Postural sway and joint kinematics during quiet standing are affected by lumbar extensor fatigue

The purpose of this study was to investigate changes in postural sway and strategy elicited by lumbar extensor muscle fatigue. Specifically, changes in center of mass (COM), center of pressure (COP), and joint kinematics during quiet standing were determined, as well as selected cross correlations between these variables that are indicative of movement strategy. Twelve healthy male participants stood quietly both before and after exercises that fatigued the lumbar extensors. Whole-body movement and ground reaction force data were recorded and used to calculate mean body posture and variability of COM, COP, and joint kinematics during quiet standing. Three main findings emerged. First, participants adopted a slight forward lean post-fatigue as evidenced by an anterior shift of the COM and COP. Second, post-fatigue increases in joint angle variability were observed at multiple joints including joints distal to the fatigued musculature. Despite these increases, anterior-posterior (AP) ankle angle correlated well with AP COM position, suggesting the body still behaved similar to an inverted pendulum. Third, global measures of sway based on COM and COP were not necessarily indicative of changes in individual joint kinematics. Thus, in trying to advance our understanding of how localized fatigue affects movement patterns and the postural control system, it appears that joint kinematics and/or multivariate measures of postural sway are necessary.     

Effects of joint immobilization on standing balance

We investigated the effect of joint immobilization on the postural sway during quiet standing. We hypothesized that the center of pressure (COP), rambling, and trembling trajectories would be affected by joint immobilization. Ten young adults stood on a force plate during 60 s without and with immobilized joints (only knees constrained, CK; knees and hips, CH; and knees, hips, and trunk, CT), with their eyes open (OE) or closed (CE). The root mean square deviation (RMS, the standard deviation from the mean) and mean speed of COP, rambling, and trembling trajectories in the anterior–posterior and medial–lateral directions were analyzed. Similar effects of vision were observed for both directions: larger amplitudes for all variables were observed in the CE condition. In the anterior–posterior direction, postural sway increased only when the knees, hips, and trunk were immobilized. For the medial–lateral direction, the RMS and the mean speed of the COP, rambling, and trembling displacements decreased after immobilization of knees and hips and knees, hips, and trunk. These findings indicate that the single inverted pendulum model is unable to completely explain the processes involved in the control of the quiet upright stance in the anterior–posterior and medial–lateral directions.     

Does the "eyes lead the hand" principle apply to reach-to-grasp movements evoked by unexpected balance perturbations?

A fundamental principle that has emerged from studies of natural gaze behavior is that goal-directed arm movements are typically guided by a saccade to the target. In this study, we evaluated a hypothesis that this principle does not apply to rapid reach-to-grasp movements evoked by sudden unexpected balance perturbations. These perturbations involved forward translation of a large (2 × 6 m) motion platform configured to simulate a “real-life” environment. Subjects performed a common “daily-life” visuo-cognitive task (find a telephone and make a call) that required walking to the end of the platform, which was triggered to move as they approached a handrail mounted alongside the travel path. A deception was used to ensure that the perturbation was truly unexpected. Eleven of 18 healthy young-adult subjects (age 22-30) reached to grasp or touch the rail in response to the balance perturbation. In support of the hypothesis, none of these arm reactions was guided by concurrent visual fixation of the handrail. Seven of the 11 looked at the rail upon first entering the environment, and hence may have used “stored” central-field information about the handrail location to guide the subsequent arm reaction. However, the other four subjects never looked directly at the rail, indicating a complete reliance on peripheral vision. These findings add to previous evidence of distinctions in the CNS control of volitional and perturbation-evoked arm movements. Future studies will determine whether similar visuo-motor behavior occurs when the available handhold is smaller or when subjects are not engaged in a concurrent visuo-cognitive task.     

Acute effect of stretching modalities on global coordination and kicking accuracy in 12–13 year-old soccer players

The aim of the study was to compare the effect of stretching procedures on global coordination and accuracy in instep soccer kicks achieved in different stress conditions. Twenty male young soccer players completed the global coordination test (GC), the instep kicking accuracy test in free (FKA) and in time-pressure (TPKA) conditions, either after static (SS), dynamic (DS), ballistic (BS) or no-stretching (CTR) protocols, on nonconsecutive days and in a randomized order. After performing a 5 min standardized intensity jogging (70% of MAV), followed by stretching exercises for 10 min, each participant completed, successively, the GC, FKA and TPKA tests. Accuracy data, heart-rate, rating of perceived exertion and task difficulty perception were recorded and analyzed using a two-way ANOVA. GC scores were analyzed using one way ANOVA with repeated measures. The results showed higher GC and TPKA performances after DS and BS procedures. However, there was no effect of the stretching procedures on FKA. The GC scores increased by 10.8% and 7.2% after DS and BS, respectively, but were not affected by SS. Compared to FKA, the TPKA accuracy significantly decreased by 20.2% after CTR (p < 0.01) and 30.7% SS (p < 0.001) with no significant difference after DS (10.1%; p > 0.05) and BS (11.0%; p > 0.05). The use of dynamic and ballistic stretching yielded to better GC scores and helped reducing the adverse effect of time-pressure on instep kicking accuracy. Consequently, dynamic and ballistic exercises can be recommended before practicing activities requiring coordination and lower limbs speed and accuracy.     

