Hans Sjöholm

Energy metabolism and contraction force of human skeletal muscle in situ during electrical stimulation.

The quadriceps femoris muscles of nine volunteers were stimulated with intramuscular electrodes for 50 s. The stimulation frequency was 20 Hz and the voltage adjusted to produce an initial tension of 50‐75% of the maximum voluntary contraction force. The force decreased after 25‐30 s of contraction to reach a mean of 78% of the initial value after 50 s. During the stimulation up to four muscle biopsy samples were taken from each leg. The ATP turnover rate was initially 5.6 mmol kg‐1 dry wt. s‐1 and decreased during the later stages of contraction to 4.0 mmol kg‐1 dry wt. s‐1. The force declined at approximately the same rate as the ATP turnover during the contraction. The phosphocreatine (PCr) store decreased exponentially during contraction and was practically depleted at 50 s. Glycolysis began within 5 s after initiation of contraction. The rate of glycolysis measured as lactate accumulation increased with time throughout the entire contraction period.

The contents of high-energy phosphates in different fibre types in skeletal muscles from rat, guinea-pig and man.

1. The contents of high‐energy phosphates at rest have been measured in skeletal muscles with different fibre‐type composition from rat, guinea‐pig and man. All muscles studied biochemically have been characterized histochemically. 2. Fast‐twitch muscles had a higher ATP/ADP ratio than slow‐twitch muscles and, with the exception of the tongue in the rat, higher contents of ATP and phosphocreatine. 3. There was an inverse relationship between the content of phosphocreatine and the stainability for succinyl dehydrogenase, which is a marker enzyme for oxidative capacity. 4. The biochemical and histochemical data are discussed in relation to known morphological and functional properties of the different muscle‐fibre types. It is concluded that fast‐twitch fibres have a high ATP/ADP ratio favouring a fast acceleration of energy production. The content of phosphocreatine seems to be related to the glycolytic capacity but not to the contraction time. In addition to being an immediate energy source, phosphocreatine functions as a buffer against lactic acidosis.