Ryschon, T. W., J. C. Jarvis, S. Salmons, and R. S. Balaban. High-energy phosphates and tension production in rabbit tibialis anterior/extensor digitorum longus muscles. J. Appl. Physiol. 82(3): 1024-1029, 1997.The effects of repetitive muscle contraction on energy state and tension production were studied in rabbit tibialis anterior/extensor digitorum longus muscles that had been subjected to 90 days of continuous indirect electrical stimulation at 10 Hz. Anesthetized chronically stimulated and control rabbits were challenged with 15 min of stimulation at 4 and 15 tetani/min. P_i-to-phosphocreatine (PCr) ratio (P_i/PCr) was measured in vivo before, during, and after acute stimulation by ³¹P-magnetic resonance spectroscopy, and tension was recorded at the same time. Although P_i/PCr was low at rest, it was significantly higher in chronically stimulated muscle than in control muscle (0.20 ± 0.02 vs. 0.05 ± 0.01, P < 0.05). Stimulation of control muscle for 15 min at both 4 and 15 tetani/min induced a significant rise in P_i/PCr, whereas the same conditions in chronically stimulated muscle did not produce any significant departure from initial levels. The tension produced by control muscle fell to 93 ± 3% of its initial value during stimulation at 4 tetani/min and to 61 ± 7% at 15 tetani/min, respectively. In chronically stimulated muscle, on the other hand, tension was potentiated above its initial level at both stimulation rates (135 ± 15 and 138 ± 11%, respectively) and remained significantly elevated throughout each trial. The ability of chronically stimulated muscle to sustain high levels of activity with minimal perturbations in P_i/PCr or decrement in tension is attributable to cellular adaptations that include a well-documented increase in oxidative capacity.