Paulo Barbosa de Freitas

Step initiation requires the coordinated preparation of posture and movement, yet how first step constraints influence multi-muscle synergies stabilizing the center of pressure (COP), anticipatory synergy adjustments (ASAs), and early postural adjustments (EPAs) remains poorly understood. This study examined the effects of stepping surface height and step length on these outcomes in healthy young adults. Sixteen participants initiated a step under five conditions: level, step-up, step-down, short-step, and long-step. Surface electromyography was recorded from 16 leg and trunk muscles bilaterally, and COP coordinates were obtained from a force platform. The uncontrolled manifold approach was used to quantify multi-muscle synergies and ASAs stabilizing COP in the anteroposterior (AP) and mediolateral (ML) directions during steady-steady and around the foot lift-off {−25:+25 ms}. Multi-muscle synergies stabilizing COP-AP were present across all conditions, and their index did not differ significantly across stepping conditions, supporting the view that steady-state synergic organization reflects the demands of upright stance rather than anticipation of the upcoming step. Synergies stabilizing COP-ML were absent in all conditions. ASAs were not detected in any condition or direction, suggesting that step initiation relies more on EPA-based rather than synergy-level anticipatory mechanisms. Stepping surface height significantly influenced EPA onset timing and COP displacement magnitude in both directions, with the step-down condition producing earlier EPA onset and smaller displacements than the step-up condition. Step length did not significantly affect EPAs. These findings reveal a dissociation between synergic muscle organization and COP trajectory control and highlight the asymmetric postural demands of descending versus ascending steps during step initiation.

BACKGROUND:Levodopa effects on postural sway may vary between motor subtypes Parkinson’s disease (PD). AIM:This study investigated the impact of Levodopa on postural sway and identified clinical predictors of postural sway in individuals with PD classified as either postural instability/gait difficulty (PIGD) or tremor dominant (TD). METHODS:Thirty individuals with PD were classified as PIGD (n=12) or TD (n=18) subtypes. A subgroup of TD-participants without freezing of gait (FOG) was also analyzed (n=14) to control for FOG as a confounding factor. Participants stood as still as possible on a force plate for 30-seconds with closed-eyes both On- and Off-medication. Repeated-measures analyses of variance were used to compare groups and assess the effect of medication on postural sway. Backward regression analyses were conducted to identify the clinical predictors of postural sway in both medication states for each subtype. RESULTS:Medication state significantly influenced postural sway in the PIGD group, with increased sway observed in the On-medication condition, while the sway in the TD group remained unchanged across medication states. In the TD group, postural sway under medication was predicted by disease duration and cognition performance. In the PIGD group, sway in the Off-medication state was predicted by motor severity and cognition performance. FOG was confirmed as a confounding factor in postural sway among TD participants. INTERPRETATION:Levodopa affected postural balance only PIGD-subtype. Despite subtype differences in Levodopa effects, cognition emerged as a common predictor of postural sway between groups. Excluding participants with FOG strengthened the relationship between clinical characteristics and postural sway in the TD-subtype.

Introduction: Jumping is a fundamental element of classical ballet, often performed with the feet turned out, in a position known as en dehors , achieved through hip external rotation. Experience level and impact attenuation strategies may play a crucial role in reducing injury risk during jump landings. This study aimed to investigate the impact during drop jump landings in en dehors versus parallel foot orientation in experienced ballet dancers. Method: Seventeen experienced ballet dancers (aged 18-31 years, with an average of 13 years of experience) and 15 controls (aged 20-32 years old) who were not dancers and did not engage in regular physical activity, participated in the study. Participants performed drop jumps from a 30 cm high block, landed on two force platforms, and immediately executed a maximum vertical jump with feet in en dehors position or parallel. Maximum vertical ground reaction force, time to reach this maximum, maximum rate of force development during drop landing, and maximum height of the jump performed immediately after drop landing were calculated and compared between groups and foot orientations. Results: Time to reach the maximum vertical ground reaction force was shorter in the en dehors compared to parallel foot orientation, with no significant group differences. Ballet dancers achieved higher vertical jumps after the drop landing compared to controls. Conclusions: Long-term classical ballet practice does not lead to lower impact landings in en dehors compared to parallel foot orientation. Ballet dancers may take more advantage of the impact and rebounding for performing the subsequent vertical jump.

