Abstract
The rambling and trembling analysis separates the center of pressure (COP) fluctuations into two components: rambling (supraspinal contribution) and trembling (muscle stiffness / reflexive properties contribution). We examined whether the trembling component is correlated to the contractile properties (muscle stiffness and contraction time) of lower limb superficial skeletal muscles to experimentally test the rambling and trembling hypothesis. We hypothesized that muscle stiffness and contraction time, would be: (a) more correlated with; and (b) have a greater impact on the trembling component compared to the rambling component. Thirty-two healthy young adults were recruited for the study and tensiomyography was used to assess mechanical muscle responses to a single electrical stimulus to calculate muscle stiffness and contraction time based on radial muscle belly displacement measurements of lower limb muscles unilaterally. Moreover, upright postural control was assessed using a force plate to record ground reaction forces and moments and calculate the COP fluctuations during two 30 seconds trials. From the COP fluctuations, rambling and trembling time series were extracted, and all fluctuation time series were described using a number of different time-domain and frequency-domain parameters in both the anterior-posterior and medial-lateral directions. Our results demonstrated that both muscle stiffness and contraction time were moderately correlated with time-domain and frequency-domain parameters of the trembling component, as compared with those of the rambling component which was not as well correlated. Moreover, they also predicted the trembling component better. Overall, these results imply that postural control during quiet stance is, in part, related to intrinsic muscle stiffness in the lower extremities. Moreover, we showed that the rambling and trembling hypothesis is effective in separating postural sway fluctuations during upright posture to extract the contributions of muscle stiffness / reflexive properties (trembling), and likely the supraspinal contribution (rambling).
Highlights
Postural control is a complex mechanism in which multiple sensory systems, muscular activations, and passive dynamics are coordinated simultaneously within the central nervous system (CNS) [1]
It is proposed that the Rambling component reflects the processes of the CNS that are controlled by the supraspinal centers, whereas the Trembling component reflects the peripheral mechanisms of postural control system, such as spinal reflexes and/or passive mechanical properties of the muscles, ligaments and joints [5, 6]
The mean (SD) of contraction properties for all participants were: (i) rectus femoris (RF) Dm = 7.05 (2.25) and Tc = 25.31 (5.03); (ii) vastus medialis (VM) Dm = 6.82 (1.67) and Tc = 22.26 (4.05); (iii) vastus lateralis (VL) Dm: 5.21 (1.55) and Tc = 22.55 (4.29); (iv) biceps femoris (BF) Dm = 3.91 (2.52) and Tc = 36.39 (19.25); (v) gluteus maximus (GT) Dm = 7.31 (2.69) and Tc = 36.35 (8.97); and (vi) ST Dm = 6.15 (2.17) and Tc = 38.47 (10.34), where Dm was measured in mm and Tc in ms
Summary
Postural control is a complex mechanism in which multiple sensory systems (i.e., visual, vestibular, and somatosensory), muscular activations, and passive dynamics (i.e., ligament and joint stiffness) are coordinated simultaneously within the central nervous system (CNS) [1]. Previous studies demonstrated experimentally that Rambling and Trembling trajectories, separated from the COP fluctuations, reflect these separate components [5,6,7,8,9] These studies utilized a variety of conditions to manipulate feedback or task constraints, including visual feedback [7], joint fixation [8], and support surface area [9] during upright standing. Shin and Sosnoff [12] tested whether Trembling fluctuations could reflect impairment levels in individuals with spinal cord injury (SCI) during sitting balance, with the underlying hypothesis that people with different levels of SCI are more spastic compared to the control group due Muscle contractile properties and postural control to increased stiffness [13, 14]. Further work is warranted to directly test whether Rambling and Trembling fluctuations are correlated to stiffness during standing balance control
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