## 1 Introduction  
Force control capabilities are essential for successfully conducting various activities of daily living [1, 2]. Moreover, the isometric force control paradigm may be a viable option to estimate sensorimotor integration functions in the motor system [3, 4] because a performer can produce and maintain isometric forces around the targeted force level via continuous online motor corrections by processing visual feedback. During the isometric force control task, quantifying inconsistency of motor outputs (i.e., force variability) provides information regarding how the motor system controls and regulates motor-related neural oscillations for successful task performance [5, 6, 7]. A recent literature review raised the possibility that force variability properties in the motor system may be associated with changes in the low-frequency oscillations of motor outputs [8]. For healthy young adults, when the concurrent visual information was removed, force oscillations below 0.5 Hz increased with greater force variability during the index finger's abduction force control tasks [9]. Moreover, older adults and patients with stroke revealed more low force oscillations with higher motor variability than those in the control groups [9, 10]. While processing a greater amount of visual information (i.e., higher visual gain conditions), elderly people especially elevated force frequency properties below 0.5 Hz that predicted more force variability across force control tasks using upper and lower limbs [9, 11]. Interestingly, these greater force oscillations below 0.5 Hz were additionally associated with increased oscillation of muscle activity below 0.5 Hz (i.e., EMG burst power) for both younger and older adult groups [11, 12]. Taken together, these findings support an idea that force frequency power below 0.5 Hz may be a crucial predictor indicating altered neuromuscular variability in the motor system. A recent literature review proposed several possible mechanisms underlying the relationship between the greater powers in the low-frequency band and increased force variability [8]. Potentially, the central nervous system (CNS) may select a control strategy for increasing the efficiency in controlling and generating more accurate force outputs by grouping the activations in the motor neuron pools into a low-frequency unit. Next, the low-frequency oscillations in forces may be considered as neuronal noise that presumably originated from higher physiological stress in the sympathetic nervous system (e.g., breathing and heart rate) [13]. Finally, the altered voluntary drive from the brain when more difficult task requirements were provided [9; 12; 14] may be responsible for the low force frequency oscillations. Importantly, force control performances can be affected by various neuromuscular factors such as different targeted force levels. For example, force variability (i.e., standard deviation of force outputs) typically increased at higher targeted force levels [15; 16]. Given that this scaling relation between force variability and targeted force level presumably reflects neuronal changes in the central and peripheral nervous systems [17; 18; 19], increased force variability at higher targeted force levels would be influenced by greater force oscillations below 0.5 Hz. In fact, a prior study reported that increased force variability from 10% to 30% of maximum voluntary contraction (MVC) in healthy young adults was significantly associated with greater force frequency power below 0.5 Hz [12]. Previously, older adults revealed higher force variability than young adult controls across various submaximal targeted force levels (e.g., 10-50% of MVC) because of age-related deficits in the nervous systems [16; 20]. Force control at higher targeted levels is necessary to facilitate greater neural involvements in the motor system (e.g., hyperactivation across sensorimotor cortex and greater low-frequency oscillations in motor neuron discharge rates) [21; 22].