The effect of a non-steroidal anti-inflammatory drug on two important predictors for accidental falls: Postural balance and manual reaction time. A randomized, controlled pilot study

Accidental falls in older individuals are a major health and research topic. Increased reaction time and impaired postural balance have been determined as reliable predictors for those at risk of falling and are important functions of the central nervous system (CNS). An essential risk factor for falls is medication exposure. Amongst the medications related to accidental falls are the non-steroidal anti-inflammatory drugs (NSAIDs). About 1-10% of all users experience CNS side effects. These side effects, such as dizziness, headaches, drowsiness, mood alteration, and confusion, seem to be more common during treatment with indomethacin. Hence, it is possible that maintenance of (static) postural balance and swift reactions to stimuli are affected by exposure to NSAIDs, indomethacin in particular, consequently putting older individuals at a greater risk for accidental falls. The present study investigated the effect of a high indomethacin dose in healthy middle-aged individuals on two important predictors of falls: postural balance and reaction time. Twenty-two healthy middle-aged individuals (59.5 ± 4.7 years) participated in this double-blind, placebo-controlled, randomized crossover trial. Three measurements were conducted with a week interval each. A measurement consisted of postural balance as a single task and while concurrently performing a secondary cognitive task and reaction time tasks. For the first measurement indomethacin 75 mg (slow-release) or a visually identical placebo was randomly assigned. In total, five capsules were taken orally in the 2.5 days preceding assessment. The second measurement was without intervention, for the final one the first placebo group got indomethacin and vice versa. Repeated measures GLM revealed no significant differences between indomethacin, placebo, and baseline in any of the balance tasks. No differences in postural balance were found between the single and dual task conditions, or on the performance of the dual task itself. Similarly, no differences were found on the manual reaction time tasks. The present study showed that a high indomethacin dose does not negatively affect postural balance and manual reaction time in this healthy middle-aged population. Although the relatively small and young sample limits the direct ability to generalize the results to a population at risk of falling, the results indicate that indomethacin alone is not likely to increase fall risk, as far as this risk is related to abovementioned important functions of the CNS, and not affected by comorbidities.     

Is social inhibition of return due to action co-representation?

When two individuals alternate reaching responses to visual targets presented on a shared workspace, one individual is slower to respond to targets occupying the same position as their partner’s previous response. This phenomenon is thought to be due to processes that inhibit the initiation of a movement to a location recently acted upon. However, two distinct forms of the inhibition account have been posited, one based on inhibition of an action, the other based on inhibition of an action and location. Furthermore, an additional recent explanation suggests the phenomenon is due to mechanisms that give rise to action congruency effects. Thus the three different theories differ in the degree to which action co-representation plays a role in the effect. The aim of the present work was to examine these competing accounts. Three experiments demonstrated that when identical actions are made, the effect is modulated by the configuration of the visual stimuli acted upon and the perceptual demands of the task. In addition, when the co-actors perform different actions to the same target, the effect is still observed. These findings support the hypothesis that this particular joint action phenomenon is generated via social cues that induce location-based inhibition of return rather than being due to shared motor co-representations.     

Postural coordination patterns as a function of dynamics of the support surface

The present study investigated the compensatory postural coordination patterns that emerge in the face of dynamic changes in the surface of support. Adult subjects stood on a moving platform that was sinusoidally translated in the anterior-posterior direction. The frequency and amplitude of the support surface translation were manipulated over a wide range of parameter values. The results revealed that as the frequency of platform motion increased, the postural system systematically exploited the available joint-space degrees of freedom and generated four distinct postural coordination modes (a rigid mode --> ankle mode --> ankle-hip mode --> ankle-hip-knee mode). It appears that upright standing posture has a small set of coordination patterns that are particular to the dynamics of the surface of support.     

The functional role of central and peripheral vision in the control of posture

Three experiments were conducted to investigate the role of central and peripheral vision (CV and PV) in postural control. In Experiment 1, either the central or peripheral visual field were selectively stimulated using a circular random dot pattern that was either static or alternated at 5 Hz. Center of foot pressure (CoP) was used to examine postural sway during quiet standing under both CV and PV conditions. The results showed that, when the visual stimulus was presented in the periphery, the CoP area decreased and more so in the anterior-posterior (AP) than in the medio-lateral (ML) direction, indicating a characteristic directional specificity. There was no significant difference between the static and dynamic (alternating) conditions. Experiment 2 investigated the directional specificity of body sway found in Experiment 1 by having the trunk either be faced toward the stimulus display or perpendicularly to it, with the head always facing the display. The results showed that the stabilizing effect of peripheral vision was present in the direction of stimulus observation (i.e., the head/gaze direction), irrespective of trunk orientation. This suggested that head/gaze direction toward the stimulus presentation, rather than a biomechanical factor like greater mobility of the ankle joint in AP direction than in ML direction, was essential to postural stability. Experiment 3 further examined whether the stabilizing effect of peripheral vision found in Experiments 1 and 2 was caused because more dots (500) were presented as visual cues to the peripheral visual field than to the central visual field (20 dots) by presenting the same number of dots (20) in both conditions. It was found that, in spite of the equal number of dots, the postural sway amplitudes were larger for the central vision conditions than for the peripheral vision conditions. In conclusion, the present study showed that peripheral rather than central vision contributes to maintaining a stable standing posture, with postural sway being influenced more in the direction of stimulus observation, or head/gaze direction, than in the direction of trunk orientation, which suggests that peripheral vision operates primarily in a viewer-centered frame of reference characterized by the head/gaze direction rather than in a body-centered frame of reference characterized by the anatomical planes of the body.     

