O2 uptake and developed tension during and after fatigue, curare block, and ischemia

The purpose of this study was to investigate the effect of changes in developed tension on the ratio between O2 uptake and isometric developed tension in the in situ dog gastrocnemius-plantaris muscle group. O2 uptake and isometric developed tension of the muscle were measured during contractions at 1 twitch/s before and after fatigue with both single and twin impulses (6.5 V, 0.2-ms duration; the twins were separated by 10--20 ms). Twin impulses prior to fatigue raised developed tension to two times the tension developed with single impulses. Fatigue was obtained by stimulation at 10--20 impulses/s for 30--40 min. Twin impulses after fatigue returned developed tension to the level of single impulses before fatigue. O2 uptake and developed tension were also measured during the slower development of fatigue produced by continuous stimulation (3, 4, 5, and 6 impulses/s) as well as during "fatigue" induced by partial neuromuscular block with curare or ischemia. In all cases, there was no change in the O2 uptake per unit of tension developed, indicating a constant coupling between O2 uptake and developed tension.