## 1 Introduction  
Children with neurodevelopmental disorders such as cerebral palsy (CP) and spina bifida, among other neurodevelopmental disorders, exhibit sensorimotor impairment on one or both sides of their body. These conditions can lead to challenges in motor control, affecting the ability to perform functional activities of daily living (ADLs) [1]. Cerebral palsy, including unilateral, bilateral, and dyskinetic types, is characterized by abnormal muscle tone and movement stemming from disruptions in early brain development [2]. Similarly, CP and spina bifida lead to mobility and sensory deficits. While spina bifida affects both mobility and sensory function, leading to fine and gross motor deficits, typical upper extremity problems in children include muscle weakness, impaired coordination, and reduced fine motor control. These conditions pose unique challenges in tasks that require varied levels of strength, precision, and motor control [3; 4; 5]. Despite extensive research on motor impairments in CP and spina bifida, there is limited knowledge about how these impairments affect positional stability during force-dependent tasks, such as lifting objects. Positional performance is critical for stability and precision, and its deficits may contribute to increased difficulty in handling objects and performing coordinated movements [6]. These impairments present notable challenges to motor function, especially for tasks involving upper limb functionality, which are essential for daily independence. ADLs require various levels of muscle force depending on the task. These force levels are categorized along a 0-100% scale of maximum voluntary contraction (MVC): very low (0-20%), low (20-40%), medium (40-60%), high (60-80%), and very high (80-100%) [7]. For example, fine motor tasks like eating and dressing typically require lower MVC levels, while lifting objects demands higher MVC levels. Research shows that motor unit control strategies adapt to these force demands. Small motor units are recruited during low-force, high-precision activities, while large motor units are activated for high-force tasks [8; 9]. For children with neurodevelopmental disabilities, tasks that typically require low or medium MVC levels, like eating, dressing, or typing, can be especially difficult due to muscle weakness, impaired coordination, and reduced fine motor control. As task intensity increases (e.g., lifting a box), fast-twitch muscle fibers are recruited, leading to increased fatigue and reduced precision, especially for those with compromised motor units [10]. Thus, low-intensity precision and high-intensity strength tasks can be challenging, limiting their independence in ADLs. Effective bimanual motor control is, however, critical for ADLs, especially in tasks like lifting a box or zippering a jacket that require the coordinated use of both hands [11] and bimanual dexterity [12]. Such coordination is vital for enhancing independence and improving quality of life [13]. In tasks involving object positioning (e.g., manipulating a knife, fork, or box), adequate strength enables precise, coordinated movements [14]. Optimal object positioning is essential for task execution [15]. When lifting a box, for example, its position relative to the lifter, height from the ground, and orientation affect stability and control strategies [16]. While various methods exist for assessing motor performance, Inertial Measurement Units (IMUs) offer an effective tool for capturing positional stability in different movement planes. Prior studies have demonstrated the use of IMUs in motor assessments for individuals with CP and other neuromuscular conditions, providing a reliable way to evaluate dynamic movement patterns [17; 18; 19]. This study evaluates positional performance in children with neurodevelopmental disabilities using IMUs to measure tilt angles during a bimanual box-lifting task. Stable positional performances are crucial to prevent spillage or dropping, yet how these challenges vary with force levels remains unclear.