Fig. 4. Inter-individual (top) and intra-individual (bottom) variation in cumulative energy production by lateral gastrocnemius (LG, left) and digital flexor-4 (DG-IV, right) over the locomotor cycle, shown for level running at 1.3 m s^-1. The final value is the net work generated per stride. The traces of average cumulative work output shown across individuals (birds 4, 5 and 6, top) encompass the range of inter-individual variation for both muscles. The mean ± S.D. of energy production over the normalized stride from one individual (bird 4, bottom) illustrates typical levels of stride-to-stride variation for each muscle.