a–f, Data from wildebeest. g–k, Data from cows. Relationship between stimulus phase and efficiency during three cycles of movement at 0.5 Hz for stimulus duty cycles (DC). Circle, DC = 0.4; square, DC = 0.3; triangle, DC = 0.2; diamond, DC = 0.1. Efficiency = power per rate of heat + work output. a, b, d, Data are from a different muscle fibre bundle each from a different wildebeest. c, Data are the average of the values shown in e and f, which are results for two fibre bundles from the same wildebeest. g–k, Data are from a different fibre bundle from a different cow.