Initial evidence for the mixing and soft assembly of the ankle, suspensory, and hip muscle patterns

The onset latencies of automatic muscle responses to backward translations of the support surface in standing subjects were assessed to test two longstanding hypotheses: that (1) the ankle, hip, and suspensory muscle patterns represent discrete synergies; and (2) the suspensory pattern cannot be mixed with the ankle and the hip patterns (based on theoretical modeling). Muscle responses were recorded at the leg (medial gastrocnemius and tibialis anterior), thigh (hamstring and quadriceps), lower trunk (paraspinals and abdominals), and the upper trapezius muscles. A principal factor analysis with orthogonal varimax rotation was conducted to describe the variation among the muscle onset latencies by means of the smallest possible set of newly defined variables, or factors. Muscles that were correlated with each other were grouped into a factor. The analysis produced three factors with eigenvalues > or =1, which accounted for 81% of the total variance. Based on the first muscle response in each factor, the muscle patterns resembled the hip, ankle, and suspensory synergies. Muscle responses within a synergy were activated before the completion of the muscle responses in the other synergies, indicating that the synergies, though mixed, are triggered separately. This is consistent with the hypothesis that the ankle, hip, and suspensory patterns represent discrete synergies. On the other hand, the results do not support the hypothesis that the three patterns cannot be combined.