POSSIBLE EFFECTS OF FATIGUE ON MUSCLE EFFICIENCY

The efficiency of energy transduction is defined as the ratio of the w ork done by a muscle to the free energy change of the chemical process es driving contraction. Two examples of the experimental measurement o f muscle efficiency are: (1) the classical method of Hill which measur es the value during a steady state of shortening, (2) measuring the ov erall efficiency during a complete cycle of a sinusoidal process, whic h comes closer to the situation during natural locomotion. The reasons why fatigue might lower efficiency are the following. (1) The reducti on in PCr concentration and increase in Pi and Cr concentration which are characteristic of fatigued muscle, reduce the free energy of PCr s plitting. This will reduce the efficiency of the recovery process. It is not known whether the efficiency of the initial process is increase d to compensate. (2) There is a general conflict between efficiency an d power output when motor units are chosen for a task or when the timi ng of activation is decided. During fatigue more powerful units have t o be used to achieve a task which is no longer within the scope of les s powerful units. (3) The slowing of relaxation that is sometimes foun d with fatigue may make it impossible to achieve the short periods of activity required for optimum efficiency during rapid cyclical movemen ts. A reason why fatigue might increase efficiency is that muscles are thought to be more efficient energy converters when not fully activat ed than when fully active. Full activation is often not achieved in mu scle which is considerably fatigued. Available observations do not all ow us to find where the balance between these factors lies. The conclu sion is thus that experiments of both the types discussed here should be performed.