Biomechanical reorganisation of stepping initiation during acute dorsiflexor fatigue

Influence of Swing-Foot Strike Pattern on Balance Control Mechanisms during Gait Initiation over an Obstacle to Be Cleared

Gait initiation (GI) over an obstacle to be cleared is a functional task that is highly challenging for the balance control system. Two swing-foot strike patterns were identified during this task—the rearfoot strike (RFS), where the heel strikes the ground first, and the forefoot strike (FFS), where the toe strikes the ground first. This study investigated the effect of the swing-foot strike pattern on the postural organisation of GI over an obstacle to be cleared. Participants performed a series of obstacle clearance tasks with the instruction to strike the ground with either an FFS or RFS pattern. Results showed that anticipatory postural adjustments in the frontal plane were smaller in FFS than in RFS, while stability was increased in FFS. The vertical braking of the centre of mass (COM) during GI swing phase was attenuated in FFS compared to RFS, leading to greater downward centre of mass velocity at foot contact in FFS. In addition, the collision forces acting on the foot were smaller in FFS than in RFS, as were the slope of these forces and the slope of the C7 vertebra acceleration at foot contact. Overall, these results suggest an interdependent relationship between balance control mechanisms and foot strike pattern for optimal stability control.

Anticipatory postural control differs between low back pain and pelvic girdle pain patients in the absence of visual feedback

PURPOSE

The aim of this study was to examine the effect of vision on anticipatory postural control (APA) responses in two groups of clinically diagnosed chronic low back pain patients, those with Posterior Pelvic Girdle pain and those with Non-Specific Low Back Pain compared to a matched group of healthy controls during the modified Trendelenburg task.

METHODS

Seventy-eight volunteer participants (60 females and 18 males) gave informed consent to take part in this study. 39 with confirmed LBP or PGP lasting longer than 12 weeks and 39 healthy matched controls performed 40 single leg lift tasks (hip flexion to 90° as quickly as possible) with their non-dominant lower limb. A force plate was used to determine the medial-lateral displacement of the center of pressure, and the initiation of weight shift; kinematics was used to determine initiation of leg lift; and electromyography was used to determine onset times from the external oblique (EO), internal oblique (IO) and lumbar multifidus (MF), gluteus maximus (GM) and biceps femoris (BF).

RESULTS

The PGP group showed significantly longer muscle onset latencies in the BF, EO MF with visual occlusion (F_2,746 = 4.51, p < .0001).

CONCLUSION

The muscle onset delays identified between the two LBP sub-groups suggests that pain may not be the primary factor in alteration of APA response. The PGP group show a greater reliance on vision which may signal impairment in multiple feedback channels.

Acute Effects of Whole-Body Vibration on the Postural Organization of Gait Initiation in Young Adults and Elderly: A Randomized Sham Intervention Study

Whole-body vibration (WBV) is a training method that exposes the entire body to mechanical oscillations while standing erect or seated on a vibrating platform. This method is nowadays commonly used by clinicians to improve specific motor outcomes in various sub-populations such as elderly and young healthy adults, either sedentary or well-trained. The present study investigated the effects of acute WBV application on the balance control mechanisms during gait initiation (GI) in young healthy adults and elderly. It was hypothesized that the balance control mechanisms at play during gait initiation may compensate each other in case one or several components are perturbed following acute WBV application, so that postural stability and/or motor performance can be maintained or even improved. It is further hypothesized that this capacity of adaptation is altered with aging. Main results showed that the effects of acute WBV application on the GI postural organization depended on the age of participants. Specifically, a positive effect was observed on dynamic stability in the young adults, while no effect was observed in the elderly. An increased stance leg stiffness was also observed in the young adults only. The positive effect of WBV on dynamic stability was ascribed to an increase in the mediolateral amplitude of "anticipatory postural adjustments" following WBV application, which did overcompensate the potentially destabilizing effect of the increased stance leg stiffness. In elderly, no such anticipatory (nor corrective) postural adaptation was required since acute WBV application did not elicit any change in the stance leg stiffness. These results suggest that WBV application may be effective in improving dynamic stability but at the condition that participants are able to develop adaptive changes in balance control mechanisms, as did the young adults. Globally, these findings are thus in agreement with the hypothesis that balance control mechanisms are interdependent within the postural system, i.e., they may compensate each other in case one component (here the leg stiffness) is perturbed.

