Figure 4. Varimax factors derived from muscle activity patterns by principle component analysis (PCA) during locomotion.

A, 5 varimax factors derived for each of 6 subjects (different colours) from the recordings made with 4 different speeds (1, 2, 3 and 5kmh⁻¹). Stance phase was somewhat different across subjects and speeds as indicated by hatched area of the stance–swing bar. B, the factors derived from the recordings made from one subject for the 9 leg muscles plus 6 trunk muscles at one speed (5kmh⁻¹) are compared using either the intramuscular or surface recordings of leg muscle activity (Fig. 1). C, red dashed traces show the factors averaged across the 6 subjects in A. Results are compared with the Winter factors (derived from the activity of 25 leg and trunk muscles illustrated in Fig. 2) (black traces) and with the result of a PCA of EMGs recorded from 16 leg muscles (Davis & Vaughan, 1993) (blue traces) and from 8 muscles in each leg (Olree & Vaughan, 1995) (dotted traces). D, the cumulative percentage of variance explained by each factor is shown for each subject and for the Winter factors.