The uncontrolled manifold (UCM) framework was employed to investigate the presence of a multi-joint synergy stabilizing foot height during the swing phase of treadmill running and its potential dependence on running speed. Experienced runners (N = 28; aged 22-51) ran on an instrumented treadmill set at three different speeds: 2.5, 3.5, and 4.5 m/s. Kinematic data were utilized to calculate UCM outcomes: variances in the joint space that had no effect (V_UCM) and those that influenced (V_ORT) foot height, and the synergy index (ΔV_Z, the normalized difference between V_UCM and V_ORT). They were computed for each normalized frame (1-100%) of the swing phase and averaged in 10% intervals. ΔV_Z was greater than zero and varied across the swing phase, being lowest at 51-60% and highest at 81-100%. ΔV_Z was the lowest at the slowest speed in the second half of the swing phase, because of a low V_UCM. The findings indicate that the CNS organizes a multi-joint synergy to stabilize foot height, with the strongest synergy at the end of the swing phase to ensure safe foot placement and landing. Faster running speeds enhance this synergy, allowing greater adaptability to perturbations, while slower speeds lead to a more cautious approach, reducing overall variance.

Background: Isometric maximum force (MaxF) and rate of force development (MaxRFD) obtained from sustained contractions are two important outcomes used in clinical and research settings that evaluate strength and power of an individual, respectively. These outcomes are often impaired in people with multiple sclerosis (PwMS) and are associated with functional limitations. The values of these isometric outcomes can be influenced by the verbal instructions given to the participants prior to contraction. Objective: We investigated the effects of two common instructions - “as hard as possible” (AH) and “as fast and as hard as possible” (AFH) - on MaxF and MaxRFD in both upper and lower extremities in people with multiple sclerosis (PwMS) and in controls. Methods: Twenty-seven PwMS and age- and gender-matched healthy controls completed isometric assessments of grip force (GF) and knee extension (KE) under both AH and AFH instructions. Results: Instructions did not significantly affect MaxF in either group or muscle group (p>0.05). However, AFH instruction yielded a significantly higher MaxRFD compared to AH in both GF and KE across both groups (p<0.001). Conclusions: MaxF is unaffected by instruction type, while MaxRFD is sensitive to instruction that emphasizes both speed and magnitude of a contraction. Therefore, AFH instruction enables efficient and simultaneous assessment of both outcomes in neuromuscular evaluations, including clinical populations like PwMS where both outcomes may be impaired, and where the effects of disease could be more prevalent in the lower extremity than the upper extremity.

BACKGROUND: Stabilization of the vertical body oscillation is important to running efficiency and performance and might be affected by running speed. AIM: We used the uncontrolled manifold (UCM) framework to test whether there would be a multi-joint synergy stabilizing the vertical body oscillation on stance phase of treadmill running and the effect of running speed. METHOD: Experienced runners (N=28; 22-51 years old) ran on an instrumented treadmill set at 2.5, 3.5, and 4.5 m/s. Ankle, knee, and hip angles in the sagittal plane and pelvis obliquity were used as elemental variables (DOF=4) to calculate the synergy index (DVZ), which is the normalized difference between the variances in the joint space that did not affect (VUCM) and affected (VORT) the vertical body oscillation (i.e., center of mass) during the stance phase (1-100%). Statistical parametric mapping (SPM) analyses were performed to detect the presence of synergy and the effect of running speed. RESULTS: SPM analyses showed that independent of running speed, synergy was always present (DVZ>0), except between 13% and 17% in the slowest speed. Also, results showed that the synergy index was unaffected by speed. Additionally, while no effect of speed was seen in the VORT, VUCM in the fastest running speed was the highest between 7 and 43% of the stance phase. INTERPRETATION: The findings indicated the existence of a synergy stabilizing the vertical body oscillation during the stance phase in running and the running speed did not affect the stabilization of the vertical body oscillation during the stance phase.