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Molly B. Johnson and Richard E.A. Van Emmerik

Sensory feedback from the vestibular system and neck muscle stretch receptors is critical for the regulation of postural control. The postural relationship of the head to the trunk is a major factor determining the integration of sensory feedback and can be interfered with by varying head orientation. This study assessed how 60-s of standing with the head neutral, flexed, or extended impacted postural stability during upright stance and during forward lean in 13 healthy participants (26 ±5 years old). During both quiet upright stance and maximal forward lean, head extension increased postural center of pressure (COP) velocity and decreased the COP time-to-contact the anterior stability boundary compared with the head neutral condition. Head flexion did not differ from head neutral for either of the stance conditions. This study demonstrates that interfering with the head-trunk relationship by adopting extended, but not flexed, head orientations interferes with postural control that may impact postural stability during both quiet upright stance and maximal forward lean conditions.

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Erik B. Simonsen, Katrine L. Cappelen, Ragnhild í Skorini, Peter K. Larsen, Tine Alkjær and Poul Dyhre-Poulsen

A hip joint flexor moment in the last half of the stance phase during walking has repeatedly been reported. However, the purpose of this moment remains uncertain and it is unknown how it is generated. Nine male subjects were instructed to walk at 4.5 km/h with their upper body in three different positions: normal, inclined and reclined. Net joint moments were calculated about the hip, knee and ankle joint. The peak hip joint flexor moment during late stance was significantly lower during inclined walking than in the two other conditions. During normal walking the iliacus muscle showed no or very weak activity and first at the transition from stance to swing. When walking reclined, a clear but rather low activity level of the iliacus muscle was seen in the first half of the stance phase, which could contribute to the hip moment. In the inclined condition the iliacus showed much increased activity but only in the swing phase. It is concluded that the hip flexor moment in question is largely generated by passive structures in the form of ligaments resisting hip joint extension.

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Eric Yiou, Malha Mezaour and Serge Le Bozec

This study investigated how young healthy subjects control their equilibrium in situations of instability specifically elicited by a reduced capacity of force production in the postural muscle system. Ten subjects displaced a bar forward with both hands at maximal velocity toward a target while standing on the dominant leg (UNID), on the nondominant leg (UNIND), or on both legs. In each stance condition, anticipatory postural adjustments (APAs) were elicited. Along the anteroposterior axis, APAs were two-times longer in UNID and UNIND than in bipedal stance, while the anticipatory inertia forces remained equivalent. The focal performance was maintained without any additive postural perturbation. A small effect of leg dominance could be detected on APAs along the mediolateral axis (i.e., anticipatory inertia forces were higher in UNIND than in UNID). These results stress the adaptability of the central nervous system to the instability specifically elicited by reduced postural muscle system efficiency.

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Olivier Oullier, Benoît G. Bardy, Thomas A. Stoffregen and Reinoud J. Bootsma

Surfaces shorter in extent than the feet elicit multi-joint coordination that differs from what is elicited by stance on extensive surfaces. This well-known effect arises from the mechanics of the actor-environment interaction. Multi-joint control of stance is also known to be influenced by non-mechanical aspects of a situation, including participants' task or intention. Intentional constraints do not originate in mechanics, and for this reason one might suppose that constraints imposed by mechanics would dominate constraints imposed by intentions, when the two were in conflict. We evaluated this hypothesis by varying participants' supra-postural task during stance on a short surface. While standing on a 10-cm wide beam, participants were exposed to optic flow generated by fore-aft oscillations of a moving room. Participants faced a target attached to the front wall of the moving room and were asked either to look at the target (with no instruction to move) or intentionally to track it with their head (i.e., to keep the target-head distance constant). Within trials, we varied the frequency of room (and target) motion, from 0.15 to 0.75 Hz, in steps of 0.05 Hz. In both conditions, ankle and hip rotations exhibited antiphase coordination, but behavior was not identical across conditions. Coupling between motion of the room and the head was stronger for the tracking task than for the looking task, and the stability of ankle-hip coordination was greater during tracking than during looking. These results indicate that the influence of support surface mechanics did not eliminate the influence of the supra-postural task. Environment-based and task-based constraints interacted in determining the coordination of hips and ankles during stance.

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D.S. Blaise Williams III, Denis Brunt and Robert J. Tanenberg

The majority of plantar ulcers in the diabetic population occur in the forefoot. Peripheral neuropathy has been related to the occurrence of ulcers. Long-term diabetes results in the joints becoming passively stiffer. This static stiffness may translate to dynamic joint stiffness in the lower extremities during gait. Therefore, the purpose of this investigation was to demonstrate differences in ankle and knee joint stiffness between diabetic individuals with and without peripheral neuropathy during gait. Diabetic subjects with and without peripheral neuropathy were compared. Subjects were monitored during normal walking with three-dimensional motion analysis and a force plate. Neuropathic subjects had higher ankle stiffness (0.236 N·m/ deg) during 65 to 80% of stance when compared with non-neuropathic subjects (−0.113 N·m/deg). Neuropathic subjects showed a different pattern in ankle stiffness compared with non-neuropathic subjects. Neuropathic subjects demonstrated a consistent level of ankle stiffness, whereas non-neuropathic subjects showed varying levels of stiffness. Neuropathic subjects demonstrated lower knee stiffness (0.015 N·m/deg) compared with non-neuropathic subjects (0.075 N·m/deg) during 50 to 65% of stance. The differences in patterns of ankle and knee joint stiffness between groups appear to be related to changes in timing of peak ankle dorsiflexion during stance, with the neuropathic group reaching peak dorsiflexion later than the non-neuropathic subjects. This may partially relate to the changes in plantar pressures beneath the metatarsal heads present in individuals with neuropathy.

