Effects of global postural alignment on posture-stabilizing synergy and intermuscular coherence in bipedal standing

Clinicians frequently assess and intervene on postural alignment; however, notions of what constitutes good postural alignment are variable. Furthermore, the majority of current evidence appeals either to population norms or defines good postural alignment as the negation of what has been observed to correlate with pathology. The purpose of this study was to identify affirmative indicators of good postural alignment in reference to motor control theory. Electromyography (anterior leg, posterior leg, and trunk muscles) and motion capture data were acquired from 13 participants during 4 min bipedal standing trials in 4 conditions: control, - 10%, + 30%, and + 60% of subject-specific anterior limits of stability. Synergistic kinematic coordination was quantified via the uncontrolled manifold framework, and correlated neural drive was quantified in posture-relevant muscle groups (anterior, posterior, and trunk) via intermuscular coherence. Multilevel models assessed the effects of sagittal plane alignment on both outcomes. We observed a within-subjects fixed effect in which kinematic synergistic coordination decreased as subjects became more misaligned. We also observed within-subjects fixed effects for middle- and high-frequency intermuscular coherence in the posterior group (increased coherence with increased misalignment) and for trunk intermuscular coherence across all frequency bands (decreased coherence with increased misalignment). Our findings indicate that it may be possible to describe healthy postural alignment in light of referent control theory. Greater misalignment with respect to vertical is associated with compromises in synergistic control of posture and increased corticospinal drive to specific muscle groups. These results suggest that postural alignment may not simply be an empirical phenomenon.