PROGRESSIVE EFFECT OF ENDURANCE TRAINING ON METABOLIC ADAPTATIONS IN WORKING SKELETAL-MUSCLE

We investigated the hypothesis that a program of prolonged endurance t raining, previously shon to decrease metabolic pertubations to acute e xcercise within 5 days of training, would result in greater metabolic adaptations after a longer training duration. Seven healthy male volun teers [O-2 consumption equals 3.52+/-0.20(SE l/min] engaged in a train ing program consiswting of 2h of cycle excersise at 59% of pertaing pe ak O-2 consumption (V(O-2)peak) 5-6 times/wk. Responses to a 90-min su bmaximal excersise challenge were assessed pretraining (PRE) and after 5 and 31 days of training. On the basis of biopsies obtained from the vastus lateralis muscle, it was found that, after 5 days of training, muscle lactate concentration, phosphocreatine (PCr) hydrolysis, and g lycogen depletion were reduced vs. PRE (all P<0.01). Further training (26 days) showed that, at 31 days, the reduction in PCr and tha accumu lation of muscle lactate was even less than at 5 days (P<0.01). Muscle oxidative potential, estimated from the maximal activity of succinate dehydrogenase, was increased only after 31 days of training (+41%; P< 0.01). In addition, (V(O-2)peak) was only increased (10%) by 31 says ( P<0.05). The results show that a period of short-term training results in many characteristic training adaptations but that these adaptation s occured before increases in mitochondrial potential. However, a furt her period of training resulted in further adaptations in muscle metab olism and muscle phosphorylation potential, which were linked to the i ncrease in muscle mitochondrial capacity.