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Thomas Dos’Santos, Christopher Thomas, Paul A. Jones and Paul Comfort

Purpose:

To investigate the within-session reliability of bilateral- and unilateral-stance isometric midthigh-pull (IMTP) force–time characteristics including peak force (PF), relative PF, and impulse at time bands (0–100, 0–200, 0–250, and 0–300 milliseconds) and to compare isometric force–time characteristics between right and left and dominant (D) and nondominant (ND) limbs.

Methods:

Professional male rugby league and multisport male college athletes (N = 54; age, 23.4 ± 4.2 y; height, 1.80 ± 0.05 m; mass, 88.9 ± 12.9 kg) performed 3 bilateral IMTP trials and 6 unilateral-stance IMTP trials (3 per leg) on a force plate sampling at 600 Hz.

Results:

Intraclass correlation coefficients (ICCs) and coefficients of variation (CVs) demonstrated high within-session reliability for bilateral and unilateral IMTP PF (ICC = .94, CV = 4.7–5.5%). Lower reliability measures and greater variability were observed for bilateral and unilateral IMTP impulse at time bands (ICC = .81–.88, CV = 7.7–11.8%). Paired-sample t tests and Cohen d effect sizes revealed no significant differences for all isometric force–time characteristics between right and left limbs in male college athletes (P >.05, d ≤ 0.32) and professional rugby league players (P > .05, d ≤ 0.11); however, significant differences were found between D and ND limbs in male college athletes (P < .001, d = 0.43–0.91) and professional rugby league players (P < .001, d = 0.27–0.46).

Conclusion:

This study demonstrated high within-session reliability for unilateral-stance IMTP PF, revealing significant differences in isometric force–time characteristics between D and ND limbs in male athletes.

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Sivan Almosnino, David Kingston and Ryan B. Graham

The purpose of this investigation was to assess the effects of stance width and foot rotation angle on three-dimensional knee joint moments during bodyweight squat performance. Twenty-eight participants performed 8 repetitions in 4 conditions differing in stance or foot rotation positions. Knee joint moment waveforms were subjected to principal component analysis. Results indicated that increasing stance width resulted in a larger knee flexion moment magnitude, as well as larger and phase-shifted adduction moment waveforms. The knee’s internal rotation moment magnitude was significantly reduced with external foot rotation only under the wide stance condition. Moreover, squat performance with a wide stance and externally rotated feet resulted in a flattening of the internal rotation moment waveform during the middle portion of the movement. However, i is speculated that the differences observed across conditions are not of clinical relevance for young, healthy participants.

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Jennifer E. Earl

Context:

Gluteus medius (GM) contraction during single-leg stance prevents the contralateral pelvis from “dropping,” providing stability for lower extremity motion.

Objective:

To determine which combination of hip rotation and abduction exercise results in the greatest activity of the GM and whether the GM responds to increased loads in these exercises.

Design and Setting:

Repeated measures, laboratory.

Subjects:

20 healthy volunteers.

Interventions:

Resistance (2.26 and 4.53 kg) was provided to 3 variations of a single-leg-stance exercise: hip abduction only, abduction-internal rotation (ABD-IR), and abduction-external rotation.

Measurements:

Muscle activity was recorded from the anterior and middle portions of the GM using surface electromyography.

Results:

ABD-IR produced the most activity in the anterior and middle sections of the GM muscle. The 4.53-kg load produced significantly more activity than the 2.26-kg load (P < .05).

Conclusions:

The GM is most active when performing abduction and internal rotation of the hip. This information could be used to develop GM-strengthening exercises.

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Rodolfo B. Parreira, Marcela C. Boer, Lucas Rabello, Viviane de Souza P. Costa, Eros de Oliveira Jr. and Rubens A. da Silva

The aim of this study was to evaluate the changes in center of pressure (COP) movement in four time intervals (5, 10, 15 and 30 s) during a one-leg stance test performed by young and elderly adults. Twelve young adults (mean 20 years) and 12 elderly subjects (mean 68 years) participated in this study. The subjects performed three 30 s trials of an eyes open one-leg stance test on a force platform, in which the COP parameter was computed at four points in time from same original COP signal. Significant differences were found between the young and elderly adults (P < .007) only at the 10, 15 and 30 s intervals. For both groups, COP changes were significantly different between the 5 s time interval and other intervals (10, 15 and 30 s). In conclusion, these results pointed out that age-related difference in COP changes were time dependent. This suggests that the use of longer durations increases the possibility of distinguishing more subtle differences in postural strategy among different groups of subjects.

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Christopher McCrum, Katrin Eysel-Gosepath, Gaspar Epro, Kenneth Meijer, Hans H.C.M. Savelberg, Gert-Peter Brüggemann and Kiros Karamanidis

Posturography is used to assess balance in clinical settings, but its relationship to gait stability is unclear. We assessed if dynamic gait stability is associated with standing balance in 12 patients with unilateral vestibulopathy. Participants were unexpectedly tripped during treadmill walking and the change in the margin of stability (MoSchange) and base of support (BoSchange) relative to nonperturbed walking was calculated for the perturbed and first recovery steps. The center of pressure (COP) path during 30-s stance with eyes open and closed, and the distance between the most anterior point of the COP and the anterior BoS boundary during forward leaning (ADist), were assessed using a force plate. Pearson correlations were conducted between the static and dynamic variables. The perturbation caused a large decrease in the BoS, leading to a decrease in MoS. One of 12 correlations was significant (MoSchange at the perturbed step and ADist; r = −.595, P = .041; nonsignificant correlations: .068 ≤ P ≤ .995). The results suggest that different control mechanisms may be involved in stance and gait stability, as a consistent relationship was not found. Therefore, posturography may be of limited use in predicting stability in dynamic situations.