## 4 Discussion
### Changes in Force Control Capabilities and Low-Frequency Oscillations at Different Targeted Levels  
Not surprisingly, older women revealed force control deficits such as increased force error and absolute force variability at 40% of MVC as compared with those at 10% of MVC consistent with most prior findings [12, 15, 16, 35]. Presumably, simultaneously maintaining and correcting force outputs via the processing of visual information at higher targeted levels is challenging so that these procedures required more neural involvements in the motor system, such as greater brain activation across sensorimotor areas and low-frequency oscillations in motor neuron discharge rates [21, 22]. Importantly, the neural involvements for the elderly people tended to increase to compensate for age-related impairments in the motor system [23]. These greater neural resources during the task may increase difficulty in controlling the degrees of freedom of motor actions for the older adults, resulting in more erroneous and variable forces [24]. Moreover, higher force regularity at 40% of MVC than at 10% of MVC is in line with previous studies [16, 36, 37]. The prior findings indicated that the motor system may decrease motor adaptations (e.g., increased motor optimality) rather than identify various motor options (e.g., increased motor flexibility) while performing more difficult tasks (e.g., tasks that required stronger force productions and tasks without visual feedback) [37, 38]. Beyond the prior findings that showed the contribution of low-frequency oscillations in force signals to force variability [8], we found that these relationship patterns were observed in both force accuracy and regularity across different targeted force level. A recent study reported that increased relative force power below 0.25 Hz was significantly associated with higher task error (\(R^{2}\) = 0.36) and force variability (\(R^{2}\) = 0.33) when healthy young participants executed more challenging dynamic force control tasks (i.e., tracking 1 Hz of the sinusoidal target line) [14]. Common findings from two studies that focused on healthy young adults suggested that task constraints  
Figure 3: Regression models showing relationship between change of low-frequency oscillations in force and force control performances across two targeted force levels. (**A**) force frequency oscillations below 0.5 Hz (absolute power in 0–0.5 Hz) vs. force accuracy (RMSE), (**B**) force frequency oscillations below 1.0 Hz (absolute powers in 0–0.5 Hz and 0.5–1.0 Hz) vs. force variability (SD), and (**C**) force frequency oscillations below 0.5 Hz (absolute power in 0–0.5 Hz) vs. force regularity (ApEn). (e.g., more difficult task requirements such as higher targeted force level and more oscillatory targeted force level) influenced changes in the low force frequency band (<0.5 Hz) resulting in impaired force control performances [12; 14]. Taken together, we confirmed that the potential effects of task constraint on low-frequency oscillation in force signals additionally appeared in the older women group in addition to the young adults [12]. Interestingly, the current findings expanded prior findings by showing a significant relationship between force frequency power below 0.5 Hz and force regularity across different targeted force levels. [PERSON] and colleagues posited that greater force oscillations below 0.5 Hz may be related to the CNS's motor control strategies [8; 14]. When higher force outputs are generated, a greater number of spinal motor neurons are recruited [39]. For example, producing and controlling isometric force outputs at a higher targeted force level (>30% of MVC) may require the recruitment of large motor neurons that innervate fast-twitch, fatigue muscle fibers, in addition to the early recruitment of small motor neurons that innervate slow-twitch muscle fibers [39].