Mitochondrial coupling control ratios of permeabilized skeletal muscle fibers from gluteus medius and triceps brachii. A: L/P coupling control ratio (LEAK/PCI+II) of gluteus medius and triceps brachii from young (n = 18 for gluteus, n = 12 for triceps) and aged (n = 9 for gluteus, n = 8 for triceps) American Quarter Horses. B: P/E coupling control ratio (PCI+II/ETS capacity) of gluteus medius and triceps brachii from young (n = 18 for gluteus, n = 12 for triceps) and aged (n = 9 for gluteus, n = 9 for triceps) American Quarter Horses. Values are means ± SE. Open bars represent young horses; solid bars, aged horses. Young vs. aged: †P < 0.1.