Postural adaptations to unilateral knee joint hypomobility induced by orthosis wear during gait initiation

Balance control and whole-body progression during gait initiation (GI) involve knee-joint mobility. Single knee-joint hypomobility often occurs with aging, orthopedics or neurological conditions. The goal of the present study was to investigate the capacity of the CNS to adapt GI organization to single knee-joint hypomobility induced by the wear of an orthosis. Twenty-seven healthy adults performed a GI series on a force-plate in the following conditions: without orthosis ("control"), with knee orthosis over the swing leg ("orth-swing") and with the orthosis over the contralateral stance leg ("orth-stance"). In orth-swing, amplitude of mediolateral anticipatory postural adjustments (APAs) and step width were larger, execution phase duration longer, and anteroposterior APAs smaller than in control. In orth-stance, mediolateral APAs duration was longer, step width larger, and amplitude of anteroposterior APAs smaller than in control. Consequently, step length and progression velocity (which relate to the "motor performance") were reduced whereas stability was enhanced compared to control. Vertical force impact at foot-contact did not change across conditions, despite a smaller step length in orthosis conditions compared to control. These results show that the application of a local mechanical constraint induced profound changes in the global GI organization, altering motor performance but ensuring greater stability.

Short-Term Effects of Thoracic Spine Manipulation on the Biomechanical Organisation of Gait Initiation: A Randomized Pilot Study

Speed performance during gait initiation is known to be dependent on the capacity of the central nervous system to generate efficient anticipatory postural adjustments (APA). According to the posturo-kinetic capacity (PKC) concept, any factor enhancing postural chain mobility and especially spine mobility, may facilitate the development of APA and thus speed performance. "Spinal Manipulative Therapy High-Velocity, Low-Amplitude" (SMT-HVLA) is a healing technique applied to the spine which is routinely used by healthcare practitioners to improve spine mobility. As such, it may have a positive effect on the PKC and therefore facilitate gait initiation. The present study aimed to investigate the short-term effect of thoracic SMT-HVLA on spine mobility, APA and speed performance during gait initiation. Healthy young adults (n = 22) performed a series of gait initiation trials on a force plate before ("pre-manipulation" condition) and after ("post-manipulation" condition) a sham manipulation or an HVLA manipulation applied to the ninth thoracic vertebrae (T9). Participants were randomly assigned to the sham (n = 11) or the HVLA group (n = 11).The spine range of motion (ROM) was assessed in each participant immediately after the sham or HVLA manipulations using inclinometers. The results showed that the maximal thoracic flexion increased in the HVLA group after the manipulation, which was not the case in the sham group. In the HVLA group, results further showed that each of the following gait initiation variables reached a significantly lower mean value in the post-manipulation condition as compared to the pre-manipulation condition: APA duration, peak of anticipatory backward center of pressure displacement, center of gravity velocity at foot-off, mechanical efficiency of APA, peak of center of gravity velocity and step length. In contrast, for the sham group, results showed that none of the gait initiation variables significantly differed between the pre- and post-manipulation conditions. It is concluded that HVLA manipulation applied to T9 has an immediate beneficial effect on spine mobility but a detrimental effect on APA development and speed performance during gait initiation. We suggest that a neural effect induced by SMT-HVLA, possibly mediated by a transient alteration in the early sensory-motor integration, might have masked the potential mechanical benefits associated with increased spine mobility.

