Swaying to the complex motion of a visual target affects postural sway variability

Background Voluntary shifting body weight in the anteroposterior direction is an important element of daily life activities, such as rising from a chair or initiating a step. In order to accommodate the daily-life challenges of such tasks, voluntary postural sway needs to be flexible and variable. Research question In this study we asked how whole-body tracking of a complex visual target motion with the concurrent provision of feedback modulates the variability of voluntary sway. Methods Twenty young adults (age: 27.10 ± 9.15years, height: 170.73 ± 9.40 cm, mass: 62.84 ± 11.48 kg) performed 132 cycles of voluntary antero-posterior sway, on a force platform, under two conditions: a) self-paced sway and b) swaying while tracking the complex motion of a visual target. Magnitude and temporal structure of variability of postural sway were investigated with the Coefficient of Variance (CoV) and the fractal exponent α, respectively. This analysis was performed for sway cycle duration, amplitude and velocity. The cross-correlation function between the target and sway cycle parameters was computed as a measure of visuo-postural coupling. Results The CoV of sway cycle amplitude, duration and velocity increased during active tracking of the complex target. Fractal exponent α increased for sway cycle amplitude but decreased for cycle duration and remained unchanged for sway velocity. The cross-correlation function revealed a consistent peak at lag+1 indicating an asynchrony between the target and sway cycle duration, while the peak cross-correlation for cycle amplitude was noted at lag 0. Significance Swaying to the complex motion of a visual target improves the variability of sway cycle amplitude, at the cost of cycle duration. This is associated with a more synchronous spatial than temporal coupling to the visual target motion. This knowledge could inform the design of postural tracking paradigms as appropriate exercise interventions, for improving voluntary sway in populations with reduced limits of stability (i.e. older adults)

Age induced modifications in the persistency of voluntary sway when actively tracking the complex motion of a visual target

Movement persistency, reflected in systematic cycle to cycle fluctuations of a rhythmical task such as walking or voluntary sway, is compromised with increasing age, making older adults more susceptible to falls. In the present study, we tested whether it is possible to improve rhythmic voluntary sway persistency in old age by actively tracking the complex (i.e. persistent) motion of a visual target. Twenty healthy young and 20 older adults performed 132 cycles of anterior-posterior sway under two conditions: a) self-paced sway and b) sway while tracking the vertical motion of a complex visual target. The persistency of sway cycle amplitude and duration, detected from the center of pressure displacement, was quantified using the Fractal exponent α. We also recorded body kinematics in order to assess the intersegmental coordination that was quantified in the Mean Absolute Relative Phase (MARP) and the Deviation Phase (DPh) between the trunk and the lower limbs. In self-paced sway, older adults showed a lower persistency of cycle duration and a higher MARP and DPh between the trunk and the lower limbs compared to young adults. Tracking the complex visual target motion increased the persistency of cycle amplitude, in young but not in older adults, when compared to the self-paced sway while it decreased the persistency of cycle duration in both groups. The relative phase measures showed a moderate to strong relationship with the persistency of cycle amplitude and duration when older adults swayed in their self-pace. These findings suggest older adults cannot exploit active tracking of the complex visual motion cue to improve voluntary sway persistency. This could be related to the less stable and out of phase intersegmental coordination characterizing rhythmic voluntary sway in old age

Standing on unstable surface challenges postural control of tracking tasks and modulates neuromuscular adjustments specific to task complexity

Understanding the modulations of motor control in the presence of perturbations in task conditions of varying complexity is a key element towards the design of effective perturbation-based balance exercise programs. In this study we investigated the effect of mechanical perturbations, induced by an unstable surface, on muscle activation and visuo-postural coupling, when actively tracking target motion cues of different complexity. Four postural tasks following a visual oscillating target of varying target complexity (periodic-sinusoidal vs. chaotic-Lorenz) and surface (stable-floor vs. unstable-foam) were performed. The electromyographic activity of the main plantarflexor and dorsiflexor muscles was captured. The coupling between sway and target was assessed through spectral analysis and the system’s local dynamic stability through the short-term maximum Lyapunov exponent. We found that external perturbations increased local instability and deteriorated visuo-motor coupling. Visuo-motor deterioration was greater for the chaotic target, implying that the effect of the induced perturbations depends on target complexity. There was a modulation of the neuromotor system towards amplification of muscle activity and coactivation to compensate surface-related perturbations and to ensure robust motor control. Our findings provide evidence that, in the presence of perturbations, target complexity induces specific modulations in the neuromotor system while controlling balance and posture.Peer Reviewe

