Influence of constrained visual and somatic senses on controlling centre of mass during sit-to-stand
Highlights
► We examined sit-to-stand movement under constraining vision and somatic senses. ► The variability of velocity and position of Center of mass (COM) were computed. ► Velocity's variability in two major directions decreased when constraining two senses. ► Position's variability in the right-left direction increased. ► The nervous system sets priorities for COM control in the moving direction.
Introduction
Perception of postural orientation, which integrates and assesses afferent information from peripheral sensors, is essential for executing motor tasks [1]. With this perception, healthy individuals can perform daily activities unfailingly in different body and environmental situations despite an inherently unstable body structure [2]. Particularly, perception–action coupling is well coordinated during daily routine movements such as stepping to walk and grasping, which are well-practised and skilfully performed [3], [4]. In contrast, patients with motor disorders frequently demonstrate inappropriate coordination between perception and action, which results in an inability to perform motor tasks even with minimal disturbance or minimally changing environmental situations [5], [6], [7]. Therefore, understanding perception–action coupling under conditions of perceptual constraints is important to improve performance of motor tasks and to treat patients with motor disorders.
Previous authors have investigated the relationship between static posture control and perception [8], [9]. In addition, most previous studies on the relationship between perception and action have focused on focal movements of the restrained upper extremities in stable sitting positions [10], [11], and not on whole-body movements performed during daily living activities. Whole-body dynamics have often been studied via a mechanical approach, such as analyses of joint motion at a physical level [12], [13]. However, using the uncontrolled manifold (UCM) hypothesis, Reisman et al. analysed a dynamic whole-body movement under varying support surface conditions which influenced the mechanical and perceptual situation [14]. In the Bernstein problem [15], motor variability is organised such that important task-related variables are preferentially controlled by the nervous system. However, they focused on the mechanics of motor coordination to achieve the task goal rather than on the relationship between perception and action.
The close relationship between perception and action, particularly for postural orientation, is believed to transform the unstable structure into a controllable system [1]. Multiple modalities including visual, somatosensory, and vestibular systems are redundantly involved to perceive postural orientation [16]. For example, the visual system provides information on the position and motion of the body with respect to surrounding objects, and the somatosensory system provides position and motion information about the body with respect to the supporting surfaces [2], [16]. For this reason, utilising this type of information is important for motor regulation. Among whole-body tasks, the sit-to-stand (STS) motion is one of the most mechanically demanding daily activities [7]. Centre of mass (COM) control is assumed to be a goal of postural responses [17]. Scholz et al. [18] suggested that the body COM is a control variable for the postural system quantitatively. COM control is important during STS, the period when the body mass is raised from relatively stable support to a position of lower mechanical stability [19].
This study aims to investigate the influence of visual and somatosensory constraints on COM control during STS. The concept of stability in the control-theoretical sense suggested by Scholz [20] was applied to evaluate COM control in this study. Using this concept, stability can be assessed experimentally by using the variability of the corresponding variable in time such as the fluctuation of a fixed point for postural states [21], [22] or by using the reproducibility of that variable from trial to trial such as kinematics at matching points in time during movement [23]. Those are assumed to assess nervous system control. The lesser the value of the measured variable, the more the central nervous system controls that variable [20]. It was hypothesised that the variability in position and velocity of COM in the anterior–posterior and upward–downward directions, which are the major moving directions during STS, should be reduced when both visual and somatic senses are constrained during a critical controlling period.
Section snippets
Participants
Twelve healthy adults aged 30.7 ± 6.4 years (six females and six males) participated in this study. Weight and height of the participants were 61.0 ± 11.4 kg and 167.7 ± 6.8 cm, respectively. None of the participants had a neurological disease or visual problems. All participants gave written consent, which was approved by the institutional ethics committee, before participating in the experiments.
Equipment and set-up
A MAC 3D System (Motion Analysis, Santa Rosa, CA, USA) motion measurement device and four force plates
Experimental task achievement
There were no participants who felt anxiety, and all participants maintained their balance in all trials. Four trials were deemed unsuccessful because of stool movement, starting before the buzzer and unclear end of movement. Because those trials were eliminated, 476 of the 480 trials in12 participants were analysed.
The mean movement time was 1120–2440 ms, whereas the mean SD of the movement time was 90–190 ms. The variability within individual participants was much smaller than the variability
Discussion
In this experiment, the length of the movement time and timing of T1 and T2 was almost the same within each participant for all four conditions. This finding suggests that the unstable wooden board did not affect performance despite various perceptual constraints. During STS, COM was accelerated forward in the sitting position before T1 and was decelerated upward within the base of support in the standing position after T2. Movements during both periods were relatively stable and simple.
Conflict of interest statement
None.
Acknowledgements
We would like to thank Mr Akira Motokawa and Ms Rikako Isawa for their assistance with data collection, Dr Sumiko Yamamoto, Mr Takeshi Seki, Dr. Sanae Asahara, Mr Kazunori Sasaki and Dr Yoshinobu Ehara for their technical advice, and Dr Kazutoshi Kudo, Dr Haruhiko Sato, Dr Motoko Tanabe, Dr Yoshito Furusawa and Dr Yoshimi Suzukamo for their helpful comments.
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