# Aging Impairs Unimanual and Bimanual Hand-Grip Force Control Capabilities
## 4 Discussion  
We examined unimanual and bimanual hand-grip force control performances between older and younger groups by estimating absolute and relative variables. The absolute force accuracy was not significantly different between the younger and older adult groups, and further, younger adults revealed greater absolute force variability for unimanual and bimanual hand conditions than older adults at 40% of MVC. However, the older adult group showed lower relative force accuracy and higher relative force variability than the younger adult group at 10% MVC for all hand conditions. The ROC curve analysis identified that the values of relative force accuracy in unimanual dominant and bimanual hand conditions at 10% MVC were favorable for distinguishing force control deficits in older adults. Moreover, secondary analysis revealed that increased body mass index values in older adults were positively correlated with higher relative force accuracy and lower relative force variability at 10% MVC in the unimanual non-dominant hand condition. Absolute force control variables found no significant age-related differences, whereas younger adults produced greater absolute force variability at 40% MVC. These findings can account for the relationship between increased force control deficits estimated by absolute force accuracy and variability and greater force productions [21; 25; 26; 27]. For example, younger and older adults showed greater SDs of forces as targeted force levels increased, and younger adult groups produced greater absolute force variability than older adults at 80% MVC [42; 43]. Presumably, the application of absolute force control variables may be insufficient for differentiating age-related motor control changes from those in younger adults because of decreased maximal and submaximal force production in older adults. Relative variables on force accuracy and variability identified significant impaired force control capabilities in older adults at 10% MVC. These results support the proposition that age-related unimanual force control deficits appear in tasks requiring lower force modulation (e.g., 2-10% of MVC) [13; 44; 45]. The relative measurement of force variability is frequently used to identify potential force control deficits observed in older adults [10; 16; 28; 46]. In contrast, previous studies on force accuracy frequently used absolute outcome measures (e.g., RMSE), and they failed to identify altered force control capabilities in older adults [14; 24; 31; 47]. Considering the dependence of absolute force control error on the levels of force outputs [26; 48], age-related muscle weakness may account for underestimating the ability to precisely sustain forces. For example, a significant reduction in relative force accuracy appeared in older adults across all hand (dominant vs. non-dominant) and task (stationary vs. forward-reaching vs. backward-reaching) conditions, whereas absolute force accuracy was only reduced in the stationary task with the non-dominant hand [14]. Presumably, the level of relative force accuracy may be an effective indicator of age-induced changes in force control capabilities across various experimental conditions [47]. The ROC analysis additionally confirmed that relative outcome variables on force accuracy and variability produced by unimanual and bimanual hands at 10% MVC successfully identified age-related force control deficits. Moreover, we observed that relative force accuracy showed a relatively good diagnostic quality (i.e., AUC \(\geq\) 0.8). Force control deficits in older adults at lower targeted levels may be related to greater variability in neural activation in motor neuron pools [17; 18]. Older adults showed higher inter-spike interval variability across multiple motor units during index finger force control at 10% MVC that significantly correlated with increased force variability [18].