Computing the COM from the COP in postural sway movements

A method is described for computing the centre of mass (COM) from empirical estimates of the centre of pressure (COP) obtained by means of a force platform. The method is based on a biomechanical model of sway movements in quiet standing, according to which the horizontal acceleration of the COM is approximately proportional to the COM–COP difference. The equations are solved by approximating the solution with best fitting spline functions. The implications for movement control are discussed.     

Visual movement perception in deaf and hearing individuals

A number of studies have investigated changes in the perception of visual motion as a result of altered sensory experiences. An animal study has shown that auditory-deprived cats exhibit enhanced performance in a visual movement detection task compared to hearing cats (Lomber, Meredith, & Kral, 2010). In humans, the behavioural evidence regarding the perception of motion is less clear. The present study investigated deaf and hearing adult participants using a movement localization task and a direction of motion task employing coherently-moving and static visual dot patterns. Overall, deaf and hearing participants did not differ in their movement localization performance, although within the deaf group, a left visual field advantage was found. When discriminating the direction of motion, however, deaf participants responded faster and tended to be more accurate when detecting small differences in direction compared with the hearing controls. These results conform to the view that visual abilities are enhanced after auditory deprivation and extend previous findings regarding visual motion processing in deaf individuals.     

团队异质性的平衡性对团队创造力的影响

团队异质性的平衡性是指团队中异质性属性的相对分布情况, 是一个从不平衡到平衡的连续谱概念。作为对团队构成维度的补充, 本研究探究其如何通过团队信息深化施加影响于团队创造力。通过对浙江理工大学451名学生的团队实验研究, 得出结论认为：团队知识异质性的平衡性能够积极影响团队创造力, 且团队信息深化在这一过程中起中介作用, 性别异质性的平衡性与团队创造力之间的关系不显著。这一研究结果将有助于打开团队构成和团队创造力研究的新局面。     

遮挡范式下运动速度和方向对碰撞时间估计绩效指标的影响

为探索运动速度和方向对碰撞时间（TTC）估计的影响,采用3运动速度×4运动方向的双因素设计开展了实验研究。结果发现,在相对慢速条件下,受试者TTC估计的准确性显著差于相对中速和相对快速条件;仅在相对慢速条件下受试者对竖直方向的TTC估计准确度显著差于水平方向、低估TTC的比例显著高于高估TTC的比例。结果表明,运动速度对TTC估计绩效指标的影响相对较强,运动方向对其影响相对较弱;两个因素的影响存在交互作用。     

Anticipatory postural adjustments during a Fitts’ task: Comparing young versus older adults and the effects of different foci of attention

Anticipatory postural adjustments (APAs) are an integral part of standing balance. Previous research with balance control has shown that adopting an external focus of attention, compared to an internal focus of attention, yields better performance during motor skills. Despite the importance of APAs, especially among older adults, and the potential benefits of adopting an external focus of attention, studies investigating methods for improving APAs are limited. The aim of this study was to compare behavioral, kinematic and APAs measures while adopting different foci of attention among young and older adults when performing a lower extremity Fitts’ task. Ten young adults (mean age 24 years ± 4.37) and ten older adults (mean age 75 years ± 5.85) performed a lower-extremity reaching task (Fitts’ task) while adopting an external focus (focus on target) and an internal focus (focus on limb) in a within-subject design. A motion capture system was used to record participants’ movement data. Custom software derived movement time (MT), peak velocity (PV), time to peak velocity (ttPV) and variability at target (SD_T). Electromyography (EMG) was used to determine APAs onset and magnitude. The findings showed that an external focus of attention led to significantly shorter MT, higher PV, shorter ttPV and more accuracy when reaching the target (SD_T) for both age groups. Also, EMG results showed that, with an external focus, APAs onset occurred earlier and APAs magnitude was more efficient. As predicted by Fitts’ Law, participants spent more time executing movements to targets with higher indices of difficulty. Older adults compared to young adults were more adversely affected by the increase of difficulty of the Fitts’ task, specifically, on measures of APAs. In conclusion, adopting an external focus of attention led to better overall movement performance when performing a lower extremity Fitts’ task. The task used in the present study can distinguish between APAs for older and young adults. We recommend that future studies expand on our findings in order to establish a performance-based objective measure of APAs to assess clinical interventions for postural control impairment.