Footwear and Foam Surface Alter Gait Initiation of Typical Subjects

Gait initiation is the task commonly used to investigate the anticipatory postural adjustments necessary to begin a new gait cycle from the standing position. In this study, we analyzed whether and how foot-floor interface characteristics influence the gait initiation process. For this purpose, 25 undergraduate students were evaluated while performing a gait initiation task in three experimental conditions: barefoot on a hard surface (barefoot condition), barefoot on a soft surface (foam condition), and shod on a hard surface (shod condition). Two force plates were used to acquire ground reaction forces and moments for each foot separately. A statistical parametric mapping (SPM) analysis was performed in COP time series. We compared the anterior-posterior (AP) and medial-lateral (ML) resultant center of pressure (COP) paths and average velocities, the force peaks under the right and left foot, and the COP integral x force impulse for three different phases: the anticipatory postural adjustment (APA) phase (Phase 1), the swing-foot unloading phase (Phase 2), and the support-foot unloading phase (Phase 3). In Phase 1, significantly smaller ML COP paths and velocities were found for the shod condition compared to the barefoot and foam conditions. Significantly smaller ML COP paths were also found in Phase 2 for the shod condition compared to the barefoot and foam conditions. In Phase 3, increased AP COP velocities were found for the shod condition compared to the barefoot and foam conditions. SPM analysis revealed significant differences for vector COP time series in the shod condition compared to the barefoot and foam conditions. The foam condition limited the impulse-generating capacity of COP shift and produced smaller ML force peaks, resulting in limitations to body-weight transfer from the swing to the support foot. The results suggest that footwear and a soft surface affect COP and impose certain features of gait initiation, especially in the ML direction of Phase 1.

Influence of temporal pressure constraint on the biomechanical organization of gait initiation made with or without an obstacle to clear

Many daily motor tasks have to be performed under a temporal pressure constraint. This study aimed to explore the influence of such constraint on motor performance and postural stability during gait initiation. Young healthy participants initiated gait at maximal velocity under two conditions of temporal pressure: in the low-pressure condition, gait was self-initiated (self-initiated condition, SI); in the high-pressure condition, it was initiated as soon as possible after an acoustic signal (reaction-time condition, RT). Gait was initiated with and without an environmental constraint in the form of an obstacle to be cleared placed in front of participants. Results showed that the duration of postural adjustments preceding swing heel-off ("anticipatory postural adjustments", APAs) was shorter, while their amplitude was larger in RT compared to SI. These larger APAs allowed the participants to reach equivalent postural stability and motor performance in both RT and SI. In addition, the duration of the execution phase of gait initiation increased greatly in the condition with an obstacle to be cleared (OBST) compared to the condition without an obstacle (NO OBST), thereby increasing lateral instability and thus involving larger mediolateral APA. Similar effects of temporal pressure were obtained in NO OBST and OBST. This study shows the adaptability of the postural system to temporal pressure in healthy young adults initiating gait. The outcome of this study may provide a basis for better understanding the aetiology of balance impairments with the risk of falling in frail populations while performing daily complex tasks involving a whole-body progression.

Adaptability of anticipatory postural adjustments associated with voluntary movement

The control of balance is crucial for efficiently performing most of our daily motor tasks, such as those involving goal-directed arm movements or whole body displacement. The purpose of this article is twofold. Firstly, it is to recall how balance can be maintained despite the different sources of postural perturbation arising during voluntary movement. The importance of the so-called “anticipatory postural adjustments” (APA), taken as a “line of defence” against the destabilizing effect induced by a predicted perturbation, is emphasized. Secondly, it is to report the results of recent studies that questioned the adaptability of APA to various constraints imposed on the postural system. The postural constraints envisaged here are classified into biomechanical (postural stability, superimposition of motor tasks), (neuro) physiological (fatigue), temporal (time pressure) and psychological (fear of falling, emotion). Overall, the results of these studies point out the capacity of the central nervous system (CNS) to adapt the spatio-temporal features of APA to each of these constraints. However, it seems that, depending on the constraint, the “priority” of the CNS was focused on postural stability maintenance, on body protection and/or on maintenance of focal movement performance.