## 4 Discussion
### Changes in Force Control Capabilities and Low-Frequency Oscillations at Different Targeted Levels
Thus, the CNS is necessary to control numerous spinal motor neurons in an efficient way (i.e., grouping spinal motor neurons into low-frequency units) that may increase force frequency power below 0.5 Hz instead of individually modulating spinal motor neurons. In fact, a prior simulation study found that motor unit synchronization patterns, defined as a tendency for comparable temporal intervals of discharge rate between motor units, increased with more efforts toward producing forces leading to deterioration of motor actions (e.g., greater force variability and low force frequency oscillations) [40]. [PERSON] and colleagues additionally reported simulated results that more regular force generation patterns (i.e., lower values of ApEn) appeared with a greater number of motor neurons involvements [41]. Importantly, the inter-spike intervals in aging muscles tended to be less complex (i.e., higher regularity), and furthermore elderly people hardly continued task stability along with more flexibility [42]. Overall, this motor control strategy may be responsible for producing more stereotyped motor outputs (i.e., increased force regularity) rather than generating more flexible force control. Potential Mechanisms underlying the Contribution of Low-Frequency Oscillations to Force Control Capabilities  
Potentially, the effects of changes in low force frequency oscillations on force control performances may originate from neural involvements of the central and peripheral nervous systems. Given that producing stronger muscle forces increased corticomuscular coherences across various frequency bands (e.g., beta and gamma bands) [43], greater low-frequency oscillations at higher targeted force level may be influenced by voluntary drive from supraspinal level to activations in the spinal motor neurons [8]. Moreover, despite no muscle activation findings in this study, several studies reported a potential relationship between whole muscle activity oscillations in the gamma band and low-frequency burst (<1 Hz) in muscle activity contributing to a force frequency structure below 0.5 Hz [11; 12; 14]. Intriguingly, several studies have focused on low-frequency oscillations of brain activity (i.e., electroencephalogram: EEG frequency power below 0.5-4 Hz) that potentially appeared in specific pathologies (e.g., epileptic seizure, attention deficit hyperactivity disorder, and stroke) [44; 45; 46]. Indeed, a recent study that examined postural variability during a forward lean task reported greater low-frequency of center of pressure (COP) oscillations below 0.5 Hz in the older adult group than in the young adult group, and furthermore found these changes were associated with greater COP variability collapsed across the two groups [47]. Presumably, low-frequency oscillations of various biological signals may be related to motor variability in human movements. Thus, future studies should determine whether low-frequency oscillations in force signals are the results of intentional motor control strategies from the brain or inherent noise components of biological signals in the motor system.