Segmental trunk and head dynamics during frontal plane tilt stimuli in healthy sitting adults

Abstract

A more detailed understanding of trunk behavior during upright sitting is needed to create a foundation to address functional posture impairments. Therefore, we characterized the dynamics of the trunk and head during perturbed sitting. A three-link inverted pendulum model of head and trunk segments was used to analyze kinematics of eight healthy sitting adults. Magnetic sensors were placed at the head and two locations of the trunk (C7 and T7). Six surface tilt stimuli (two spontaneous sway tests [no surface stimulus; eyes open, EO/eyes closed, EC] and four tests with continuous pseudorandom surface tilts [2 peak-to peak amplitudes of 2° or 8°; EO/EC]) were applied in the frontal plane. We used frequency-response functions (FRFs) to analyze sway across ~0.045–3Hz and found systematic differences in sway dynamics across segments. Superior segments exhibited larger fluctuations in gain and phase values across frequencies. FRF gains in superior segments were attenuated compared to other segments only at low frequencies but were larger at the higher frequencies. We also tested the influence of stimulus amplitude and visual availability on FRFs. Across all segments, increasing stimulus amplitude and visual availability (EO) resulted in lower gains, however, these effects were most prominent in superior segments. These changes in gain were likely influenced by changes in sensory reliance across test conditions. In conclusion, these results provide a benchmark for future comparisons to segmental responses from individuals with impaired trunk control. We suggest that a frequency-based approach provides detail needed to characterize multi-segment dynamics related to sensorimotor control.

Specificity and variability of trunk kinematics on a mechanical horse

“…Synergistic activation could account for the stereotypical and directionally specific responses (Torres-Oviedo & Ting 2007). Also, these linear responses are consistent with previous posture studies that elicited sway using a distinct mode of stimuli (surface tilt, visual tilt, or galvanic vestibular stimulation) in either the sagittal or frontal plane and found responses at discrete frequencies corresponding to the stimuli in that plane of motion (Cenciarini & Peterka 2006;Kiemel et al 2008;Goodworth & Peterka 2009;Peterka 2002;Mergner et al 2005;Wu et al 2016). In response to either frontal or sagittal plane stimuli, the typical pattern of gains and phases across frequency involve gain increases from about 0.01 to 0.8Hz followed by gain decreases at frequencies above 1Hz (Goodworth & Peterka 2010;Peterka 2002).…”

Sensorimotor control of the trunk in sitting sway referencing

“…All subjects reported the backboard to be comfortable. The backboard provided an unambiguous kinematic signal of body tilt to control bench motion during sway referencing (compared with multisegmental motion where the head and trunk segments may be moving differently; Wu et al 2016). The backboard facilitated interpretation of results, because in a single-link upper body and head model, center of mass motion is directly related to visual and vestibular cues.…”

“…The human trunk (head, arms, torso, and pelvis) comprises over half of the body's mass (de Leva 1996), and posture control of the trunk is a foundational motor skill because of its importance in nearly all voluntary activities (Edwards et al 2017;Karthikbabu et al 2012;Rachwani et al 2015;Teng and Powers 2014). Trunk posture requires sensory feedback from visual, vestibular, and somatosensory systems (Andreopoulou et al 2015;Goodworth and Peterka 2010b;Maaswinkel et al 2015;Wu et al 2016) and can be influenced by reflexes and intrinsic biomechanical properties of the trunk (Brown and McGill 2009;van Drunen et al 2015). Impairments in trunk control can have a major debilitating effect on daily activities and socialization.…”