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Alberto Pardo-Ibáñez, Jose L. Bermejo, Sergio Gandia, Julien Maitre, Israel Villarrasa-Sapiña, Isaac Estevan, and Xavier Garcia-Masso

A cross-sectional, prospective, between-subjects design was used in this study to establish the differences in sensory reweighting of postural control among different ages during adolescence. A total of 153 adolescents (five age groups; 13–17 years old) performed bipedal standing in three sensory conditions (i.e., with visual restriction, vestibular disturbance, and proprioceptive disturbance). Center of pressure displacement signals were measured in mediolateral and anteroposterior directions to characterize reweighting in the sensory system in static postural control when sensory information is disturbed or restricted during adolescent growth. The results indicate a development of postural control, showing large differences between subjects of 13–14 years old and older adolescents. A critical change was found in sensory reweighting during bipedal stance with disturbance of proprioceptive information at 15 years old. Adolescents of 13–14 years old showed less postural control and performance than older adolescents during the disturbance of proprioceptive information. Moreover, the results demonstrated that the visual system achieves its development around 15–16 years old. In conclusion, this research suggests that a difference of sensory reweighting under this type of sensorial condition and sensory reweight systems would seem to achieve stabilization at the age of 15.

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Patrice R. Rougier, Thibaud Coquard, Thierry Paillard, Clément Ankaoua, Jeanne Dury, Corentin Barthod, and Dominic Perennou

Healthy young subjects were instructed to modify their weight-bearing asymmetry when standing on a double-seesaw device. The results indicated decreased and unchanged amplitudes in the center-of-pressure movements under the unloaded and loaded legs, respectively. In addition, a concomitant increased contribution of the more loaded leg and a decreased contribution of the pressure distribution mechanism along the mediolateral axis were observed in the production of the resultant center of pressure, its amplitude remaining constant. Thus, contrary to what was previously reported for stance control on solid ground, one of the main characteristics of a double-seesaw device, by preventing increased amplitudes on the loaded side during weight-bearing asymmetry, would be to facilitate a greater independency of the feet in the stance control process.

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Gustavo Sandri Heidner, Patrick M. Rider, J.C. Mizelle, Caitlin M. O’Connell, Nicholas P. Murray, and Zachary J. Domire

Virtual reality (VR) paradigms have proved to be a valid method to challenge and perturb balance. There is little consensus in the literature on the best protocol design to perturb balance and induce postural sway. Current VR interventions still lack a well-defined standardized metric to generate a virtual environment that can perturb balance in an efficacious, lifelike, and repeatable manner. The objective of this study was to investigate different configurations of amplitude and frequency in an anterior–posterior translation VR environment, that is, lifelike and scaled. Thirteen young adults with no conditions affecting balance were recruited. Balance was challenged by anterior–posterior sinusoidal movement of the lab image within the VR headset. Four different amplitudes of the sinusoidal movement were tested: 1, 5, 10, and 20 cm, with each amplitude being presented at 2 test frequencies : 0.5 and 0.25 Hz. Mean center of pressure velocity was significantly greater than baseline at 0.5 Hz and amplitudes of 10 and 20 cm. Mean center of pressure at approximate entropy was greater than baseline at 0.5 Hz and amplitude of 20 cm. The results suggest that sinusoidal movement of a realistic VR environment produces altered balance compared with baseline quiet standing, but only under specific movement parameters.

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Elizabeth Coker and Terry Kaminski

The purpose of this study was to investigate the effects of visual condition (low light, full light, and full light with mirror) on balance control and technical form during two technical dance movements in a group of elite collegiate dance students. Dancers demonstrated higher center of pressure velocity indicating lower control while performing a static dance task (parallel relevé retiré) and a dynamic dance task (fondu relevé en croix) under low light conditions than either lighted condition. Measures of Western ballet technique (pelvic obliquity, knee extension, and ankle plantar flexion) showed no decrement under low light conditions. No effect of concurrent mirror feedback was found on either center of pressure velocity or technical requirements of the dance tasks.

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Daniel C. McFarland, Alexander G. Brynildsen, and Katherine R. Saul

Most upper-extremity musculoskeletal models represent the glenohumeral joint with an inherently stable ball-and-socket, but the physiological joint requires active muscle coordination for stability. The authors evaluated sensitivity of common predicted outcomes (instability, net glenohumeral reaction force, and rotator cuff activations) to different implementations of active stabilizing mechanisms (constraining net joint reaction direction and incorporating normalized surface electromyography [EMG]). Both EMG and reaction force constraints successfully reduced joint instability. For flexion, incorporating any normalized surface EMG data reduced predicted instability by 54.8%, whereas incorporating any force constraint reduced predicted instability by 43.1%. Other outcomes were sensitive to EMG constraints, but not to force constraints. For flexion, incorporating normalized surface EMG data increased predicted magnitudes of joint reaction force and rotator cuff activations by 28.7% and 88.4%, respectively. Force constraints had no influence on these predicted outcomes for all tasks evaluated. More restrictive EMG constraints also tended to overconstrain the model, making it challenging to accurately track input kinematics. Therefore, force constraints may be a more robust choice when representing stability.

