A simple comparison between a marathon runner and a weight lifter shows that an increase in contractile activity can trigger very different adaptations in skeletal muscle. We investigated whether a selective activation of AMP kinase (AMPK) or protein kinase B (PKB; Akt) and its downstream translation factor p70 S6 kinase (p70 S6k) could be responsible for phenotypic or growth adaptations to different forms of exercise.Male rats were humanely killed and extensor digitorum longus (EDL) or soleus muscles were placed in oxygenated Krebs-Henseleit buffer. Muscles were electrically stimulated either continuously at 10 Hz for 3 h (endurance training-like stimulation; ETS) or for 10 sets of 6 repetitions of 100 Hz bursts, each of 3 s duration, with 10 s recovery between repetitions and 1 minute of recovery between sets (resistance training-like stimulation; RTS). Repeated RTS was previously shown to induce hypertrophy in vivo whereas ETS resulted in endurance training-like adaptations (Nader & Esser, 2001). The phosphorylation of AMP kinase (AMPK) at thr172, of protein kinase B (PKB/Akt) at thr308, and of the translation regulator p70 S6k at thr389 were estimated by densitometry of Western blots on protein extracts and calculated relative to control. The selective activation of AMPK by ETS is consistent with the AMPK-dependent activation of mitochondrial biogenesis and other adaptations commonly observed in response to endurance training. In contrast, the selective activation of PKB and p70 S6k by RTS is consistent with the increase in protein synthesis that is observed after resistance training. To conclude, the AMPK-PKB switch is likely to be an important switch that regulates specific adaptation to endurance and resistance training. The results also indicate that stimulation of isolated skeletal muscle is a suitable model for investigating signalling responses to exercise.