Optimal Drop Height in Prepubertal Boys Is Revealed by the Performance in Squat Jump

Drop jump (DJ) performance gain with increasing drop height is well documented in adults, but there is still no clear evidence of such gain in children. This study aimed to examine the differences in DJ performance gain in male adults and prepubescent boys by comparing drop heights tailored to each individual’s performance and expressed as a percentage of their squat jump (SJ) performance. Fifteen boys (9–11 y) and 15 men (19–27 y) executed DJs from drop heights that were set at 75%, 100%, 125%, and 150% of their best performance in SJ (DJ75, DJ100, DJ125, and DJ150, respectively). Vertical ground reaction force (vGRF), contact time and kinematics of the lower extremities were captured. The results showed that boys jumped significantly lower than adults in DJs, and both age groups presented jumping gain with increasing drop height, up to DJ125. Boys demonstrated longer total contact time, lower angular velocity and vGRF during the propulsive phase, as well as smaller knee flexion at touchdown and lower reactive strength index. vGRF in DJ75 and DJ100 was lower than in DJ125 and DJ150. The highest value for maximum knee flexion was also presented at DJ150. It is concluded that in prepubescent boys, the appropriate drop height for an effective DJ is linked to their performance in SJ and might be between 75% and 125% of their maximum SJ performance

The effect of the two bouts of descending (filled circles, solid line) and ascending (open circles, dash line) exercise on Insulin (A), glucose (B) and HOMA (C) (mean ± SEM).

<p>* Significantly different from the pre-exercise value in the same session (<i>P</i><0.05). ^#Significant difference between session 1 and session 2 at the same time-point (<i>P</i><0.05). ^¶Significant difference between ascending and descending group in the same session at the same time point (<i>P</i><0.05).</p

The automatic escalator device was invented, designed and constructed by our group and it is the first of its kind offering both stair descending (eccentric-biased) and stair ascending (concentric-biased) exercise.

<p>The automatic escalator device was invented, designed and constructed by our group and it is the first of its kind offering both stair descending (eccentric-biased) and stair ascending (concentric-biased) exercise.</p

The effect of the two bouts of descending (filled circles, solid line) and ascending (open circles, dash line) exercise on torque (A), ROM (B) DOMS (C) and CK (D) (mean ± SEM).

<p>*Significantly different from the pre-exercise value in the same session (<i>P</i><0.05). ^#Significant difference between session 1 and session 2 at the same time-point (<i>P</i><0.05). ^¶Significant difference between ascending and descending group in the same session at the same time point (<i>P</i><0.05).</p

Redox status indices at pre exercise, immediately post exercise and at day 2 and day 4 post exercise after the stair descending and stair ascending exercise (mean ± SEM).

<p>GSH, reduced glutathione; GSSG, oxidized glutathione; TAC, total antioxidant capacity; TBARS, thiobarbituric acid–reactive substances. G×B×T: 3-way interaction for group, bout and time; G×B: 2-way interaction for group and bout; G×T: 2-way interaction for group and time; B×T: 2-way interaction for bout and time; G: Main effect of training group; B: Main effect of bout; T: Main effect of time.</p>*<p>Significantly different from the pre-exercise value in the same bout (P<0.05).</p>#<p>Significant difference between bout 1 and bout 2 at the same time-point (P<0.05).</p>¶<p>Significant difference between ascending and descending group in the same bout at the same time point (P<0.05).</p

The effect of the two bouts of descending (filled circles, solid line) and ascending (open circles, dash line) exercise on TG (A), TC (B) HDL (C) and LDL (D) concentration as well as TC/HDL (E) ratio (mean ± SEM).

<p>*Significantly different from the pre-exercise value in the same session (<i>P</i><0.05). ^#Significant difference between session 1 and session 2 at the same time-point (<i>P</i><0.05).</p

Body composition and analysis of daily energy intake of stairs descending and ascending group in the first and in the second bout of exercise (mean ± SEM).

<p>RE, retinol equivalents; >-TE, alpha-tocopherol equivalents.</p