Differential activation of the plantar flexor muscles in balance control across different feet orientations on the ground.

The ankle plantar flexor muscles act synergistically to control quiet and dynamic body balance. Previous research has shown that the medial (MG) and lateral (LG) gastrocnemii, and soleus (SOL) are differentially activated as a function of motor task requirements. In the present investigation, we evaluated modulation of the plantar flexors' activation from feet orientation on the ground in an upright stance and the ensuing reactive response to a perturbation. A single group of young participants (n = 24) was evaluated in a task requiring initial stabilization of body balance against a backward pulling load (5% or 10% of body weight) attached to their trunk, and then the balance was suddenly perturbed, releasing the load. Four feet orientations were compared: parallel (0°), outward orientation at 15° and 30°, and the preferred orientation (M = 10.5°). Results revealed a higher activation magnitude of SOL compared to MG-LG when sustaining quiet balance against the 10% load. In the generation of reactive responses, MG was characterized by earlier, steeper, and proportionally higher activation than LG-SOL. Feet orientation at 30° led to higher muscular activation than the other orientations, while the activation relationship across muscles was unaffected by feet orientation. Our results support the conclusion of task-specific differential modulation of the plantar flexor muscles for balance control.

Automatic postural responses are generated according to feet orientation and perturbation magnitude.

This investigation aimed to assess the effect of feet orientation angle in upright stance on automatic postural responses (APRs) to mechanical perturbations of different magnitudes. Perturbation was produced by releasing suddenly a load attached to the participant's trunk, leading to forward body sway. We evaluated APRs to loads corresponding to 5% (low) and 10% (high) of the participant's body weight, comparing the following feet orientations: parallel, preferred (M=10.46°), 15° and 30° for each foot regarding the body midline. Results showed that APRs were sensitive to perturbation magnitude, with the high load leading to increased amplitudes of center of pressure displacement and joints rotation, in addition to stronger and earlier muscular responses. Feet orientation at 30° led to a greater amplitude of center of pressure displacement than the other feet orientations. The low perturbation magnitude led to similar responses both at the hip and ankle across feet orientations, whereas the high load induced increased rotation amplitudes in both joints for feet orientation at 30°. Our results suggest that APRs are generated by the nervous system taking into consideration the biomechanical constraints in the response production. Relevant for standardization of feet placement in evaluations of balance recovery, our results indicated that a moderate range of outward feet orientation angles in stance lead to comparable APRs, while increased outward feet orientation angles lead to distinct postural responses.

Measuring cortical motor hemodynamics during assisted stepping - An fNIRS feasibility study of using a walker.

Walkers are commonly prescribed worldwide to individuals unable to walk independently. Walker usage leads to improved postural control and voluntary movement during step. In the present study, we aimed to provide a concept-proof on the feasibility of an event-related protocol integrating the analyses of biomechanical variables of step initiation and functional near-infrared spectroscopy (fNIRS) to measure activation of the supplementary motor area (SMA) while using a walker. Healthy young participants were tested while stepping with versus without the use of the walker. Behavioral analysis showed that anticipatory postural adjustments (APA) decreased when supporting the body weight on the walker. Delta (without-with) of activation magnitude of the muscle tibialis anterior was positively correlated to the delta of deoxyhemoglobin concentration changes in the SMA. The novelty of this study is the development of a protocol to assess brain function together with biomechanical analysis during the use of a walker. The method sheds light to the potential utility of combining fNIRS and biomechanical assessment during assistive step initiation, which can represent a new opportunity to study populations with mobility deficits.

Aging increases flexibility of postural reactive responses based on constraints imposed by a manual task.

This study compared the effect of stability constraints imposed by a manual task on the adaptation of postural responses between 16 healthy elderly (mean age = 71.56 years, SD = 7.38) and 16 healthy young (mean age = 22.94 years, SD = 4.82) individuals. Postural stability was perturbed through unexpected release of a load attached to the participant's trunk while performing two versions of a voluntary task: holding a tray with a cylinder placed with its flat side down (low constraint) or with its rolling round side down (high constraint). Low and high constraint tasks were performed in alternate blocks of trials. Results showed that young participants adapted muscular activation and kinematics of postural responses in association with previous experience with the first block of manual task constraint, whereas the elderly modulated postural responses based on the current manual constraint. This study provides evidence for flexibility of postural strategies in the elderly to deal with constraints imposed by a manual task.