| Summary: | Purpose. Sensory feedback from the vestibular system and neck muscle stretch receptors is critical for the regulation of posture. The relationship of the head to the trunk is a major factor determining the availability and integration of sensory feedback and can be interfered with by varying head orientation. The goal of this research was to assess (1) how adopting different head-on-trunk orientations would impact postural stability, particularly in relation to the stability boundary, during static balance tasks and (2) how adopting different head-on-trunk and head-in-space orientations would impact postural stability, movement characteristics, and multi-segmental torso coordination during a dynamic postural transition task in healthy, young participants. Methods. Healthy, young participants were asked to maintain 30 seconds of upright stance and forward lean or to move from sitting to standing with extended, flexed, and neutral head orientations. Dual force plates were used to assess postural stability from center of pressure variability, range, velocity, or time-to-contact. Six motion capture cameras were used to assess kinematics. During the sit-to-stand task, head velocities, trunk flexion, and movement phase durations were calculated. Segment cross-correlation and joint range of motion were calculated for six torso segments. Results. Extended head-on-trunk orientations decreased postural stability during upright stance, forward lean, and the sit-to-stand movement compared to flexed or neutral orientations. During the sit-to-stand task, head-on-trunk extension, with or without head-in-space extension, led to reduced head velocities, trunk flexion, movement duration, and transition phase duration. Head extension led to increased inter-segmental torso motion, and decreased temporal coordination of torso segments. Conclusions. This study demonstrated that interfering with head-trunk posture by adopting head extended orientations impairs balance and leads to sit-to-stand strategy changes that may interfere with movement and coordination. Results show that head-on-trunk extension is more critical than head-in-space extension for determining postural and movement changes. The findings suggest that vestibular system interference may not be the main route through which head extension impacts postural control, but that extensor muscle stretch receptors may be a factor in the posture and movement changes associated with head-on-trunk extension. It is possible tonic neck muscle activity is a critical factor for regulating balance and movement.
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