Figure 3.

Muscle activity during air stepping and muscle groupings. The top left panel (A) is a pictorial representation of extension and flexion during air stepping. The corresponding EMG muscle bursts are shown in the bottom left plot over one step for each muscle recorded during air stepping and referred to by their abbreviations given in Table 1. The polar equivalent muscle burst durations are seen in the top right plot (B); the first half of the step cycle is taken up by the average extensor burst duration (of all 4 extensor muscles), whereas the last third of the step cycle contains the average flexor burst duration (between all 3 flexor muscles). The bottom right panel (C) shows that muscles clustered into two groups based on their burst onset and offset times. This tight grouping allowed us to use one extensor EMG and one flexor EMG as representative signals for each phase of air stepping.