Effect of training on the activity of five muscle enzymes studied on elite cross-country skiers.

This study examines the effect of training intensity on the activity of enzymes in m. vastus lateralis. Elite junior cross-country skiers of both sexes trained 12-15 h weeks -1 for 5 months at either moderate (60-70% of Vo 2max , MIG) or high training intensity (80-90% of the Vo 2max , close to the lactate threshold; HIG). Muscle biopsies for enzyme analyses and fibre typing were taken before and after the training period. Histochemical analyses on single fibres were done for three enzymes (succinate dehydrogenase ISDH], hydroxybutyrate dehydrogenase [HBDH], glycerol-3-phosphate dehydrogenase IGPDHI), while the activity of citrate synthase [CS] and phosphofructokinase [PFK] was measured on whole biopsies. The activity of GPDH was low in ST fibres and high in FT fibres. The activity of SDH and HBDH was high in both ST and FTa fibres but low in the FTb fibres The HIG increased their performance more than the MIG did during the training period as judged from scores on a 20-min run test. The SDH activity rose by 6% for the HIG (P 0.27). There were positive correlations between the activity of three of the enzymes (CS. SDH and GPDH) and the performance parameters (Vo 2max , cross-country skiing and running performance; r > 0.6, P 0.05). This study suggests that intensities near the lactate threshold affect biochemical and physiological parameters examined in this study as well as the performance of elite skiers, and that the rate-limiting enzymes may be more sensitive to training than non-rate-limiting enzymes.