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Gregory S. Walsh, Daniel C. Low, and Marco Arkesteijn

The aim of this study was to investigate the effects of backpack load carriage on quiet standing postural control and limits of stability (LOS) of older adults. Fourteen older adults (65 [6] y) performed quiet standing and a forward, right, and left LOS test in 3 conditions, unloaded, stable, and unstable backpack loads while activity of 4 leg muscles was recorded. Stable and unstable loads decreased postural sway (main effect ηp2=.84, stable P < .001, unstable P < .001), mediolateral (main effect ηp2=.49, stable P = .002, unstable P = .018) and anterior–posterior (main effect ηp2=.64, stable P < .001, unstable P = .001) fractal dimension, and LOS distance (main effect ηp2=.18, stable P = .011, unstable P = .046) compared with unloaded. Rectus femoris (main effect ηp2=.39, stable P = .001, unstable P = .010) and gastrocnemius (main effect ηp2=.30, unstable P = .027) activity increased in loaded conditions during LOS and quiet standing. Gastrocnemius–tibialis anterior coactivation was greater in unstable load than stable loaded quiet standing (main effect ηp2=.24, P = .040). These findings suggest older adults adopt conservative postural control strategies minimizing the need for postural corrections in loaded conditions. Reduced LOS may also increase fall risk when carrying a load. However, there was no difference between unstable and stable loads for postural control variables.

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Leila Ghamkhar, Somayeh Amiri Arimi, and Amir H. Kahlaee

While proprioceptive functioning of the cervical extensor muscles has been proposed to be associated with chronic neck pain (NP), the mechanism by which such impairment might contribute to NP is not clear. The aim of this cross-sectional study was to investigate the relevance of proprioception, extensor muscles size, and endurance to chronic NP. A total of 60 participants with (n = 30) or without (n = 30) chronic NP participated in this cross-sectional study. Joint repositioning error (JRE), ultrasonographic parameters of the cervical extensors, and clinical extensor endurance were assessed. Multivariate analysis of variance and logistic regression tests were used to compare the groups and test the predictive value of the dependent variables for chronic NP, respectively. Patients showed comparable JRE scores but smaller multifidus size and lower extensor endurance (Cohen d = 0.66 for both). JRE in the transverse plane (β = 1.20), multifidus muscle size (β = 0.02), and endurance (β = 0.99) were significant predictors for chronic NP odds ratio. The results found multifidus size as the most relevant factor to NP by showing both between-groups difference and considerable odds ratio. As JRE in transverse plane was not different between the groups and extensor endurance demonstrated minimal odds ratio, these 2 factors may be considered as less relevant to NP comparing multifidus muscle size.

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J.D. DeFreese and Alan L. Smith

Person-centered investigations of athlete burnout have utility to unearth novel information about this developmental experience within the social environment of competitive sport. Guided by extant theory, conceptually proposed developmental patterns of athlete burnout were examined across a season as expressed in profiles of emotional and physical exhaustion, reduced accomplishment, and sport devaluation perceptions. Athlete social perceptions were also explored as predictors of profile membership. Collegiate athletes (N = 129) completed established assessments of study variables at four in-season time points. Latent profile analysis revealed profiles characterized by athletes experiencing the three burnout dimensions similarly at any given time point, with the notable exception of exhaustion being more frequently experienced in some profiles. Social support perceptions predicted profile membership with moderate success. Trends in profile stability provide some support for consideration of exhaustion-driven burnout experiences. Results shed light on the theoretical pathways of burnout development and inform continued longitudinal burnout research efforts.

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Mitchell Tillman and Satyajit Ambike

The authors examined how the stability of the current total isometric force (F T) produced by four fingers is influenced by previous and expected voluntary changes in F T. The authors employed the synergy index obtained from the across-trial uncontrolled manifold analysis to quantify the stability of F T. The authors compared two tasks with similar histories of F T changes; one in which participants expected changes in F T in the future, and one in which they expected no changes in F T. The stability of F T was lower in the former task, indicating the existence of a novel type of anticipatory synergy adjustment. Disparate histories of F T changes yield inconsistent changes in stability, driven by individual differences in the covariation in the finger forces that leave F T invariant. Future research should focus on exploring these individual differences to better understand how previous and expected behavior changes influence the stability of the current motor behavior.

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Jocelyn F. Hafer and Katherine A. Boyer

The link between age-related changes in muscle strength and gait is unclear. We tested if knee extensor functional demand differs by age and physical activity status and if functional demand increases with walking speed or after exercise. Gait and knee extensor muscle torque were collected from young adults and highly and less active older adults before and after treadmill walking. Functional demand was the ratio of knee moments during gait to knee extensor muscle torques estimated from participant-specific torque–velocity curves. Functional demand at the peak knee flexion moment was greater in less active older adults than young adults (29.3% [14.3%] vs 24.6% [12.1%]) and increased with walking speed (32.0% [13.9%] vs 22.8% [10.4%]). Functional demand at both knee extension moments increased ∼2% to 3% after exercise. The low functional demand found in this study suggests that healthy adults maintain a reserve of knee extensor strength.