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Energy Sources for Muscular Activity

Key words

energy continuum

energy sources for exercise

aerobic energy sources

anaerobic energy sources

protein synthesis

protein degradation

1.1 Adenosine Triphosphate: The Energy Currency

In order for muscles to contract and provide movement, energy is required. Such energy is provided by adenosine triphosphate (ATP) and is the only energy capable of being used for muscle contraction in humans. Figure 1.1 provides the structure of an ATP molecule. As you can see from this diagram, ATP consists of a base (adenine) attached to a sugar (ribose), to which is attached three phosphate molecules. The phosphates are attached by ‘high energy’ bonds which, when removed, provide energy.

The process is reversible, which means that ATP may be re-formed from adenosine diphosphate (ADP) as long as there is sufficient energy to restore the missing phosphate molecule on to the ADP. The latter can be achieved by phosphocreatine (PCr) or by processes such as anaerobic glycolysis, and aerobic processes.

The stores of ATP in muscle tissue are rather limited, so there is a constant need to resynthesize it for survival, let alone movement. The amount of ATP in a muscle cell amounts to 25 mM/kg dry muscle or about 40–50 g in total, which is sufficient to enable high intense activity for around 2–4 seconds if it is the only useable source of energy available. This is not a great amount – hence the importance of resynthesis of ATP at rates sufficient to enable appropriate levels of exercise to ensue, i.e. fast rates of resynthesis for sprinting and slower rates for prolonged exercise.

1.2 Energy Continuum

The major energy sources for exercise are dependent on the intensity and duration of the activity. Examination of Figure 1.2 highlights that there appears to be three such sources, i.e. PCr, glycolytic and aerobic. These energy-producing processes predominate exercise from 1 to 10 seconds, 10 to 60 seconds and beyond 60 seconds respectively.

Another way of expressing the energy continuum is represented in Figure 1.3, which shows the major energy sources for running events of varying distances. Note that short, highly intense sprinting bouts lasting 1–10 seconds use PCr predominantly, while events such as the 400 m mainly use anaerobic glycolysis, and thereafter aerobic metabolism predominates.

Figure 1.1 Adenosine triphosphate (ATP).

Figure 1.2 Energy continuum.

Figure 1.3 Primary energy sources for different running distances.

1.3 Energy Supply for Muscle Contraction

ATP is not stored to a great degree in muscle cells. Therefore, once muscle contraction starts, the regeneration of ATP must occur rapidly. There are three primary sources of ATP; these, in order of their utilization, are PCr, anaerobic glycolysis and aerobic processes.

Energy from ATP derives from cleaving the terminal phosphate of the ATP molecule. The resulting molecule is ADP. PCr converts ADP back to ATP by donating its phosphate in the presence of the enzyme creatine kinase (CK), and in turn the PCr forms creatine (Cr), i.e. the dephosphorylated form of PCr.

The reaction of PCr with ADP to form ATP is very rapid, but is short-lived since the cell does not store high amounts of PCr (the muscle concentration of PCr is about 80 mM/kg dry muscle or 120 g in total). However, during short, high-intensity contractions, PCr serves as the major source of energy. This form of energy generation is sometimes referred to as anaerobic alactic, because it neither produces lactic acid nor requires oxygen. It is of paramount importance in sports requiring bursts of speed or power, such as sprints of 1 to10 seconds, lifting weights, engaging in a high/long jump or a throw in an athletics field event.

Figure 1.4 provides a schematic to show the synthesis of ATP from ADP using PCr at the muscle crossbridge, as well as the regeneration of PCr from Cr by ATP at the mitochondria. This is known as the ‘PCr shuttle’.

