, its conversion to glycogen in the muscle via gluconeogenesis and its role in providing energy for the recovery process. Active recovery exercise should not exceed 40% VO2max. This intensity can result in between 40-50% of lactate removal occurring within 15 minutes of recovery.


Work-to-rest ratios

Work-to-rest ratios are the time spent in exercise and the corresponding recovery time allocated to bring the body back to its normal functioning capacity. This ratio can occur between bouts of exercise, between exercise days or even between training cycles. The greatest training gains occur when a program is structured to allow for wave like progressions. Overloading sections should always be followed by a period of leveling off.

Aerobic prescription instructs you to follow a hard exercise day with an easier day or an exercise activity dissimilar to the one previously used. By using cross-training activities you are employing a form of active rest. Recruiting antagonistic muscles can accelerate the recovery process. Complete rest days should be planned within the program. For general health and fitness improvements no more than four sessions per week in any one training activity is necessary.

In more sophisticated aerobic exercise programs three progressively harder aerobic exercise sessions can be followed by a fourth easier recovery session. This type of overload is called a step overload. It encourages a small increase in workload for three sessions and unload for the last session of the week. Other programs will maintain the same training activity at the same workloads for the week and then increase the overall workload by between 5-15% the following week.


As a means of recovery athletes often use a wide range of physiotherapy modalities, including flotation tanks, spa baths, saunas, hydromassage, hot/cold contrast showers, whirlpools, ultrasound, heat, jacuzzi etc. Their main function is to relax the skeletal muscle and promote local blood flow.

Massage and Relaxation

A popular form of recovery is massage. Massage can take various forms including shaking, percussion, kneading, stroking and friction. Massage can reduce muscle tension, reduce stress levels, assist in venous and lymph drainage, increase blood flow, break up scar tissue, stretch soft tissue etc. Yoga and mental imagery have been used successfully to promote both physiological and psychological recovery.

Skill Level

Improving your skill level will reduce fatigue and enhance recovery. This is mainly due to the less energy needed per exercise movement in a more skilled performance. Some aerobic exercise modes will take longer to recover form than others. If you have selected running as a main training mode you will generally take longer to recover than those who are involved in non-weight-bearing exercises. This can be seen in the 21-day Tour de France where the equivalent amount of work could not be tolerated if running was the main training activity.


Sleep is regarded as one of the main methods of recovery. Normal sleep patterns should be maintained between workouts. In the event that two training sessions are scheduled on the same day, recovery can be enhanced by short naps between each workout.