Muscular exercise

The speed at which someone can run is determined in part by the speed with which they can convert their stores of chemical energy into mechanical energy. These stores may be within the muscle or outside it, and they may or may not require oxygen from outside the cell in order to be utilized. The three main energy stores are as follows.

Fig. 9.15. The relation between energy production (heat plus work) and creatine phosphate breakdown in frog sartorius muscles poisoned with iodoacetate and nitrogen. Each point represents a determination on one muscle after the end of a series of contractions, with different symbols for different types of contraction. From Wilkie (1968).

Fig. 9.15. The relation between energy production (heat plus work) and creatine phosphate breakdown in frog sartorius muscles poisoned with iodoacetate and nitrogen. Each point represents a determination on one muscle after the end of a series of contractions, with different symbols for different types of contraction. From Wilkie (1968).

1 ATP and creatine phosphate in the muscle. This is the short term energy store, amounting to about 16 kJ or so in the human body, perhaps enough for a minute of brisk walking.

2 Glycogen in the muscle and the liver. This provides a medium term store of very variable size: a value of 4000 kJ would not be out of the ordinary, providing enough energy for some hours of moderate exercise.

3 Fat in the adipose tissue. This provides a long term store; 300000 kJ might be a typical value.

The high energy phosphate store can be immediately utilised by the contractile apparatus of the muscle. The energy in the fat store and most of that in the glycogen store can only be utilised by aerobic respiration, for which oxgyen has to be transported to the cell. The rate at which these two energy stores can be tapped is therefore limited by the rate at which oxygen can be supplied to the cell. It is for this reason that the maximum running speeds for short distances cannot be maintained over long distances (Fig. 9.16).

The muscles' increased need for oxygen during steady exercise is served by the well-known physiological changes which occcur in the body during exercise. The rate and depth of breathing increases, the heart rate and stroke

Fig. 9.16. How the average speed varies with distance run. The points are determined from world records for men as they were in October 1987, with the distances plotted on a logarithmic scale.

volume increase, and the blood supply to the muscles is increased. The concentration of free fatty acids in the blood rises, as a result of the hydrolysis of some of the fat stored in the adipose tissue. There is also some mobilisation of the glycogen reserves of the liver.

Some of the glycogen in the muscle can contribute to the short term energy store since it can be utilized without oxygen supplied from outside. Anaerobic respiration, producing lactic acid, supplies 3 moles of ATP per glucose unit used. This is a much less effective process than aerobic respiration, which supplies 37 moles of ATP for each glucose unit. There is also some oxgyen bound to myoglobin in the muscle; this might amount to 0.41 or so, enough for a few seconds of maximal exercise.

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