Figure 5.

Muscle synergies extracted using rank two nonnegative matrix factorization (NMF) from a static knee extension task at 4 internal angles of the knee (0°, 20°, 60°, and 90°) [n = 17, mixed gender, female = 8, age range of 18–30 yr (24.4 ± 2.57 yr)]. Subjects performed 6 contractions of 5 s with the subject being asked to maximize rectus femoris activity. NMF was performed on the normalized surface EMG (sEMG) of each subject’s 6 contractions. The experiment was repeated across2 positions inactivating (red values) or activating (blue) contralateral hip flexors. Line charts are activation patterns identified by NMF as underlying structure in the original sEMG time series. Filled areas show SD. Bar charts show the contribution of the associated activation pattern to the activity of each of the 5 muscles in arbitrary units. Error bars represent SD. A: synergy 1 demonstrates the coordinated contraction across muscle groups in line with what is observed in the rectified and smoothed sEMG data. B: synergy 2 captures the inverse of the range of sEMG activity in each muscle. For both positions, at 0°, the antagonist muscles have significantly less activity than the agonists, which results in a high values for the antagonists. RF, rectus femoris; VL, vastus lateralis; VM, vastus medialis; ST, semitendinosus; BF, biceps femoris.