Thus, Cr is produced from PCr during intense bouts of exercise, while Cr is re-phosphorylated to PCr by ATP produced in the mitochondria during an aerobic recovery phase. Oxygen is needed for recovery of PCr, as can be seen in Figure 1.5, which clearly demonstrates that recovery of exercise-depleted PCr only happens when the blood supply to the exercising muscle is not occluded, i.e. there is an intact blood supply taking oxygen to the cells. If the blood supply is occluded (e.g. via a tourniquet), then PCr resynthesis fails. As a consequence, there is the need for a low level (so-called active) recovery in between bouts of intense exercise.

The enzyme CK, which regulates PCr activity, exists in a number of forms known as isoforms (this will be dealt with later). Note that not only is there a CK which favours the formation of ATP from PCr, but there is also another form, CKmito, which is present at the mitochondria and favours the synthesis of PCr from Cr using ATP. In effect, the same enzyme (CK) but in different isoforms which results in either the breakdown or synthesis of PCr.

You should also note from Figure 1.5 that there is a rapid loss of PCr during intense exercise and that it is rapidly recovered (PCr stores may even be depleted if the exercise is sufficiently intense or prolonged). Nearly 75% of PCr is resynthesized within the first minute of recovery and the rest over the next 3–5 minutes. The graph is biphasic, i.e. rapid restoration at first, then a second, slower phase.

Figure 1.4 PCr shuttle (mi-CK is mitochondrial CK; mm-CK is skeletal muscle CK; CrT is creatine transporter).

Figure 1.5 Resynthesis of PCr after exercise with and without an occluded blood supply

(adapted from Hultman et al., 1990).

As soon as muscle contraction starts, the process of anaerobic glycolysis also begins. Anaerobic glycolysis does not contribute as large an amount of energy as PCr in the short term, but its contribution is likely to predominate from 10 to 60 seconds.

During glycolysis, locally stored muscle glycogen, and possibly some blood-borne glucose, supply the substrate for energy generation. Glycolysis takes place in the cytoplasm, where no oxygen is required, so the process is called anaerobic. It has been referred to as ‘anaerobic lactic’, since lactic acid is formed as the end product. Sufficient lactic acid formation can lower the pH of the cell (i.e. make it more acid) to the extent that further energy production may be reduced.

The major substrate for anaerobic glycolysis (see equation below) is glycogen stored within the muscle, so prior hard exercise without adequate repletion of glycogen will limit further high-intensity short-term work.

Exercise beyond 60 seconds requires mainly aerobic energy sources, such as the complete oxidation of glucose or fatty acids to carbon dioxide and water. These processes necessitate oxygen and take place in the mitochondria of the cells. The equations below illustrate the essence of aerobic metabolic reactions:

More detail about these processes are presented in Chapters 5 and 6.

Figure 1.6 Carbohydrate and fat use at three exercise intensities

(adapted from Romijn et al., 1993).

Aerobic activities invariably occur at lower exercise intensities (which are those lasting longer than one minute), and the contributions of carbohydrate and fat at these levels of intensity can be realized in Figure 1.6. Note that fats contribute a greater percentage (and amount) of energy at 25% VO2max (i.e. walking pace) than carbohydrate, around 50% of the energy at 65% VO2max (i.e. steady state pace), and around 25% of the energy at 85% VO2max (i.e. an intense aerobic bout with some significant anaerobic energy involved).

1.4 Energy Systems and Running Speed

Based on world record times, humans can maintain maximum sprinting speed for approximately 200 m. The average speeds for the 100 and 200 m world records are similar, at 22.4 and 21.6 mph, respectively. However, with increasing distances, average speeds decline. The average speed for the marathon world record is about 12 mph, which is 55% of the world sprint record. This is quite a remarkable pace, since the marathon distance is more than 200 times the length of a 200 m race.

Although natural selection plays a crucial role in elite sprinting and marathon performance, the energy systems must also be highly trained and exercise-specific to be successful. For example, the energy needed to maintain an average sprinting speed of 22 mph for 200 m or less, and that required for an average running speed of 12 mph for the marathon, are acquired by two very different systems (the predominant energy systems required for running at different speeds can be seen in Figure 1.3). The primary energy source for sprinting...