# Aging Impairs Unimanual and Bimanual Hand-Grip Force Control Capabilities
## 1 Introduction  
Aging generally results in neuromuscular deficits including fewer numbers of motor units and muscle atrophy [1]. Furthermore, these age-related motor impairments interfere with activities of daily living, such as holding a cup, buttoning clothes, or driving vehicles [2; 3; 4]. Specifically, older adults frequently experience a loss of strength and fine motor control capabilities during hand-grip movements [5], associated with the incidence of dementia and depression [6; 7]. Thus, estimating the ability to produce and modulate hand-grip force outputs may be an effective way to predict aging progression [8]. The isometric force control paradigm has been frequently used to estimate sensorimotor processing capabilities [9; 10]. Participants were required to match their isometric forces to the submaximal targeted force level using online visual feedback (e.g., simultaneous visual information on their force outputs and a targeted level). Thus, successful isometric force control performances suggest advanced visuomotor correction functions necessary for independent living in older adults [11; 12]. For unimanual hand-grip force control tasks, older women produced greater force errors using the dominant hand than younger women [13], and other studies reported age-induced hand-grip force control deficits for unimanual dominant and non-dominant hands, respectively [14; 15]. Furthermore, force control impairments in older adults increased at lower targeted force levels (e.g., 2.5-10%maximal voluntary contraction; MVC) because of potential changes in motor unit activation patterns such as more variable inter-spike intervals [16; 17; 18]. Moreover, these sensorimotor impairments may increase in bimanual force control tasks because of altered neural resources in the brains of older adults while executing inter-limb motor actions [19; 20; 21]. During bimanual visuomotor tasks, older adults showed higher brain connectivity between the interhemispheric dorsal and ventral premotor areas [20]. Furthermore, hyperactivations across the supplementary motor area, dorsolateral prefrontal cortex, and secondary somatosensory cortex were observed in older adults because additional cognitive and sensorimotor processes may be required for interacting and coordinating motor actions between hands [19; 22]. In fact, older adults showed impaired bimanual hand-grip force control and interlimb coordination patterns [23; 24]. Although isometric hand-grip force control capabilities may be related to motor impairments caused by aging progression, whether age-related force control deficits predominantly appear in either unimanual or bimanual hands (i.e., using both hands simultaneously) is still unclear. Typically, force control capabilities are estimated by quantifying the accuracy and variability of force outputs [10]. Two perspectives on force accuracy and variability assessments include (a) absolute variables (e.g., absolute force error = root mean square error, RMSE, and force variability = standard deviation, SD) and (b) relative variables (i.e., RMSE relative to a targeted force level and SD relative to a mean force). In comparison to absolute force error and variability increased with greater force production [25; 26; 27], relative force control variables may effectively indicate fine motor control functions because of the minimal effects of altered targeted levels on force control performances [28]. Considering aging-induced muscle weakness [29; 30], relative variables may be more appropriate for comparing changes in force control capabilities between younger and older adults [10; 28]. Previous studies have reported no significant differences in force accuracy and variability estimated by absolute variables for older groups compared with younger groups [24; 31]. Although several studies have revealed lower hand-grip force variability as assessed by relative variables in older adults at relatively low targeted force levels [13; 24; 32], there is a lack of studies that have used relative variables to examine force accuracy [14].