Multi-Muscle Synergy Preceding Upward and Downward Stepping in Young Adults

• Grant number: 25/02453-0
• Support Opportunities: Scholarships abroad - Research Internship - Master's degree
• Start date: June 15, 2025
• End date: December 14, 2025
• Field of knowledge: Health Sciences - Physical Education
• Principal Investigator: Paulo Barbosa de Freitas Júnior
• Grantee: Mateus Santos Dias
• Supervisor: Mark Latash
• Host Institution: Pró-Reitoria de Pós-Graduação e Pesquisa. Universidade Cruzeiro do Sul (UNICSUL). São Paulo , SP, Brazil
• Institution abroad: Pennsylvania State University, United States
• Associated to the scholarship: 24/04054-2 - Synergic control of foot position during stair ascent in young adults, BP.MS

Abstract

Maintaining an upright posture during movement requires the motor control system to stabilize key performance variables essential for balance. Strong candidates for these variables include the center of mass (COM) position, center of pressure (COP) trajectory, and trunk orientation. The vertical projection of the COM must remain within the base of support defined by the feet, which necessitates synergistic control of movement, particularly in regulating COP and COM positioning to minimize variability and prevent destabilizing fluctuations. Gait initiation (GI) is a fundamental motor task that transitions the body from standing to walking. This process begins before the leading foot lifts off the ground, requiring coordinated whole-body postural adjustments to overcome inertia and generate forward propulsion. During the preparatory phase of GI, early postural adjustments (EPAs) occur, involving COM shifts and COP displacements in both mediolateral (ML) and anteroposterior (AP) directions. These adjustments are crucial for unloading the leading leg while simultaneously propelling the body forward. Importantly, to initiate movement, the synergistic control that stabilizes COP and COM positions must be temporarily interrupted and disturbed -a process known as anticipatory synergy adjustment (ASA). While the first step in gait initiation typically occurs on a level surface, real-life scenarios often require stepping upward or downward, introducing additional biomechanical challenges. Compared to level walking, stepping upward demands greater angular excursions of lower limb joints and the pelvis, increasing task complexity and fall risk. Similarly, stepping downward involves distinct postural adjustments to control descent and ensure stability. This study investigates how the location of the first step (level, upward, or downward) affects muscle synergy indices and early postural adjustments prior to gait initiation. Sixteen healthy, right-leg dominant adults (18-40 years old) will perform a self-paced single-step gait initiation task under three conditions: (i) stepping forward at the same level, (ii) stepping upward (17 cm), and (iii) stepping downward (17 cm). We will identify muscle groups with parallel activation patterns (muscle modes) and quantify synergy indices stabilizing COP trajectory in AP and ML directions. The analysis will focus on three key metrics: Synergy index at steady state (-1000 to -800 ms before step onset); timing and magnitude of synergy drop (ASA) at step initiation and timing and characteristics of EPAs. We hypothesize that stepping upward and downward will require greater and earlier ASAs and EPAs compared to stepping forward at the same level, due to the increased postural and motor demands imposed by these conditions.