How do limited familial ties and community relationships shape health-seeking in older adulthood? Evidence from Indonesia.

Human movement demonstrates some key typical features, one of which is the adherence to smooth, coordinated trajectories that are thought to reflect internal optimization processes within the motor system. A foundational theory in this domain, proposed by Hogan and colleagues [], suggests that the central nervous system minimizes the third derivative of position (jerk) when planning point-to-point movements. This optimization leads to the minimum-jerk trajectory (MJT), which is obtained by minimizing the integral of the squared jerk over the duration of movement: Solving this optimization yields analytically defined position and velocity profiles []:
where and denote position and velocity at time , and denote start and finish times, is the movement duration, and represent the start and end positions, and is normalized time. These equations define the smooth sigmoidal position and bell-shaped velocity profiles characteristic of natural movement. Subsequent studies have demonstrated that MJT-like behavior is not limited to arm reaching but is also observed across a range of behaviors, including vertical arm movements, drawing, catching, head movements, chewing, and saccadic eye movements [–]. These findings suggest that the nervous system may apply a consistent planning strategy that generalizes across joints, effectors, and tasks. In addition, MJT-like behavior has been reported during both simple and complex multi-joint upper-limb movements [, ]. The presence of bell shaped velocity profiles and MJT-like trajectories mainly during point-to-point reaching movements has been extensively documented, including in the seminal works of Flash and Hogan [, ], which demonstrated that unconstrained reaching movements retain smooth trajectory structure even when direct visual information about the arm is reduced. These studies established that movement trajectories exhibit approximately straight hand paths and bell shaped tangential velocity profiles consistent with minimum jerk predictions. Subsequent studies have similarly demonstrated that these invariant movement features are preserved across a variety of motor tasks and sensorimotor conditions. Most studies of MJT-like trajectories focus on limb kinematics, leaving open the question of whether these smooth movement patterns persist when movement is produced without a limb, using alternative biosignals as the control signal. In our recent work [], we addressed this gap by examining whether MJT-like behavior is preserved when the control signal is derived directly from muscle activity rather than from overt limb motion. Using sonomyography [–], a technique that measures real-time muscle deformation via ultrasound imaging, we developed a proportional control interface in which participants generated one-dimensional cursor movements by modulating deep and superficial flexor muscles of the forearm during wrist flexion and extension movements. This framework allowed us to characterize movement trajectories within a control domain that precedes overt kinematic execution and is directly associated with muscle activation. While sonomyography served as the measurement modality in our previous work, our objective was not to evaluate the utility of sonomyography per se, but rather to determine whether control derived from muscle-based activity adheres to the same fundamental principles of motor coordination observed in conventional reaching.