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Fig. 3 The effects of increasing stimulation frequency on cardiac contractility variables. (A and D) twitch force, (B and E) Raterise, and (C and F) Rate50% in the 4 wk of adult hypoxia and lifelong hypoxia studies. Control data are represented by the open symbols and experimental groups represented by filled symbols. *Significant difference from the values at 2 Hz; **significant difference from the immediately previous step of stimulation frequency in the control fish. #Significant difference from the values at 2 Hz; ##significant difference from the immediately previous step of stimulation frequency in the hypoxic fish only. Data are presented as means ± SE. Sample size for this protocol was 8 each for control fish and those that experienced 4 wk of 10 kPa of O2. Sample size was also 8 for the control fish and 9 for the fish that were maintained in 10 kPa O2 over their entire development.